21 files changed, 7294 insertions, 0 deletions
diff --git a/src/lib-dict/Makefile.am b/src/lib-dict/Makefile.am
new file mode 100644
index 0000000..97e5eb6
--- /dev/null
+++ b/src/lib-dict/Makefile.am
@@ -0,0 +1,73 @@
+noinst_LTLIBRARIES = \
+	libdict.la
+
+AM_CPPFLAGS = \
+	-I$(top_srcdir)/src/lib \
+	-I$(top_srcdir)/src/lib-test \
+	-I$(top_srcdir)/src/lib-settings
+
+base_sources = \
+	dict.c \
+	dict-client.c \
+	dict-file.c \
+	dict-memcached.c \
+	dict-memcached-ascii.c \
+	dict-redis.c \
+	dict-fail.c \
+	dict-transaction-memory.c
+
+libdict_la_SOURCES = \
+	$(base_sources)
+
+headers = \
+	dict.h \
+	dict-client.h \
+	dict-private.h \
+	dict-transaction-memory.h
+
+# Internally, the dict methods yield via lua_yieldk() as implemented in Lua
+# 5.3 and newer.
+if DLUA_WITH_YIELDS
+noinst_LTLIBRARIES += libdict_lua.la
+
+libdict_lua_la_SOURCES = \
+	dict-lua.c \
+	dict-iter-lua.c \
+	dict-txn-lua.c
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+	$(AM_CPPFLAGS) \
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+	-I$(top_srcdir)/src/lib-lua
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+	libdict.la
+
+headers += \
+	dict-lua.h \
+	dict-lua-private.h
+endif
+
+pkginc_libdir=$(pkgincludedir)
+pkginc_lib_HEADERS = $(headers)
+
+test_programs = \
+	test-dict
+
+noinst_PROGRAMS = $(test_programs) test-dict-client
+
+test_libs = \
+	../lib-test/libtest.la \
+	../lib/liblib.la
+
+test_dict_SOURCES = test-dict.c
+test_dict_LDADD = libdict.la $(test_libs)
+test_dict_DEPENDENCIES = $(noinst_LTLIBRARIES) $(test_libs)
+
+test_dict_client_SOURCES = test-dict-client.c
+test_dict_client_LDADD = $(noinst_LTLIBRARIES) ../lib/liblib.la
+test_dict_client_DEPENDENCIES = $(noinst_LTLIBRARIES) $(test_libs)
+
+check-local:
+	for bin in $(test_programs) $(check_PROGRAMS); do \
+	  if ! $(RUN_TEST) ./$$bin; then exit 1; fi; \
+	done
diff --git a/src/lib-dict/Makefile.in b/src/lib-dict/Makefile.in
new file mode 100644
index 0000000..3c55975
--- /dev/null
+++ b/src/lib-dict/Makefile.in
@@ -0,0 +1,970 @@
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+	-rm -f TAGS ID GTAGS GRTAGS GSYMS GPATH tags
+
+distdir: $(BUILT_SOURCES)
+	$(MAKE) $(AM_MAKEFLAGS) distdir-am
+
+distdir-am: $(DISTFILES)
+	@srcdirstrip=`echo "$(srcdir)" | sed 's/[].[^$$\\*]/\\\\&/g'`; \
+	topsrcdirstrip=`echo "$(top_srcdir)" | sed 's/[].[^$$\\*]/\\\\&/g'`; \
+	list='$(DISTFILES)'; \
+	  dist_files=`for file in $$list; do echo $$file; done | \
+	  sed -e "s|^$$srcdirstrip/||;t" \
+	      -e "s|^$$topsrcdirstrip/|$(top_builddir)/|;t"`; \
+	case $$dist_files in \
+	  */*) $(MKDIR_P) `echo "$$dist_files" | \
+			   sed '/\//!d;s|^|$(distdir)/|;s,/[^/]*$$,,' | \
+			   sort -u` ;; \
+	esac; \
+	for file in $$dist_files; do \
+	  if test -f $$file || test -d $$file; then d=.; else d=$(srcdir); fi; \
+	  if test -d $$d/$$file; then \
+	    dir=`echo "/$$file" | sed -e 's,/[^/]*$$,,'`; \
+	    if test -d "$(distdir)/$$file"; then \
+	      find "$(distdir)/$$file" -type d ! -perm -700 -exec chmod u+rwx {} \;; \
+	    fi; \
+	    if test -d $(srcdir)/$$file && test $$d != $(srcdir); then \
+	      cp -fpR $(srcdir)/$$file "$(distdir)$$dir" || exit 1; \
+	      find "$(distdir)/$$file" -type d ! -perm -700 -exec chmod u+rwx {} \;; \
+	    fi; \
+	    cp -fpR $$d/$$file "$(distdir)$$dir" || exit 1; \
+	  else \
+	    test -f "$(distdir)/$$file" \
+	    || cp -p $$d/$$file "$(distdir)/$$file" \
+	    || exit 1; \
+	  fi; \
+	done
+check-am: all-am
+	$(MAKE) $(AM_MAKEFLAGS) check-local
+check: check-am
+all-am: Makefile $(PROGRAMS) $(LTLIBRARIES) $(HEADERS)
+installdirs:
+	for dir in "$(DESTDIR)$(pkginc_libdir)"; do \
+	  test -z "$$dir" || $(MKDIR_P) "$$dir"; \
+	done
+install: install-am
+install-exec: install-exec-am
+install-data: install-data-am
+uninstall: uninstall-am
+
+install-am: all-am
+	@$(MAKE) $(AM_MAKEFLAGS) install-exec-am install-data-am
+
+installcheck: installcheck-am
+install-strip:
+	if test -z '$(STRIP)'; then \
+	  $(MAKE) $(AM_MAKEFLAGS) INSTALL_PROGRAM="$(INSTALL_STRIP_PROGRAM)" \
+	    install_sh_PROGRAM="$(INSTALL_STRIP_PROGRAM)" INSTALL_STRIP_FLAG=-s \
+	      install; \
+	else \
+	  $(MAKE) $(AM_MAKEFLAGS) INSTALL_PROGRAM="$(INSTALL_STRIP_PROGRAM)" \
+	    install_sh_PROGRAM="$(INSTALL_STRIP_PROGRAM)" INSTALL_STRIP_FLAG=-s \
+	    "INSTALL_PROGRAM_ENV=STRIPPROG='$(STRIP)'" install; \
+	fi
+mostlyclean-generic:
+
+clean-generic:
+
+distclean-generic:
+	-test -z "$(CONFIG_CLEAN_FILES)" || rm -f $(CONFIG_CLEAN_FILES)
+	-test . = "$(srcdir)" || test -z "$(CONFIG_CLEAN_VPATH_FILES)" || rm -f $(CONFIG_CLEAN_VPATH_FILES)
+
+maintainer-clean-generic:
+	@echo "This command is intended for maintainers to use"
+	@echo "it deletes files that may require special tools to rebuild."
+clean: clean-am
+
+clean-am: clean-generic clean-libtool clean-noinstLTLIBRARIES \
+	clean-noinstPROGRAMS mostlyclean-am
+
+distclean: distclean-am
+		-rm -f ./$(DEPDIR)/dict-client.Plo
+	-rm -f ./$(DEPDIR)/dict-fail.Plo
+	-rm -f ./$(DEPDIR)/dict-file.Plo
+	-rm -f ./$(DEPDIR)/dict-memcached-ascii.Plo
+	-rm -f ./$(DEPDIR)/dict-memcached.Plo
+	-rm -f ./$(DEPDIR)/dict-redis.Plo
+	-rm -f ./$(DEPDIR)/dict-transaction-memory.Plo
+	-rm -f ./$(DEPDIR)/dict.Plo
+	-rm -f ./$(DEPDIR)/libdict_lua_la-dict-iter-lua.Plo
+	-rm -f ./$(DEPDIR)/libdict_lua_la-dict-lua.Plo
+	-rm -f ./$(DEPDIR)/libdict_lua_la-dict-txn-lua.Plo
+	-rm -f ./$(DEPDIR)/test-dict-client.Po
+	-rm -f ./$(DEPDIR)/test-dict.Po
+	-rm -f Makefile
+distclean-am: clean-am distclean-compile distclean-generic \
+	distclean-tags
+
+dvi: dvi-am
+
+dvi-am:
+
+html: html-am
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+maintainer-clean: maintainer-clean-am
+		-rm -f ./$(DEPDIR)/dict-client.Plo
+	-rm -f ./$(DEPDIR)/dict-fail.Plo
+	-rm -f ./$(DEPDIR)/dict-file.Plo
+	-rm -f ./$(DEPDIR)/dict-memcached-ascii.Plo
+	-rm -f ./$(DEPDIR)/dict-memcached.Plo
+	-rm -f ./$(DEPDIR)/dict-redis.Plo
+	-rm -f ./$(DEPDIR)/dict-transaction-memory.Plo
+	-rm -f ./$(DEPDIR)/dict.Plo
+	-rm -f ./$(DEPDIR)/libdict_lua_la-dict-iter-lua.Plo
+	-rm -f ./$(DEPDIR)/libdict_lua_la-dict-lua.Plo
+	-rm -f ./$(DEPDIR)/libdict_lua_la-dict-txn-lua.Plo
+	-rm -f ./$(DEPDIR)/test-dict-client.Po
+	-rm -f ./$(DEPDIR)/test-dict.Po
+	-rm -f Makefile
+maintainer-clean-am: distclean-am maintainer-clean-generic
+
+mostlyclean: mostlyclean-am
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+mostlyclean-am: mostlyclean-compile mostlyclean-generic \
+	mostlyclean-libtool
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+pdf: pdf-am
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+.MAKE: check-am install-am install-strip
+
+.PHONY: CTAGS GTAGS TAGS all all-am am--depfiles check check-am \
+	check-local clean clean-generic clean-libtool \
+	clean-noinstLTLIBRARIES clean-noinstPROGRAMS cscopelist-am \
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+	distclean-libtool distclean-tags distdir dvi dvi-am html \
+	html-am info info-am install install-am install-data \
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+	install-pkginc_libHEADERS install-ps install-ps-am \
+	install-strip installcheck installcheck-am installdirs \
+	maintainer-clean maintainer-clean-generic mostlyclean \
+	mostlyclean-compile mostlyclean-generic mostlyclean-libtool \
+	pdf pdf-am ps ps-am tags tags-am uninstall uninstall-am \
+	uninstall-pkginc_libHEADERS
+
+.PRECIOUS: Makefile
+
+
+check-local:
+	for bin in $(test_programs) $(check_PROGRAMS); do \
+	  if ! $(RUN_TEST) ./$$bin; then exit 1; fi; \
+	done
+
+# Tell versions [3.59,3.63) of GNU make to not export all variables.
+# Otherwise a system limit (for SysV at least) may be exceeded.
+.NOEXPORT:
diff --git a/src/lib-dict/dict-client.c b/src/lib-dict/dict-client.c
new file mode 100644
index 0000000..2c688ef
--- /dev/null
+++ b/src/lib-dict/dict-client.c
@@ -0,0 +1,1508 @@
+/* Copyright (c) 2005-2018 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "array.h"
+#include "llist.h"
+#include "str.h"
+#include "strescape.h"
+#include "file-lock.h"
+#include "time-util.h"
+#include "connection.h"
+#include "ostream.h"
+#include "eacces-error.h"
+#include "dict-private.h"
+#include "dict-client.h"
+
+#include <unistd.h>
+#include <fcntl.h>
+
+/* Disconnect from dict server after this many milliseconds of idling after
+   sending a command. Because dict server does blocking dict accesses, it can
+   handle only one client at a time. This is why the default timeout is zero,
+   so that there won't be many dict processes just doing nothing. Zero means
+   that the socket is disconnected immediately after returning to ioloop. */
+#define DICT_CLIENT_DEFAULT_TIMEOUT_MSECS 0
+
+/* Abort dict lookup after this many seconds. */
+#define DICT_CLIENT_REQUEST_TIMEOUT_MSECS 30000
+/* When dict lookup timeout is reached, wait a bit longer if the last dict
+   ioloop wait was shorter than this. */
+#define DICT_CLIENT_REQUEST_TIMEOUT_MIN_LAST_IOLOOP_WAIT_MSECS 1000
+/* Log a warning if dict lookup takes longer than this many milliseconds. */
+#define DICT_CLIENT_DEFAULT_WARN_SLOW_MSECS 5000
+
+struct client_dict_cmd {
+	int refcount;
+	struct client_dict *dict;
+	struct timeval start_time;
+	char *query;
+	unsigned int async_id;
+	struct timeval async_id_received_time;
+
+	uint64_t start_global_ioloop_usecs;
+	uint64_t start_dict_ioloop_usecs;
+	uint64_t start_lock_usecs;
+
+	bool reconnected;
+	bool retry_errors;
+	bool no_replies;
+	bool unfinished;
+	bool background;
+
+	void (*callback)(struct client_dict_cmd *cmd,
+			 enum dict_protocol_reply reply, const char *value,
+			 const char *const *extra_args, const char *error,
+			 bool disconnected);
+        struct client_dict_iterate_context *iter;
+	struct client_dict_transaction_context *trans;
+
+	struct {
+		dict_lookup_callback_t *lookup;
+		dict_transaction_commit_callback_t *commit;
+		void *context;
+	} api_callback;
+};
+
+struct dict_client_connection {
+	struct connection conn;
+	struct client_dict *dict;
+};
+
+struct client_dict {
+	struct dict dict;
+	struct dict_client_connection conn;
+
+	char *uri;
+	enum dict_data_type value_type;
+	unsigned warn_slow_msecs;
+
+	time_t last_failed_connect;
+	char *last_connect_error;
+
+	struct io_wait_timer *wait_timer;
+	uint64_t last_timer_switch_usecs;
+	struct timeout *to_requests;
+	struct timeout *to_idle;
+	unsigned int idle_msecs;
+
+	ARRAY(struct client_dict_cmd *) cmds;
+	struct client_dict_transaction_context *transactions;
+
+	unsigned int transaction_id_counter;
+};
+
+struct client_dict_iter_result {
+	const char *key, *const *values;
+};
+
+struct client_dict_iterate_context {
+	struct dict_iterate_context ctx;
+	char *error;
+	char *path;
+	enum dict_iterate_flags flags;
+	int refcount;
+
+	pool_t results_pool;
+	ARRAY(struct client_dict_iter_result) results;
+	unsigned int result_idx;
+
+	bool cmd_sent;
+	bool seen_results;
+	bool finished;
+	bool deinit;
+};
+
+struct client_dict_transaction_context {
+	struct dict_transaction_context ctx;
+	struct client_dict_transaction_context *prev, *next;
+
+	char *first_query;
+	char *error;
+
+	unsigned int id;
+	unsigned int query_count;
+
+	bool sent_begin:1;
+};
+
+static struct connection_list *dict_connections;
+
+static int client_dict_connect(struct client_dict *dict, const char **error_r);
+static int client_dict_reconnect(struct client_dict *dict, const char *reason,
+				 const char **error_r);
+static void client_dict_disconnect(struct client_dict *dict, const char *reason);
+static const char *dict_wait_warnings(const struct client_dict_cmd *cmd);
+
+static struct client_dict_cmd *
+client_dict_cmd_init(struct client_dict *dict, const char *query)
+{
+	struct client_dict_cmd *cmd;
+
+	io_loop_time_refresh();
+
+	cmd = i_new(struct client_dict_cmd, 1);
+	cmd->refcount = 1;
+	cmd->dict = dict;
+	cmd->query = i_strdup(query);
+	cmd->start_time = ioloop_timeval;
+	cmd->start_global_ioloop_usecs = ioloop_global_wait_usecs;
+	cmd->start_dict_ioloop_usecs = io_wait_timer_get_usecs(dict->wait_timer);
+	cmd->start_lock_usecs = file_lock_wait_get_total_usecs();
+	return cmd;
+}
+
+static void client_dict_cmd_ref(struct client_dict_cmd *cmd)
+{
+	i_assert(cmd->refcount > 0);
+	cmd->refcount++;
+}
+
+static bool client_dict_cmd_unref(struct client_dict_cmd *cmd)
+{
+	i_assert(cmd->refcount > 0);
+	if (--cmd->refcount > 0)
+		return TRUE;
+
+	i_assert(cmd->trans == NULL);
+
+	i_free(cmd->query);
+	i_free(cmd);
+	return FALSE;
+}
+
+static bool
+dict_cmd_callback_line(struct client_dict_cmd *cmd, const char *const *args)
+{
+	const char *value = args[0];
+	enum dict_protocol_reply reply;
+
+	if (value == NULL) {
+		/* "" is a valid iteration reply */
+		reply = DICT_PROTOCOL_REPLY_ITER_FINISHED;
+	} else {
+		reply = value[0];
+		value++;
+		args++;
+	}
+
+	cmd->unfinished = FALSE;
+	cmd->callback(cmd, reply, value, args, NULL, FALSE);
+	return !cmd->unfinished;
+}
+
+static void
+dict_cmd_callback_error(struct client_dict_cmd *cmd, const char *error,
+			bool disconnected)
+{
+	const char *null_arg = NULL;
+
+	cmd->unfinished = FALSE;
+	if (cmd->callback != NULL) {
+		cmd->callback(cmd, DICT_PROTOCOL_REPLY_ERROR,
+			      "", &null_arg, error, disconnected);
+	}
+	i_assert(!cmd->unfinished);
+}
+
+static struct client_dict_cmd *
+client_dict_cmd_first_nonbg(struct client_dict *dict)
+{
+	struct client_dict_cmd *const *cmds;
+	unsigned int i, count;
+
+	cmds = array_get(&dict->cmds, &count);
+	for (i = 0; i < count; i++) {
+		if (!cmds[i]->background)
+			return cmds[i];
+	}
+	return NULL;
+}
+
+static void client_dict_input_timeout(struct client_dict *dict)
+{
+	struct client_dict_cmd *cmd;
+	const char *error;
+	uint64_t msecs_in_last_dict_ioloop_wait;
+	int cmd_diff;
+
+	/* find the first non-background command. there must be at least one. */
+	cmd = client_dict_cmd_first_nonbg(dict);
+	i_assert(cmd != NULL);
+
+	cmd_diff = timeval_diff_msecs(&ioloop_timeval, &cmd->start_time);
+	if (cmd_diff < DICT_CLIENT_REQUEST_TIMEOUT_MSECS) {
+		/* need to re-create this timeout. the currently-oldest
+		   command was added when another command was still
+		   running with an older timeout. */
+		timeout_remove(&dict->to_requests);
+		dict->to_requests =
+			timeout_add(DICT_CLIENT_REQUEST_TIMEOUT_MSECS - cmd_diff,
+				    client_dict_input_timeout, dict);
+		return;
+	}
+
+	/* If we've gotten here because all the time was spent in other ioloops
+	   or locks, make sure there's a bit of time waiting for the dict
+	   ioloop as well. There's a good chance that the reply can be read. */
+	msecs_in_last_dict_ioloop_wait =
+		(io_wait_timer_get_usecs(dict->wait_timer) -
+		 dict->last_timer_switch_usecs + 999) / 1000;
+	if (msecs_in_last_dict_ioloop_wait < DICT_CLIENT_REQUEST_TIMEOUT_MIN_LAST_IOLOOP_WAIT_MSECS) {
+		timeout_remove(&dict->to_requests);
+		dict->to_requests =
+			timeout_add(DICT_CLIENT_REQUEST_TIMEOUT_MIN_LAST_IOLOOP_WAIT_MSECS -
+				    msecs_in_last_dict_ioloop_wait,
+				    client_dict_input_timeout, dict);
+		return;
+	}
+
+	(void)client_dict_reconnect(dict, t_strdup_printf(
+		"Dict server timeout: %s "
+		"(%u commands pending, oldest sent %u.%03u secs ago: %s, %s)",
+		connection_input_timeout_reason(&dict->conn.conn),
+		array_count(&dict->cmds),
+		cmd_diff/1000, cmd_diff%1000, cmd->query,
+		dict_wait_warnings(cmd)), &error);
+}
+
+static int
+client_dict_cmd_query_send(struct client_dict *dict, const char *query)
+{
+	struct const_iovec iov[2];
+	ssize_t ret;
+
+	iov[0].iov_base = query;
+	iov[0].iov_len = strlen(query);
+	iov[1].iov_base = "\n";
+	iov[1].iov_len = 1;
+	ret = o_stream_sendv(dict->conn.conn.output, iov, 2);
+	if (ret < 0)
+		return -1;
+	i_assert((size_t)ret == iov[0].iov_len + 1);
+	return 0;
+}
+
+static bool
+client_dict_cmd_send(struct client_dict *dict, struct client_dict_cmd **_cmd,
+		     const char **error_r)
+{
+	struct client_dict_cmd *cmd = *_cmd;
+	const char *error = NULL;
+	bool retry = cmd->retry_errors;
+	int ret;
+
+	*_cmd = NULL;
+
+	/* we're no longer idling. even with no_replies=TRUE we're going to
+	   wait for COMMIT/ROLLBACK. */
+	timeout_remove(&dict->to_idle);
+
+	if (client_dict_connect(dict, &error) < 0) {
+		retry = FALSE;
+		ret = -1;
+	} else {
+		ret = client_dict_cmd_query_send(dict, cmd->query);
+		if (ret < 0) {
+			error = t_strdup_printf("write(%s) failed: %s", dict->conn.conn.name,
+					o_stream_get_error(dict->conn.conn.output));
+		}
+	}
+	if (ret < 0 && retry) {
+		/* Reconnect and try again. */
+		if (client_dict_reconnect(dict, error, &error) < 0)
+			;
+		else if (client_dict_cmd_query_send(dict, cmd->query) < 0) {
+			error = t_strdup_printf("write(%s) failed: %s", dict->conn.conn.name,
+				o_stream_get_error(dict->conn.conn.output));
+		} else {
+			ret = 0;
+		}
+	}
+
+	if (cmd->no_replies) {
+		/* just send and forget */
+		client_dict_cmd_unref(cmd);
+		return TRUE;
+	} else if (ret < 0) {
+		i_assert(error != NULL);
+		/* we didn't successfully send this command to dict */
+		dict_cmd_callback_error(cmd, error, cmd->reconnected);
+		client_dict_cmd_unref(cmd);
+		if (error_r != NULL)
+			*error_r = error;
+		return FALSE;
+	} else {
+		if (dict->to_requests == NULL && !cmd->background) {
+			dict->to_requests =
+				timeout_add(DICT_CLIENT_REQUEST_TIMEOUT_MSECS,
+					    client_dict_input_timeout, dict);
+		}
+		array_push_back(&dict->cmds, &cmd);
+		return TRUE;
+	}
+}
+
+static bool
+client_dict_transaction_send_begin(struct client_dict_transaction_context *ctx,
+				   const struct dict_op_settings_private *set)
+{
+	struct client_dict *dict = (struct client_dict *)ctx->ctx.dict;
+	struct client_dict_cmd *cmd;
+	const char *query, *error;
+
+	i_assert(ctx->error == NULL);
+
+	ctx->sent_begin = TRUE;
+
+	/* transactions commands don't have replies. only COMMIT has. */
+	query = t_strdup_printf("%c%u\t%s", DICT_PROTOCOL_CMD_BEGIN,
+				ctx->id,
+				set->username == NULL ? "" : str_tabescape(set->username));
+	cmd = client_dict_cmd_init(dict, query);
+	cmd->no_replies = TRUE;
+	cmd->retry_errors = TRUE;
+	if (!client_dict_cmd_send(dict, &cmd, &error)) {
+		ctx->error = i_strdup(error);
+		return FALSE;
+	}
+	return TRUE;
+}
+
+static void
+client_dict_send_transaction_query(struct client_dict_transaction_context *ctx,
+				   const char *query)
+{
+	struct client_dict *dict = (struct client_dict *)ctx->ctx.dict;
+	const struct dict_op_settings_private *set = &ctx->ctx.set;
+	struct client_dict_cmd *cmd;
+	const char *error;
+
+	if (ctx->error != NULL)
+		return;
+
+	if (!ctx->sent_begin) {
+		if (!client_dict_transaction_send_begin(ctx, set))
+			return;
+	}
+
+	ctx->query_count++;
+	if (ctx->first_query == NULL)
+		ctx->first_query = i_strdup(query);
+
+	cmd = client_dict_cmd_init(dict, query);
+	cmd->no_replies = TRUE;
+	if (!client_dict_cmd_send(dict, &cmd, &error))
+		ctx->error = i_strdup(error);
+}
+
+static bool client_dict_is_finished(struct client_dict *dict)
+{
+	return dict->transactions == NULL && array_count(&dict->cmds) == 0;
+}
+
+static void client_dict_timeout(struct client_dict *dict)
+{
+	if (client_dict_is_finished(dict))
+		client_dict_disconnect(dict, "Idle disconnection");
+	else
+		timeout_remove(&dict->to_idle);
+}
+
+static bool client_dict_have_nonbackground_cmds(struct client_dict *dict)
+{
+	struct client_dict_cmd *cmd;
+
+	array_foreach_elem(&dict->cmds, cmd) {
+		if (!cmd->background)
+			return TRUE;
+	}
+	return FALSE;
+}
+
+static void client_dict_add_timeout(struct client_dict *dict)
+{
+	if (dict->to_idle != NULL) {
+		if (dict->idle_msecs > 0)
+			timeout_reset(dict->to_idle);
+	} else if (client_dict_is_finished(dict)) {
+		dict->to_idle = timeout_add(dict->idle_msecs,
+					    client_dict_timeout, dict);
+		timeout_remove(&dict->to_requests);
+	} else if (dict->transactions == NULL &&
+		   !client_dict_have_nonbackground_cmds(dict)) {
+		/* we had non-background commands, but now we're back to
+		   having only background commands. remove timeouts. */
+		timeout_remove(&dict->to_requests);
+	}
+}
+
+static void client_dict_cmd_backgrounded(struct client_dict *dict)
+{
+	if (dict->to_requests == NULL)
+		return;
+
+	if (!client_dict_have_nonbackground_cmds(dict)) {
+		/* we only have background-commands.
+		   remove the request timeout. */
+		timeout_remove(&dict->to_requests);
+	}
+}
+
+static int
+dict_conn_assign_next_async_id(struct dict_client_connection *conn,
+			       const char *line)
+{
+	struct client_dict_cmd *const *cmds;
+	unsigned int i, count, async_id;
+
+	i_assert(line[0] == DICT_PROTOCOL_REPLY_ASYNC_ID);
+
+	if (str_to_uint(line+1, &async_id) < 0 || async_id == 0) {
+		e_error(conn->conn.event, "Received invalid async-id line: %s",
+			line);
+		return -1;
+	}
+	cmds = array_get(&conn->dict->cmds, &count);
+	for (i = 0; i < count; i++) {
+		if (cmds[i]->async_id == 0) {
+			cmds[i]->async_id = async_id;
+			cmds[i]->async_id_received_time = ioloop_timeval;
+			return 0;
+		}
+	}
+	e_error(conn->conn.event, "Received async-id line, but all %u "
+				  "commands already have it: %s",
+		count, line);
+	return -1;
+}
+
+static int dict_conn_find_async_id(struct dict_client_connection *conn,
+				   const char *async_arg,
+				   const char *line, unsigned int *idx_r)
+{
+	struct client_dict_cmd *const *cmds;
+	unsigned int i, count, async_id;
+
+	i_assert(async_arg[0] == DICT_PROTOCOL_REPLY_ASYNC_REPLY);
+
+	if (str_to_uint(async_arg+1, &async_id) < 0 || async_id == 0) {
+		e_error(conn->conn.event, "Received invalid async-reply line: %s",
+			line);
+		return -1;
+	}
+
+	cmds = array_get(&conn->dict->cmds, &count);
+	for (i = 0; i < count; i++) {
+		if (cmds[i]->async_id == async_id) {
+			*idx_r = i;
+			return 0;
+		}
+	}
+	e_error(conn->conn.event, "Received reply for nonexistent async-id %u: %s",
+		async_id, line);
+	return -1;
+}
+
+static int dict_conn_input_line(struct connection *_conn, const char *line)
+{
+	struct dict_client_connection *conn =
+		(struct dict_client_connection *)_conn;
+	struct client_dict *dict = conn->dict;
+	struct client_dict_cmd *const *cmds;
+	const char *const *args;
+	unsigned int i, count;
+	bool finished;
+
+	if (dict->to_requests != NULL)
+		timeout_reset(dict->to_requests);
+
+	if (line[0] == DICT_PROTOCOL_REPLY_ASYNC_ID)
+		return dict_conn_assign_next_async_id(conn, line) < 0 ? -1 : 1;
+
+	cmds = array_get(&conn->dict->cmds, &count);
+	if (count == 0) {
+		e_error(conn->conn.event, "Received reply without pending commands: %s",
+			line);
+		return -1;
+	}
+
+	args = t_strsplit_tabescaped(line);
+	if (args[0] != NULL && args[0][0] == DICT_PROTOCOL_REPLY_ASYNC_REPLY) {
+		if (dict_conn_find_async_id(conn, args[0], line, &i) < 0)
+			return -1;
+		args++;
+	} else {
+		i = 0;
+	}
+	i_assert(!cmds[i]->no_replies);
+
+	client_dict_cmd_ref(cmds[i]);
+	finished = dict_cmd_callback_line(cmds[i], args);
+	if (!client_dict_cmd_unref(cmds[i])) {
+		/* disconnected during command handling */
+		return -1;
+	}
+	if (!finished) {
+		/* more lines needed for this command */
+		return 1;
+	}
+	client_dict_cmd_unref(cmds[i]);
+	array_delete(&dict->cmds, i, 1);
+
+	client_dict_add_timeout(dict);
+	return 1;
+}
+
+static int client_dict_connect(struct client_dict *dict, const char **error_r)
+{
+	const char *query, *error;
+
+	if (dict->conn.conn.fd_in != -1)
+		return 0;
+	if (dict->last_failed_connect == ioloop_time) {
+		/* Try again later */
+		*error_r = dict->last_connect_error;
+		return -1;
+	}
+
+	if (connection_client_connect(&dict->conn.conn) < 0) {
+		dict->last_failed_connect = ioloop_time;
+		if (errno == EACCES) {
+			error = eacces_error_get("net_connect_unix",
+						 dict->conn.conn.name);
+		} else {
+			error = t_strdup_printf(
+				"net_connect_unix(%s) failed: %m", dict->conn.conn.name);
+		}
+		i_free(dict->last_connect_error);
+		dict->last_connect_error = i_strdup(error);
+		*error_r = error;
+		return -1;
+	}
+
+	query = t_strdup_printf("%c%u\t%u\t%d\t%s\t%s\n",
+				DICT_PROTOCOL_CMD_HELLO,
+				DICT_CLIENT_PROTOCOL_MAJOR_VERSION,
+				DICT_CLIENT_PROTOCOL_MINOR_VERSION,
+				dict->value_type,
+				"",
+				str_tabescape(dict->uri));
+	o_stream_nsend_str(dict->conn.conn.output, query);
+	client_dict_add_timeout(dict);
+	return 0;
+}
+
+static void
+client_dict_abort_commands(struct client_dict *dict, const char *reason)
+{
+	ARRAY(struct client_dict_cmd *) cmds_copy;
+	struct client_dict_cmd *cmd;
+
+	/* abort all commands */
+	t_array_init(&cmds_copy, array_count(&dict->cmds));
+	array_append_array(&cmds_copy, &dict->cmds);
+	array_clear(&dict->cmds);
+
+	array_foreach_elem(&cmds_copy, cmd) {
+		dict_cmd_callback_error(cmd, reason, TRUE);
+		client_dict_cmd_unref(cmd);
+	}
+}
+
+static void client_dict_disconnect(struct client_dict *dict, const char *reason)
+{
+	struct client_dict_transaction_context *ctx, *next;
+
+	client_dict_abort_commands(dict, reason);
+
+	/* all transactions that have sent BEGIN are no longer valid */
+	for (ctx = dict->transactions; ctx != NULL; ctx = next) {
+		next = ctx->next;
+		if (ctx->sent_begin && ctx->error == NULL)
+			ctx->error = i_strdup(reason);
+	}
+
+	timeout_remove(&dict->to_idle);
+	timeout_remove(&dict->to_requests);
+	connection_disconnect(&dict->conn.conn);
+}
+
+static int client_dict_reconnect(struct client_dict *dict, const char *reason,
+				 const char **error_r)
+{
+	ARRAY(struct client_dict_cmd *) retry_cmds;
+	struct client_dict_cmd *cmd;
+	const char *error;
+	int ret;
+
+	t_array_init(&retry_cmds, array_count(&dict->cmds));
+	for (unsigned int i = 0; i < array_count(&dict->cmds); ) {
+		cmd = array_idx_elem(&dict->cmds, i);
+		if (!cmd->retry_errors) {
+			i++;
+		} else if (cmd->iter != NULL &&
+			   cmd->iter->seen_results) {
+			/* don't retry iteration that already returned
+			   something to the caller. otherwise we'd return
+			   duplicates. */
+			i++;
+		} else {
+			array_push_back(&retry_cmds, &cmd);
+			array_delete(&dict->cmds, i, 1);
+		}
+	}
+	client_dict_disconnect(dict, reason);
+	if (client_dict_connect(dict, error_r) < 0) {
+		reason = t_strdup_printf("%s - reconnect failed: %s",
+					 reason, *error_r);
+		array_foreach_elem(&retry_cmds, cmd) {
+			dict_cmd_callback_error(cmd, reason, TRUE);
+			client_dict_cmd_unref(cmd);
+		}
+		return -1;
+	}
+	if (array_count(&retry_cmds) == 0)
+		return 0;
+	e_warning(dict->conn.conn.event, "%s - reconnected", reason);
+
+	ret = 0; error = "";
+	array_foreach_elem(&retry_cmds, cmd) {
+		cmd->reconnected = TRUE;
+		cmd->async_id = 0;
+		/* if it fails again, don't retry anymore */
+		cmd->retry_errors = FALSE;
+		if (ret < 0) {
+			dict_cmd_callback_error(cmd, error, TRUE);
+			client_dict_cmd_unref(cmd);
+		} else if (!client_dict_cmd_send(dict, &cmd, &error))
+			ret = -1;
+	}
+	return ret;
+}
+
+static void dict_conn_destroy(struct connection *_conn)
+{
+	struct dict_client_connection *conn =
+		(struct dict_client_connection *)_conn;
+
+	client_dict_disconnect(conn->dict, connection_disconnect_reason(_conn));
+}
+
+static const struct connection_settings dict_conn_set = {
+	.input_max_size = SIZE_MAX,
+	.output_max_size = SIZE_MAX,
+	.unix_client_connect_msecs = 1000,
+	.client = TRUE
+};
+
+static const struct connection_vfuncs dict_conn_vfuncs = {
+	.destroy = dict_conn_destroy,
+	.input_line = dict_conn_input_line
+};
+
+static int
+client_dict_init(struct dict *driver, const char *uri,
+		 const struct dict_settings *set,
+		 struct dict **dict_r, const char **error_r)
+{
+	struct ioloop *old_ioloop = current_ioloop;
+	struct client_dict *dict;
+	const char *p, *dest_uri, *path;
+	unsigned int idle_msecs = DICT_CLIENT_DEFAULT_TIMEOUT_MSECS;
+	unsigned int warn_slow_msecs = DICT_CLIENT_DEFAULT_WARN_SLOW_MSECS;
+
+	/* uri = [idle_msecs=<n>:] [warn_slow_msecs=<n>:] [<path>] ":" <uri> */
+	for (;;) {
+		if (str_begins(uri, "idle_msecs=")) {
+			p = strchr(uri+11, ':');
+			if (p == NULL) {
+				*error_r = t_strdup_printf("Invalid URI: %s", uri);
+				return -1;
+			}
+			if (str_to_uint(t_strdup_until(uri+11, p), &idle_msecs) < 0) {
+				*error_r = "Invalid idle_msecs";
+				return -1;
+			}
+			uri = p+1;
+		} else if (str_begins(uri, "warn_slow_msecs=")) {
+			p = strchr(uri+11, ':');
+			if (p == NULL) {
+				*error_r = t_strdup_printf("Invalid URI: %s", uri);
+				return -1;
+			}
+			if (str_to_uint(t_strdup_until(uri+16, p), &warn_slow_msecs) < 0) {
+				*error_r = "Invalid warn_slow_msecs";
+				return -1;
+			}
+			uri = p+1;
+		} else {
+			break;
+		}
+	}
+	dest_uri = strchr(uri, ':');
+	if (dest_uri == NULL) {
+		*error_r = t_strdup_printf("Invalid URI: %s", uri);
+		return -1;
+	}
+
+	if (dict_connections == NULL) {
+		dict_connections = connection_list_init(&dict_conn_set,
+							&dict_conn_vfuncs);
+	}
+
+	dict = i_new(struct client_dict, 1);
+	dict->dict = *driver;
+	dict->conn.dict = dict;
+	dict->conn.conn.event_parent = set->event_parent;
+	dict->idle_msecs = idle_msecs;
+	dict->warn_slow_msecs = warn_slow_msecs;
+	i_array_init(&dict->cmds, 32);
+
+	if (uri[0] == ':') {
+		/* default path */
+		path = t_strconcat(set->base_dir,
+			"/"DEFAULT_DICT_SERVER_SOCKET_FNAME, NULL);
+	} else if (uri[0] == '/') {
+		/* absolute path */
+		path = t_strdup_until(uri, dest_uri);
+	} else {
+		/* relative path to base_dir */
+		path = t_strconcat(set->base_dir, "/",
+			t_strdup_until(uri, dest_uri), NULL);
+	}
+	connection_init_client_unix(dict_connections, &dict->conn.conn, path);
+	dict->uri = i_strdup(dest_uri + 1);
+
+	dict->dict.ioloop = io_loop_create();
+	dict->wait_timer = io_wait_timer_add();
+	io_loop_set_current(old_ioloop);
+	*dict_r = &dict->dict;
+	return 0;
+}
+
+static void client_dict_deinit(struct dict *_dict)
+{
+	struct client_dict *dict = (struct client_dict *)_dict;
+	struct ioloop *old_ioloop = current_ioloop;
+
+	client_dict_disconnect(dict, "Deinit");
+	connection_deinit(&dict->conn.conn);
+	io_wait_timer_remove(&dict->wait_timer);
+
+	i_assert(dict->transactions == NULL);
+	i_assert(array_count(&dict->cmds) == 0);
+
+	io_loop_set_current(dict->dict.ioloop);
+	io_loop_destroy(&dict->dict.ioloop);
+	io_loop_set_current(old_ioloop);
+
+	array_free(&dict->cmds);
+	i_free(dict->last_connect_error);
+	i_free(dict->uri);
+	i_free(dict);
+
+	if (dict_connections->connections == NULL)
+		connection_list_deinit(&dict_connections);
+}
+
+static void client_dict_wait(struct dict *_dict)
+{
+	struct client_dict *dict = (struct client_dict *)_dict;
+
+	if (array_count(&dict->cmds) == 0)
+		return;
+
+	i_assert(io_loop_is_empty(dict->dict.ioloop));
+	dict->dict.prev_ioloop = current_ioloop;
+	io_loop_set_current(dict->dict.ioloop);
+	dict_switch_ioloop(_dict);
+	while (array_count(&dict->cmds) > 0)
+		io_loop_run(dict->dict.ioloop);
+
+	io_loop_set_current(dict->dict.prev_ioloop);
+	dict->dict.prev_ioloop = NULL;
+
+	dict_switch_ioloop(_dict);
+	i_assert(io_loop_is_empty(dict->dict.ioloop));
+}
+
+static bool client_dict_switch_ioloop(struct dict *_dict)
+{
+	struct client_dict *dict = (struct client_dict *)_dict;
+
+	dict->last_timer_switch_usecs =
+		io_wait_timer_get_usecs(dict->wait_timer);
+	dict->wait_timer = io_wait_timer_move(&dict->wait_timer);
+	if (dict->to_idle != NULL)
+		dict->to_idle = io_loop_move_timeout(&dict->to_idle);
+	if (dict->to_requests != NULL)
+		dict->to_requests = io_loop_move_timeout(&dict->to_requests);
+	connection_switch_ioloop(&dict->conn.conn);
+	return array_count(&dict->cmds) > 0;
+}
+
+static const char *dict_wait_warnings(const struct client_dict_cmd *cmd)
+{
+	int global_ioloop_msecs = (ioloop_global_wait_usecs -
+				   cmd->start_global_ioloop_usecs + 999) / 1000;
+	int dict_ioloop_msecs = (io_wait_timer_get_usecs(cmd->dict->wait_timer) -
+				 cmd->start_dict_ioloop_usecs + 999) / 1000;
+	int other_ioloop_msecs = global_ioloop_msecs - dict_ioloop_msecs;
+	int lock_msecs = (file_lock_wait_get_total_usecs() -
+			  cmd->start_lock_usecs + 999) / 1000;
+
+	return t_strdup_printf(
+		"%d.%03d in dict wait, %d.%03d in other ioloops, %d.%03d in locks",
+		dict_ioloop_msecs/1000, dict_ioloop_msecs%1000,
+		other_ioloop_msecs/1000, other_ioloop_msecs%1000,
+		lock_msecs/1000, lock_msecs%1000);
+}
+
+static const char *
+dict_warnings_sec(const struct client_dict_cmd *cmd, int msecs,
+		  const char *const *extra_args)
+{
+	string_t *str = t_str_new(64);
+	struct timeval tv_start, tv_end;
+	unsigned int tv_start_usec, tv_end_usec;
+
+	str_printfa(str, "%d.%03d secs (%s", msecs/1000, msecs%1000,
+		    dict_wait_warnings(cmd));
+	if (cmd->reconnected) {
+		int reconnected_msecs =
+			timeval_diff_msecs(&ioloop_timeval,
+				&cmd->dict->conn.conn.connect_started);
+		str_printfa(str, ", reconnected %u.%03u secs ago",
+			    reconnected_msecs/1000, reconnected_msecs%1000);
+	}
+	if (cmd->async_id != 0) {
+		int async_reply_msecs =
+			timeval_diff_msecs(&ioloop_timeval, &cmd->async_id_received_time);
+		str_printfa(str, ", async-id reply %u.%03u secs ago",
+			    async_reply_msecs/1000, async_reply_msecs%1000);
+	}
+	if (extra_args != NULL &&
+	    str_array_length(extra_args) >= 4 &&
+	    str_to_time(extra_args[0], &tv_start.tv_sec) == 0 &&
+	    str_to_uint(extra_args[1], &tv_start_usec) == 0 &&
+	    str_to_time(extra_args[2], &tv_end.tv_sec) == 0 &&
+	    str_to_uint(extra_args[3], &tv_end_usec) == 0) {
+		tv_start.tv_usec = tv_start_usec;
+		tv_end.tv_usec = tv_end_usec;
+
+		int server_msecs_since_start =
+			timeval_diff_msecs(&ioloop_timeval, &tv_start);
+		int server_msecs = timeval_diff_msecs(&tv_end, &tv_start);
+		str_printfa(str, ", started on dict-server %u.%03d secs ago, "
+			    "took %u.%03d secs",
+			    server_msecs_since_start/1000,
+			    server_msecs_since_start%1000,
+			    server_msecs/1000, server_msecs%1000);
+	}
+	str_append_c(str, ')');
+	return str_c(str);
+}
+
+static void
+client_dict_lookup_async_callback(struct client_dict_cmd *cmd,
+				  enum dict_protocol_reply reply,
+				  const char *value,
+				  const char *const *extra_args,
+				  const char *error,
+				  bool disconnected ATTR_UNUSED)
+{
+	struct client_dict *dict = cmd->dict;
+	struct dict_lookup_result result;
+	const char *const values[] = { value, NULL };
+
+	i_zero(&result);
+	if (error != NULL) {
+		result.ret = -1;
+		result.error = error;
+	} else switch (reply) {
+	case DICT_PROTOCOL_REPLY_OK:
+		result.value = value;
+		result.values = values;
+		result.ret = 1;
+		break;
+	case DICT_PROTOCOL_REPLY_MULTI_OK:
+		result.values = t_strsplit_tabescaped(value);
+		result.value = result.values[0];
+		result.ret = 1;
+		break;
+	case DICT_PROTOCOL_REPLY_NOTFOUND:
+		result.ret = 0;
+		break;
+	case DICT_PROTOCOL_REPLY_FAIL:
+		result.error = value[0] == '\0' ? "dict-server returned failure" :
+			t_strdup_printf("dict-server returned failure: %s",
+			value);
+		result.ret = -1;
+		break;
+	default:
+		result.error = t_strdup_printf(
+			"dict-client: Invalid lookup '%s' reply: %c%s",
+			cmd->query, reply, value);
+		client_dict_disconnect(dict, result.error);
+		result.ret = -1;
+		break;
+	}
+
+	int diff = timeval_diff_msecs(&ioloop_timeval, &cmd->start_time);
+	if (result.error != NULL) {
+		/* include timing info always in error messages */
+		result.error = t_strdup_printf("%s (reply took %s)",
+			result.error, dict_warnings_sec(cmd, diff, extra_args));
+	} else if (!cmd->background &&
+		   diff >= (int)dict->warn_slow_msecs) {
+		e_warning(dict->conn.conn.event, "dict lookup took %s: %s",
+			  dict_warnings_sec(cmd, diff, extra_args),
+			  cmd->query);
+	}
+
+	dict_pre_api_callback(&dict->dict);
+	cmd->api_callback.lookup(&result, cmd->api_callback.context);
+	dict_post_api_callback(&dict->dict);
+}
+
+static void
+client_dict_lookup_async(struct dict *_dict, const struct dict_op_settings *set,
+			 const char *key, dict_lookup_callback_t *callback,
+			 void *context)
+{
+	struct client_dict *dict = (struct client_dict *)_dict;
+	struct client_dict_cmd *cmd;
+	const char *query;
+
+	query = t_strdup_printf("%c%s\t%s", DICT_PROTOCOL_CMD_LOOKUP,
+				str_tabescape(key),
+				set->username == NULL ? "" : str_tabescape(set->username));
+	cmd = client_dict_cmd_init(dict, query);
+	cmd->callback = client_dict_lookup_async_callback;
+	cmd->api_callback.lookup = callback;
+	cmd->api_callback.context = context;
+	cmd->retry_errors = TRUE;
+
+	client_dict_cmd_send(dict, &cmd, NULL);
+}
+
+struct client_dict_sync_lookup {
+	char *error;
+	char *value;
+	int ret;
+};
+
+static void client_dict_lookup_callback(const struct dict_lookup_result *result,
+					struct client_dict_sync_lookup *lookup)
+{
+	lookup->ret = result->ret;
+	if (result->ret == -1)
+		lookup->error = i_strdup(result->error);
+	else if (result->ret == 1)
+		lookup->value = i_strdup(result->value);
+}
+
+static int client_dict_lookup(struct dict *_dict,
+			      const struct dict_op_settings *set,
+			      pool_t pool, const char *key,
+			      const char **value_r, const char **error_r)
+{
+	struct client_dict_sync_lookup lookup;
+
+	i_zero(&lookup);
+	lookup.ret = -2;
+
+	dict_lookup_async(_dict, set, key, client_dict_lookup_callback, &lookup);
+	if (lookup.ret == -2)
+		client_dict_wait(_dict);
+
+	switch (lookup.ret) {
+	case -1:
+		*error_r = t_strdup(lookup.error);
+		i_free(lookup.error);
+		return -1;
+	case 0:
+		i_assert(lookup.value == NULL);
+		*value_r = NULL;
+		return 0;
+	case 1:
+		*value_r = p_strdup(pool, lookup.value);
+		i_free(lookup.value);
+		return 1;
+	}
+	i_unreached();
+}
+
+static void client_dict_iterate_unref(struct client_dict_iterate_context *ctx)
+{
+	i_assert(ctx->refcount > 0);
+	if (--ctx->refcount > 0)
+		return;
+	i_free(ctx->error);
+	i_free(ctx);
+}
+
+static void
+client_dict_iter_api_callback(struct client_dict_iterate_context *ctx,
+			      struct client_dict_cmd *cmd,
+			      const char *const *extra_args)
+{
+	struct client_dict *dict = cmd->dict;
+
+	if (ctx->deinit) {
+		/* Iterator was already deinitialized. Stop if we're in
+		   client_dict_wait(). */
+		dict_post_api_callback(&dict->dict);
+		return;
+	}
+	if (ctx->finished) {
+		int diff = timeval_diff_msecs(&ioloop_timeval, &cmd->start_time);
+		if (ctx->error != NULL) {
+			/* include timing info always in error messages */
+			char *new_error = i_strdup_printf("%s (reply took %s)",
+				ctx->error, dict_warnings_sec(cmd, diff, extra_args));
+			i_free(ctx->error);
+			ctx->error = new_error;
+		} else if (!cmd->background &&
+			   diff >= (int)dict->warn_slow_msecs) {
+			e_warning(dict->conn.conn.event, "dict iteration took %s: %s",
+				  dict_warnings_sec(cmd, diff, extra_args),
+				  cmd->query);
+		}
+	}
+	if (ctx->ctx.async_callback != NULL) {
+		dict_pre_api_callback(&dict->dict);
+		ctx->ctx.async_callback(ctx->ctx.async_context);
+		dict_post_api_callback(&dict->dict);
+	} else {
+		/* synchronous lookup */
+		io_loop_stop(dict->dict.ioloop);
+	}
+}
+
+static void
+client_dict_iter_async_callback(struct client_dict_cmd *cmd,
+				enum dict_protocol_reply reply,
+				const char *value,
+				const char *const *extra_args,
+				const char *error,
+				bool disconnected ATTR_UNUSED)
+{
+	struct client_dict_iterate_context *ctx = cmd->iter;
+	struct client_dict *dict = cmd->dict;
+	struct client_dict_iter_result *result;
+	const char *iter_key = NULL, *const *iter_values = NULL;
+
+	if (ctx->deinit) {
+		cmd->background = TRUE;
+		client_dict_cmd_backgrounded(dict);
+	}
+
+	if (error != NULL) {
+		/* failed */
+	} else switch (reply) {
+	case DICT_PROTOCOL_REPLY_ITER_FINISHED:
+		/* end of iteration */
+		i_assert(!ctx->finished);
+		ctx->finished = TRUE;
+		client_dict_iter_api_callback(ctx, cmd, extra_args);
+		client_dict_iterate_unref(ctx);
+		return;
+	case DICT_PROTOCOL_REPLY_OK:
+		/* key \t value */
+		iter_key = value;
+		iter_values = extra_args;
+		extra_args++;
+		break;
+	case DICT_PROTOCOL_REPLY_FAIL:
+		error = t_strdup_printf("dict-server returned failure: %s", value);
+		break;
+	default:
+		break;
+	}
+	if ((iter_values == NULL || iter_values[0] == NULL) && error == NULL) {
+		/* broken protocol */
+		error = t_strdup_printf("dict client (%s) sent broken iterate reply: %c%s",
+			dict->conn.conn.name, reply, value);
+		client_dict_disconnect(dict, error);
+	}
+
+	if (error != NULL) {
+		if (ctx->error == NULL)
+			ctx->error = i_strdup(error);
+		i_assert(!ctx->finished);
+		ctx->finished = TRUE;
+		client_dict_iter_api_callback(ctx, cmd, extra_args);
+		client_dict_iterate_unref(ctx);
+		return;
+	}
+	cmd->unfinished = TRUE;
+
+	if (ctx->deinit) {
+		/* iterator was already deinitialized */
+		return;
+	}
+
+	result = array_append_space(&ctx->results);
+	result->key = p_strdup(ctx->results_pool, iter_key);
+	result->values = p_strarray_dup(ctx->results_pool, iter_values);
+
+	client_dict_iter_api_callback(ctx, cmd, NULL);
+}
+
+static struct dict_iterate_context *
+client_dict_iterate_init(struct dict *_dict,
+			 const struct dict_op_settings *set ATTR_UNUSED,
+			 const char *path, enum dict_iterate_flags flags)
+{
+        struct client_dict_iterate_context *ctx;
+
+	ctx = i_new(struct client_dict_iterate_context, 1);
+	ctx->ctx.dict = _dict;
+	ctx->results_pool = pool_alloconly_create("client dict iteration", 512);
+	ctx->flags = flags;
+	ctx->path = i_strdup(path);
+	ctx->refcount = 1;
+	i_array_init(&ctx->results, 64);
+	return &ctx->ctx;
+}
+
+static void
+client_dict_iterate_cmd_send(struct client_dict_iterate_context *ctx)
+{
+	struct client_dict *dict = (struct client_dict *)ctx->ctx.dict;
+	const struct dict_op_settings_private *set = &ctx->ctx.set;
+	struct client_dict_cmd *cmd;
+	string_t *query = t_str_new(256);
+
+	/* we can't do this query in _iterate_init(), because
+	   _set_limit() hasn't been called yet at that point. */
+	str_printfa(query, "%c%d\t%"PRIu64, DICT_PROTOCOL_CMD_ITERATE,
+		    ctx->flags, ctx->ctx.max_rows);
+	str_append_c(query, '\t');
+	str_append_tabescaped(query, ctx->path);
+	str_append_c(query, '\t');
+	str_append_tabescaped(query, set->username == NULL ? "" : set->username);
+
+	cmd = client_dict_cmd_init(dict, str_c(query));
+	cmd->iter = ctx;
+	cmd->callback = client_dict_iter_async_callback;
+	cmd->retry_errors = TRUE;
+
+	ctx->refcount++;
+	client_dict_cmd_send(dict, &cmd, NULL);
+}
+
+static bool client_dict_iterate(struct dict_iterate_context *_ctx,
+				const char **key_r, const char *const **values_r)
+{
+	struct client_dict_iterate_context *ctx =
+		(struct client_dict_iterate_context *)_ctx;
+	const struct client_dict_iter_result *results;
+	unsigned int count;
+
+	if (ctx->error != NULL) {
+		ctx->ctx.has_more = FALSE;
+		return FALSE;
+	}
+
+	results = array_get(&ctx->results, &count);
+	if (ctx->result_idx < count) {
+		*key_r = results[ctx->result_idx].key;
+		*values_r = results[ctx->result_idx].values;
+		ctx->ctx.has_more = TRUE;
+		ctx->result_idx++;
+		ctx->seen_results = TRUE;
+		return TRUE;
+	}
+	if (!ctx->cmd_sent) {
+		ctx->cmd_sent = TRUE;
+		client_dict_iterate_cmd_send(ctx);
+		return client_dict_iterate(_ctx, key_r, values_r);
+	}
+	ctx->ctx.has_more = !ctx->finished;
+	ctx->result_idx = 0;
+	array_clear(&ctx->results);
+	p_clear(ctx->results_pool);
+
+	if ((ctx->flags & DICT_ITERATE_FLAG_ASYNC) == 0 && ctx->ctx.has_more) {
+		client_dict_wait(_ctx->dict);
+		return client_dict_iterate(_ctx, key_r, values_r);
+	}
+	return FALSE;
+}
+
+static int client_dict_iterate_deinit(struct dict_iterate_context *_ctx,
+				      const char **error_r)
+{
+	struct client_dict *dict = (struct client_dict *)_ctx->dict;
+	struct client_dict_iterate_context *ctx =
+		(struct client_dict_iterate_context *)_ctx;
+	int ret = ctx->error != NULL ? -1 : 0;
+
+	i_assert(!ctx->deinit);
+	ctx->deinit = TRUE;
+
+	*error_r = t_strdup(ctx->error);
+	array_free(&ctx->results);
+	pool_unref(&ctx->results_pool);
+	i_free(ctx->path);
+	client_dict_iterate_unref(ctx);
+
+	client_dict_add_timeout(dict);
+	return ret;
+}
+
+static struct dict_transaction_context *
+client_dict_transaction_init(struct dict *_dict)
+{
+	struct client_dict *dict = (struct client_dict *)_dict;
+	struct client_dict_transaction_context *ctx;
+
+	ctx = i_new(struct client_dict_transaction_context, 1);
+	ctx->ctx.dict = _dict;
+	ctx->id = ++dict->transaction_id_counter;
+
+	DLLIST_PREPEND(&dict->transactions, ctx);
+	return &ctx->ctx;
+}
+
+static void
+client_dict_transaction_free(struct client_dict_transaction_context **_ctx)
+{
+	struct client_dict_transaction_context *ctx = *_ctx;
+
+	*_ctx = NULL;
+	i_free(ctx->first_query);
+	i_free(ctx->error);
+	i_free(ctx);
+}
+
+static void
+client_dict_transaction_commit_callback(struct client_dict_cmd *cmd,
+					enum dict_protocol_reply reply,
+					const char *value,
+					const char *const *extra_args,
+					const char *error, bool disconnected)
+{
+	struct client_dict *dict = cmd->dict;
+	struct dict_commit_result result = {
+		.ret = DICT_COMMIT_RET_FAILED, .error = NULL
+	};
+
+	i_assert(cmd->trans != NULL);
+
+	if (error != NULL) {
+		/* failed */
+		if (disconnected)
+			result.ret = DICT_COMMIT_RET_WRITE_UNCERTAIN;
+		result.error = error;
+	} else switch (reply) {
+	case DICT_PROTOCOL_REPLY_OK:
+		result.ret = DICT_COMMIT_RET_OK;
+		break;
+	case DICT_PROTOCOL_REPLY_NOTFOUND:
+		result.ret = DICT_COMMIT_RET_NOTFOUND;
+		break;
+	case DICT_PROTOCOL_REPLY_WRITE_UNCERTAIN:
+		result.ret = DICT_COMMIT_RET_WRITE_UNCERTAIN;
+		/* fallthrough */
+	case DICT_PROTOCOL_REPLY_FAIL: {
+		/* value contains the obsolete trans_id */
+		const char *error = extra_args[0];
+
+		result.error = t_strdup_printf("dict-server returned failure: %s",
+			error != NULL ? t_str_tabunescape(error) : "");
+		if (error != NULL)
+			extra_args++;
+		break;
+	}
+	default:
+		result.ret = DICT_COMMIT_RET_FAILED;
+		result.error = t_strdup_printf(
+			"dict-client: Invalid commit reply: %c%s",
+			reply, value);
+		client_dict_disconnect(dict, result.error);
+		break;
+	}
+
+	int diff = timeval_diff_msecs(&ioloop_timeval, &cmd->start_time);
+	if (result.error != NULL) {
+		/* include timing info always in error messages */
+		result.error = t_strdup_printf("%s (reply took %s)",
+			result.error, dict_warnings_sec(cmd, diff, extra_args));
+	} else if (!cmd->background && !cmd->trans->ctx.no_slowness_warning &&
+		   diff >= (int)dict->warn_slow_msecs) {
+		e_warning(dict->conn.conn.event, "dict commit took %s: "
+			  "%s (%u commands, first: %s)",
+			  dict_warnings_sec(cmd, diff, extra_args),
+			  cmd->query, cmd->trans->query_count,
+			  cmd->trans->first_query);
+	}
+	client_dict_transaction_free(&cmd->trans);
+
+	dict_pre_api_callback(&dict->dict);
+	cmd->api_callback.commit(&result, cmd->api_callback.context);
+	dict_post_api_callback(&dict->dict);
+}
+
+
+static void
+client_dict_transaction_commit(struct dict_transaction_context *_ctx,
+			       bool async,
+			       dict_transaction_commit_callback_t *callback,
+			       void *context)
+{
+	struct client_dict_transaction_context *ctx =
+		(struct client_dict_transaction_context *)_ctx;
+	struct client_dict *dict = (struct client_dict *)_ctx->dict;
+	struct client_dict_cmd *cmd;
+	const char *query;
+
+	DLLIST_REMOVE(&dict->transactions, ctx);
+
+	if (ctx->sent_begin && ctx->error == NULL) {
+		query = t_strdup_printf("%c%u", DICT_PROTOCOL_CMD_COMMIT, ctx->id);
+		cmd = client_dict_cmd_init(dict, query);
+		cmd->trans = ctx;
+
+		cmd->callback = client_dict_transaction_commit_callback;
+		cmd->api_callback.commit = callback;
+		cmd->api_callback.context = context;
+		if (callback == dict_transaction_commit_async_noop_callback)
+			cmd->background = TRUE;
+		if (client_dict_cmd_send(dict, &cmd, NULL)) {
+			if (!async)
+				client_dict_wait(_ctx->dict);
+		}
+	} else if (ctx->error != NULL) {
+		/* already failed */
+		struct dict_commit_result result = {
+			.ret = DICT_COMMIT_RET_FAILED, .error = ctx->error
+		};
+		callback(&result, context);
+		client_dict_transaction_free(&ctx);
+	} else {
+		/* nothing changed */
+		struct dict_commit_result result = {
+			.ret = DICT_COMMIT_RET_OK, .error = NULL
+		};
+		callback(&result, context);
+		client_dict_transaction_free(&ctx);
+	}
+
+	client_dict_add_timeout(dict);
+}
+
+static void
+client_dict_transaction_rollback(struct dict_transaction_context *_ctx)
+{
+	struct client_dict_transaction_context *ctx =
+		(struct client_dict_transaction_context *)_ctx;
+	struct client_dict *dict = (struct client_dict *)_ctx->dict;
+
+	if (ctx->sent_begin) {
+		const char *query;
+
+		query = t_strdup_printf("%c%u", DICT_PROTOCOL_CMD_ROLLBACK,
+					ctx->id);
+		client_dict_send_transaction_query(ctx, query);
+	}
+
+	DLLIST_REMOVE(&dict->transactions, ctx);
+	client_dict_transaction_free(&ctx);
+	client_dict_add_timeout(dict);
+}
+
+static void client_dict_set(struct dict_transaction_context *_ctx,
+			    const char *key, const char *value)
+{
+	struct client_dict_transaction_context *ctx =
+		(struct client_dict_transaction_context *)_ctx;
+	const char *query;
+
+	query = t_strdup_printf("%c%u\t%s\t%s",
+				DICT_PROTOCOL_CMD_SET, ctx->id,
+				str_tabescape(key),
+				str_tabescape(value));
+	client_dict_send_transaction_query(ctx, query);
+}
+
+static void client_dict_unset(struct dict_transaction_context *_ctx,
+			      const char *key)
+{
+	struct client_dict_transaction_context *ctx =
+		(struct client_dict_transaction_context *)_ctx;
+	const char *query;
+
+	query = t_strdup_printf("%c%u\t%s",
+				DICT_PROTOCOL_CMD_UNSET, ctx->id,
+				str_tabescape(key));
+	client_dict_send_transaction_query(ctx, query);
+}
+
+static void client_dict_atomic_inc(struct dict_transaction_context *_ctx,
+				   const char *key, long long diff)
+{
+	struct client_dict_transaction_context *ctx =
+		(struct client_dict_transaction_context *)_ctx;
+	const char *query;
+
+	query = t_strdup_printf("%c%u\t%s\t%lld",
+				DICT_PROTOCOL_CMD_ATOMIC_INC,
+				ctx->id, str_tabescape(key), diff);
+	client_dict_send_transaction_query(ctx, query);
+}
+
+static void client_dict_set_timestamp(struct dict_transaction_context *_ctx,
+				      const struct timespec *ts)
+{
+	struct client_dict_transaction_context *ctx =
+		(struct client_dict_transaction_context *)_ctx;
+	const char *query;
+
+	query = t_strdup_printf("%c%u\t%s\t%u",
+				DICT_PROTOCOL_CMD_TIMESTAMP,
+				ctx->id, dec2str(ts->tv_sec),
+				(unsigned int)ts->tv_nsec);
+	client_dict_send_transaction_query(ctx, query);
+}
+
+static void client_dict_set_hide_log_values(struct dict_transaction_context *_ctx,
+					    bool hide_log_values)
+{
+       struct client_dict_transaction_context *ctx =
+               container_of(_ctx, struct client_dict_transaction_context, ctx);
+       const char *query;
+
+       query = t_strdup_printf("%c%u\t%s",
+                               DICT_PROTOCOL_CMD_HIDE_LOG_VALUES,
+                               ctx->id,
+                               hide_log_values ? "yes" : "no");
+
+        client_dict_send_transaction_query(ctx, query);
+}
+
+struct dict dict_driver_client = {
+	.name = "proxy",
+
+	{
+		.init = client_dict_init,
+		.deinit = client_dict_deinit,
+		.wait = client_dict_wait,
+		.lookup = client_dict_lookup,
+		.iterate_init = client_dict_iterate_init,
+		.iterate = client_dict_iterate,
+		.iterate_deinit = client_dict_iterate_deinit,
+		.transaction_init = client_dict_transaction_init,
+		.transaction_commit = client_dict_transaction_commit,
+		.transaction_rollback = client_dict_transaction_rollback,
+		.set = client_dict_set,
+		.unset = client_dict_unset,
+		.atomic_inc = client_dict_atomic_inc,
+		.lookup_async = client_dict_lookup_async,
+		.switch_ioloop = client_dict_switch_ioloop,
+		.set_timestamp = client_dict_set_timestamp,
+		.set_hide_log_values = client_dict_set_hide_log_values,
+	}
+};
diff --git a/src/lib-dict/dict-client.h b/src/lib-dict/dict-client.h
new file mode 100644
index 0000000..95e2fb0
--- /dev/null
+++ b/src/lib-dict/dict-client.h
@@ -0,0 +1,48 @@
+#ifndef DICT_CLIENT_H
+#define DICT_CLIENT_H
+
+#include "dict.h"
+
+#define DEFAULT_DICT_SERVER_SOCKET_FNAME "dict"
+
+#define DICT_CLIENT_PROTOCOL_MAJOR_VERSION 3
+#define DICT_CLIENT_PROTOCOL_MINOR_VERSION 2
+
+#define DICT_CLIENT_PROTOCOL_VERSION_MIN_MULTI_OK 2
+
+#define DICT_CLIENT_MAX_LINE_LENGTH (64*1024)
+
+enum dict_protocol_cmd {
+        /* <major-version> <minor-version> <value type> <user> <dict name> */
+	DICT_PROTOCOL_CMD_HELLO = 'H',
+
+	DICT_PROTOCOL_CMD_LOOKUP = 'L', /* <key> */
+	DICT_PROTOCOL_CMD_ITERATE = 'I', /* <flags> <path> */
+
+	DICT_PROTOCOL_CMD_BEGIN = 'B', /* <id> */
+	DICT_PROTOCOL_CMD_COMMIT = 'C', /* <id> */
+	DICT_PROTOCOL_CMD_COMMIT_ASYNC = 'D', /* <id> */
+	DICT_PROTOCOL_CMD_ROLLBACK = 'R', /* <id> */
+
+	DICT_PROTOCOL_CMD_SET = 'S', /* <id> <key> <value> */
+	DICT_PROTOCOL_CMD_UNSET = 'U', /* <id> <key> */
+	DICT_PROTOCOL_CMD_ATOMIC_INC = 'A', /* <id> <key> <diff> */
+	DICT_PROTOCOL_CMD_TIMESTAMP = 'T', /* <id> <secs> <nsecs> */
+	DICT_PROTOCOL_CMD_HIDE_LOG_VALUES = 'V', /* <id> <hide_log_values> */
+};
+
+enum dict_protocol_reply {
+	DICT_PROTOCOL_REPLY_ERROR = -1,
+
+	DICT_PROTOCOL_REPLY_OK = 'O', /* <value> */
+	DICT_PROTOCOL_REPLY_MULTI_OK = 'M', /* protocol v2.2+ */
+	DICT_PROTOCOL_REPLY_NOTFOUND = 'N',
+	DICT_PROTOCOL_REPLY_FAIL = 'F',
+	DICT_PROTOCOL_REPLY_WRITE_UNCERTAIN = 'W',
+	DICT_PROTOCOL_REPLY_ASYNC_COMMIT = 'A',
+	DICT_PROTOCOL_REPLY_ITER_FINISHED = '\0',
+	DICT_PROTOCOL_REPLY_ASYNC_ID = '*',
+	DICT_PROTOCOL_REPLY_ASYNC_REPLY = '+',
+};
+
+#endif
diff --git a/src/lib-dict/dict-fail.c b/src/lib-dict/dict-fail.c
new file mode 100644
index 0000000..101750b
--- /dev/null
+++ b/src/lib-dict/dict-fail.c
@@ -0,0 +1,134 @@
+/* Copyright (c) 2017-2018 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "dict.h"
+#include "dict-private.h"
+
+struct dict_iterate_context dict_iter_unsupported =
+{
+	.dict = &dict_driver_fail,
+};
+
+struct dict_transaction_context dict_transaction_unsupported =
+{
+        .dict = &dict_driver_fail,
+};
+
+static int dict_fail_init(struct dict *dict_driver ATTR_UNUSED,
+			  const char *uri ATTR_UNUSED,
+			  const struct dict_settings *set ATTR_UNUSED,
+			  struct dict **dict_r ATTR_UNUSED, const char **error_r)
+{
+	*error_r = "Unsupported operation (dict does not support this feature)";
+	return -1;
+}
+
+static void dict_fail_deinit(struct dict *dict ATTR_UNUSED)
+{
+}
+
+static void dict_fail_wait(struct dict *dict ATTR_UNUSED)
+{
+}
+
+static int dict_fail_lookup(struct dict *dict ATTR_UNUSED,
+			    const struct dict_op_settings *set ATTR_UNUSED,
+			    pool_t pool ATTR_UNUSED,
+			    const char *key ATTR_UNUSED, const char **value_r ATTR_UNUSED,
+			    const char **error_r)
+{
+	*error_r = "Unsupported operation (dict does not support this feature)";
+	return -1;
+}
+
+static struct dict_iterate_context *
+dict_fail_iterate_init(struct dict *dict ATTR_UNUSED,
+		       const struct dict_op_settings *set ATTR_UNUSED,
+		       const char *path ATTR_UNUSED,
+		       enum dict_iterate_flags flags ATTR_UNUSED)
+{
+	return &dict_iter_unsupported;
+}
+
+static bool dict_fail_iterate(struct dict_iterate_context *ctx ATTR_UNUSED,
+			      const char **key_r ATTR_UNUSED,
+			      const char *const **values_r ATTR_UNUSED)
+{
+	return FALSE;
+}
+
+static int dict_fail_iterate_deinit(struct dict_iterate_context *ctx ATTR_UNUSED,
+				    const char **error_r)
+{
+	*error_r = "Unsupported operation (dict does not support this feature)";
+	return -1;
+}
+
+static struct dict_transaction_context *dict_fail_transaction_init(struct dict *dict ATTR_UNUSED)
+{
+	return &dict_transaction_unsupported;
+}
+
+static void dict_fail_transaction_commit(struct dict_transaction_context *ctx ATTR_UNUSED,
+					 bool async ATTR_UNUSED,
+					 dict_transaction_commit_callback_t *callback,
+					 void *context)
+{
+	struct dict_commit_result res = {
+		.ret = DICT_COMMIT_RET_FAILED,
+		.error = "Unsupported operation (dict does not support this feature)"
+	};
+	if (callback != NULL)
+		callback(&res, context);
+}
+
+static void dict_fail_transaction_rollback(struct dict_transaction_context *ctx ATTR_UNUSED)
+{
+}
+
+static void dict_fail_set(struct dict_transaction_context *ctx ATTR_UNUSED,
+			  const char *key ATTR_UNUSED, const char *value ATTR_UNUSED)
+{
+}
+
+static void dict_fail_unset(struct dict_transaction_context *ctx ATTR_UNUSED,
+			    const char *key ATTR_UNUSED)
+{
+}
+
+static void dict_fail_atomic_inc(struct dict_transaction_context *ctx ATTR_UNUSED,
+				 const char *key ATTR_UNUSED, long long diff ATTR_UNUSED)
+{
+}
+
+static bool dict_fail_switch_ioloop(struct dict *dict ATTR_UNUSED)
+{
+	return TRUE;
+}
+
+static void dict_fail_set_timestamp(struct dict_transaction_context *ctx ATTR_UNUSED,
+				    const struct timespec *ts ATTR_UNUSED)
+{
+}
+
+struct dict dict_driver_fail = {
+	.name = "fail",
+	.v = {
+		.init = dict_fail_init,
+		.deinit = dict_fail_deinit,
+		.wait = dict_fail_wait,
+		.lookup = dict_fail_lookup,
+		.iterate_init = dict_fail_iterate_init,
+		.iterate = dict_fail_iterate,
+		.iterate_deinit = dict_fail_iterate_deinit,
+		.transaction_init = dict_fail_transaction_init,
+		.transaction_commit = dict_fail_transaction_commit,
+		.transaction_rollback = dict_fail_transaction_rollback,
+		.set = dict_fail_set,
+		.unset = dict_fail_unset,
+		.atomic_inc = dict_fail_atomic_inc,
+		.lookup_async = NULL,
+		.switch_ioloop = dict_fail_switch_ioloop,
+		.set_timestamp = dict_fail_set_timestamp
+	},
+};
diff --git a/src/lib-dict/dict-file.c b/src/lib-dict/dict-file.c
new file mode 100644
index 0000000..c9228a8
--- /dev/null
+++ b/src/lib-dict/dict-file.c
@@ -0,0 +1,709 @@
+/* Copyright (c) 2008-2018 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "array.h"
+#include "hash.h"
+#include "str.h"
+#include "strescape.h"
+#include "home-expand.h"
+#include "mkdir-parents.h"
+#include "eacces-error.h"
+#include "file-lock.h"
+#include "file-dotlock.h"
+#include "nfs-workarounds.h"
+#include "istream.h"
+#include "ostream.h"
+#include "dict-transaction-memory.h"
+#include "dict-private.h"
+
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+
+struct file_dict {
+	struct dict dict;
+	pool_t hash_pool;
+	enum file_lock_method lock_method;
+
+	char *path;
+	char *home_dir;
+	bool dict_path_checked;
+	HASH_TABLE(char *, char *) hash;
+	int fd;
+
+	bool refreshed;
+};
+
+struct file_dict_iterate_context {
+	struct dict_iterate_context ctx;
+	pool_t pool;
+
+	struct hash_iterate_context *iter;
+	const char *path;
+	size_t path_len;
+
+	enum dict_iterate_flags flags;
+	const char *values[2];
+	const char *error;
+};
+
+static struct dotlock_settings file_dict_dotlock_settings = {
+	.timeout = 60*2,
+	.stale_timeout = 60,
+	.use_io_notify = TRUE
+};
+
+static int
+file_dict_ensure_path_home_dir(struct file_dict *dict, const char *home_dir,
+			       const char **error_r)
+{
+	if (null_strcmp(dict->home_dir, home_dir) == 0)
+		return 0;
+
+	if (dict->dict_path_checked) {
+		*error_r = t_strdup_printf("home_dir changed from %s to %s "
+				"(requested dict was: %s)", dict->home_dir,
+				home_dir, dict->path);
+		return -1;
+	}
+
+	char *_p = dict->path;
+	dict->path = i_strdup(home_expand_tilde(dict->path, home_dir));
+	dict->home_dir = i_strdup(home_dir);
+	i_free(_p);
+	dict->dict_path_checked = TRUE;
+	return 0;
+}
+
+static int
+file_dict_init(struct dict *driver, const char *uri,
+	       const struct dict_settings *set ATTR_UNUSED,
+	       struct dict **dict_r, const char **error_r)
+{
+	struct file_dict *dict;
+	const char *p, *path;
+
+	dict = i_new(struct file_dict, 1);
+	dict->lock_method = FILE_LOCK_METHOD_DOTLOCK;
+
+	p = strchr(uri, ':');
+	if (p == NULL) {
+		/* no parameters */
+		path = uri;
+	} else {
+		path = t_strdup_until(uri, p++);
+		if (strcmp(p, "lock=fcntl") == 0)
+			dict->lock_method = FILE_LOCK_METHOD_FCNTL;
+		else if (strcmp(p, "lock=flock") == 0)
+			dict->lock_method = FILE_LOCK_METHOD_FLOCK;
+		else {
+			*error_r = t_strdup_printf("Invalid parameter: %s", p+1);
+			i_free(dict);
+			return -1;
+		}
+	}
+
+	/* keep the path for now, later in dict operations check if home_dir
+	   should be prepended. */
+	dict->path = i_strdup(path);
+
+	dict->dict = *driver;
+	dict->hash_pool = pool_alloconly_create("file dict", 1024);
+	hash_table_create(&dict->hash, dict->hash_pool, 0, str_hash, strcmp);
+	dict->fd = -1;
+	*dict_r = &dict->dict;
+	return 0;
+}
+
+static void file_dict_deinit(struct dict *_dict)
+{
+	struct file_dict *dict = (struct file_dict *)_dict;
+
+	i_close_fd_path(&dict->fd, dict->path);
+	hash_table_destroy(&dict->hash);
+	pool_unref(&dict->hash_pool);
+	i_free(dict->path);
+	i_free(dict->home_dir);
+	i_free(dict);
+}
+
+static bool file_dict_need_refresh(struct file_dict *dict)
+{
+	struct stat st1, st2;
+
+	if (dict->dict.iter_count > 0) {
+		/* Change nothing while there are iterators or they can crash
+		   because the hash table content recreated. */
+		return FALSE;
+	}
+
+	if (dict->fd == -1)
+		return TRUE;
+
+	/* Disable NFS flushing for now since it can cause unnecessary
+	   problems and there's no easy way for us to know here if
+	   mail_nfs_storage=yes. In any case it's pretty much an unsupported
+	   setting nowadays. */
+	/*nfs_flush_file_handle_cache(dict->path);*/
+	if (nfs_safe_stat(dict->path, &st1) < 0) {
+		e_error(dict->dict.event, "stat(%s) failed: %m", dict->path);
+		return FALSE;
+	}
+
+	if (fstat(dict->fd, &st2) < 0) {
+		if (errno != ESTALE)
+			e_error(dict->dict.event, "fstat(%s) failed: %m", dict->path);
+		return TRUE;
+	}
+	if (st1.st_ino != st2.st_ino ||
+	    !CMP_DEV_T(st1.st_dev, st2.st_dev)) {
+		/* file changed */
+		return TRUE;
+	}
+	return FALSE;
+}
+
+static int file_dict_open_latest(struct file_dict *dict, const char **error_r)
+{
+	int open_type;
+
+	if (!file_dict_need_refresh(dict))
+		return 0;
+
+	i_close_fd_path(&dict->fd, dict->path);
+
+	open_type = dict->lock_method == FILE_LOCK_METHOD_DOTLOCK ?
+		O_RDONLY : O_RDWR;
+	dict->fd = open(dict->path, open_type);
+	if (dict->fd == -1) {
+		if (errno == ENOENT)
+			return 0;
+		if (errno == EACCES)
+			*error_r = eacces_error_get("open", dict->path);
+		else
+			*error_r = t_strdup_printf("open(%s) failed: %m", dict->path);
+		return -1;
+	}
+	dict->refreshed = FALSE;
+	return 1;
+}
+
+static int file_dict_refresh(struct file_dict *dict, const char **error_r)
+{
+	struct istream *input;
+	char *key, *value;
+
+	if (file_dict_open_latest(dict, error_r) < 0)
+		return -1;
+	if (dict->refreshed || dict->dict.iter_count > 0)
+		return 0;
+
+	hash_table_clear(dict->hash, TRUE);
+	p_clear(dict->hash_pool);
+
+	if (dict->fd != -1) {
+		input = i_stream_create_fd(dict->fd, SIZE_MAX);
+
+		while ((key = i_stream_read_next_line(input)) != NULL) {
+			/* strdup() before the second read */
+			key = str_tabunescape(p_strdup(dict->hash_pool, key));
+
+			if ((value = i_stream_read_next_line(input)) == NULL)
+				break;
+
+			value = str_tabunescape(p_strdup(dict->hash_pool, value));
+			hash_table_update(dict->hash, key, value);
+		}
+		i_stream_destroy(&input);
+	}
+	dict->refreshed = TRUE;
+	return 0;
+}
+
+static int file_dict_lookup(struct dict *_dict,
+			    const struct dict_op_settings *set,
+			    pool_t pool, const char *key,
+			    const char **value_r, const char **error_r)
+{
+	struct file_dict *dict = (struct file_dict *)_dict;
+
+	if (file_dict_ensure_path_home_dir(dict, set->home_dir, error_r) < 0)
+		return -1;
+
+	if (file_dict_refresh(dict, error_r) < 0)
+		return -1;
+
+	*value_r = p_strdup(pool, hash_table_lookup(dict->hash, key));
+	return *value_r == NULL ? 0 : 1;
+}
+
+static struct dict_iterate_context *
+file_dict_iterate_init(struct dict *_dict,
+		       const struct dict_op_settings *set ATTR_UNUSED,
+		       const char *path, enum dict_iterate_flags flags)
+{
+        struct file_dict_iterate_context *ctx;
+	struct file_dict *dict = (struct file_dict *)_dict;
+	const char *error;
+	pool_t pool;
+
+	pool = pool_alloconly_create("file dict iterate", 256);
+	ctx = p_new(pool, struct file_dict_iterate_context, 1);
+	ctx->ctx.dict = _dict;
+	ctx->pool = pool;
+
+	ctx->path = p_strdup(pool, path);
+	ctx->path_len = strlen(path);
+	ctx->flags = flags;
+
+	if (file_dict_ensure_path_home_dir(dict, set->home_dir, &error) < 0 ||
+	    file_dict_refresh(dict, &error) < 0)
+		ctx->error = p_strdup(pool, error);
+
+	ctx->iter = hash_table_iterate_init(dict->hash);
+	return &ctx->ctx;
+}
+
+static bool
+file_dict_iterate_key_matches(struct file_dict_iterate_context *ctx,
+			      const char *key)
+{
+	if (strncmp(ctx->path, key, ctx->path_len) == 0)
+		return TRUE;
+	return FALSE;
+}
+
+
+static bool file_dict_iterate(struct dict_iterate_context *_ctx,
+			      const char **key_r, const char *const **values_r)
+{
+	struct file_dict_iterate_context *ctx =
+		(struct file_dict_iterate_context *)_ctx;
+	char *key, *value;
+
+	while (hash_table_iterate(ctx->iter,
+				  ((struct file_dict *)_ctx->dict)->hash,
+				  &key, &value)) {
+		if (!file_dict_iterate_key_matches(ctx, key))
+			continue;
+
+		if ((ctx->flags & DICT_ITERATE_FLAG_RECURSE) != 0) {
+			/* match everything */
+		} else if ((ctx->flags & DICT_ITERATE_FLAG_EXACT_KEY) != 0) {
+			if (key[ctx->path_len] != '\0')
+				continue;
+		} else {
+			if (strchr(key + ctx->path_len, '/') != NULL)
+				continue;
+		}
+
+		*key_r = key;
+		ctx->values[0] = value;
+		*values_r = ctx->values;
+		return TRUE;
+	}
+	return FALSE;
+}
+
+static int file_dict_iterate_deinit(struct dict_iterate_context *_ctx,
+				    const char **error_r)
+{
+	struct file_dict_iterate_context *ctx =
+		(struct file_dict_iterate_context *)_ctx;
+	int ret = ctx->error != NULL ? -1 : 0;
+
+	*error_r = t_strdup(ctx->error);
+	hash_table_iterate_deinit(&ctx->iter);
+	pool_unref(&ctx->pool);
+	return ret;
+}
+
+static struct dict_transaction_context *
+file_dict_transaction_init(struct dict *_dict)
+{
+	struct dict_transaction_memory_context *ctx;
+	pool_t pool;
+
+	pool = pool_alloconly_create("file dict transaction", 2048);
+	ctx = p_new(pool, struct dict_transaction_memory_context, 1);
+	dict_transaction_memory_init(ctx, _dict, pool);
+	return &ctx->ctx;
+}
+
+static void file_dict_apply_changes(struct dict_transaction_memory_context *ctx,
+				    bool *atomic_inc_not_found_r)
+{
+	struct file_dict *dict = (struct file_dict *)ctx->ctx.dict;
+	const char *tmp;
+	char *key, *value, *old_value;
+	char *orig_key, *orig_value;
+	const struct dict_transaction_memory_change *change;
+	size_t new_len;
+	long long diff;
+
+	array_foreach(&ctx->changes, change) {
+		if (hash_table_lookup_full(dict->hash, change->key,
+					   &orig_key, &orig_value)) {
+			key = orig_key;
+			old_value = orig_value;
+		} else {
+			key = NULL;
+			old_value = NULL;
+		}
+		value = NULL;
+
+		switch (change->type) {
+		case DICT_CHANGE_TYPE_INC:
+			if (old_value == NULL) {
+				*atomic_inc_not_found_r = TRUE;
+				break;
+			}
+			if (str_to_llong(old_value, &diff) < 0)
+				i_unreached();
+			diff +=	change->value.diff;
+			tmp = t_strdup_printf("%lld", diff);
+			new_len = strlen(tmp);
+			if (old_value == NULL || new_len > strlen(old_value))
+				value = p_strdup(dict->hash_pool, tmp);
+			else {
+				memcpy(old_value, tmp, new_len + 1);
+				value = old_value;
+			}
+			/* fall through */
+		case DICT_CHANGE_TYPE_SET:
+			if (key == NULL)
+				key = p_strdup(dict->hash_pool, change->key);
+			if (value == NULL) {
+				value = p_strdup(dict->hash_pool,
+						 change->value.str);
+			}
+			hash_table_update(dict->hash, key, value);
+			break;
+		case DICT_CHANGE_TYPE_UNSET:
+			if (old_value != NULL)
+				hash_table_remove(dict->hash, key);
+			break;
+		}
+	}
+}
+
+static int
+fd_copy_stat_permissions(const struct stat *src_st,
+			 int dest_fd, const char *dest_path,
+			 const char **error_r)
+{
+	struct stat dest_st;
+
+	if (fstat(dest_fd, &dest_st) < 0) {
+		*error_r = t_strdup_printf("fstat(%s) failed: %m", dest_path);
+		return -1;
+	}
+
+	if (src_st->st_gid != dest_st.st_gid &&
+	    ((src_st->st_mode & 0070) >> 3 != (src_st->st_mode & 0007))) {
+		/* group has different permissions from world.
+		   preserve the group. */
+		if (fchown(dest_fd, (uid_t)-1, src_st->st_gid) < 0) {
+			*error_r = t_strdup_printf("fchown(%s, -1, %s) failed: %m",
+						   dest_path, dec2str(src_st->st_gid));
+			return -1;
+		}
+	}
+
+	if ((src_st->st_mode & 07777) != (dest_st.st_mode & 07777)) {
+		if (fchmod(dest_fd, src_st->st_mode & 07777) < 0) {
+			*error_r = t_strdup_printf("fchmod(%s, %o) failed: %m",
+						   dest_path, (int)(src_st->st_mode & 0777));
+			return -1;
+		}
+	}
+	return 0;
+}
+
+static int fd_copy_permissions(int src_fd, const char *src_path,
+			       int dest_fd, const char *dest_path,
+			       const char **error_r)
+{
+	struct stat src_st;
+
+	if (fstat(src_fd, &src_st) < 0) {
+		*error_r = t_strdup_printf("fstat(%s) failed: %m", src_path);
+		return -1;
+	}
+	return fd_copy_stat_permissions(&src_st, dest_fd, dest_path, error_r);
+}
+
+static int
+fd_copy_parent_dir_permissions(const char *src_path, int dest_fd,
+			       const char *dest_path, const char **error_r)
+{
+	struct stat src_st;
+	const char *src_dir, *p;
+
+	p = strrchr(src_path, '/');
+	if (p == NULL)
+		src_dir = ".";
+	else
+		src_dir = t_strdup_until(src_path, p);
+	if (stat(src_dir, &src_st) < 0) {
+		*error_r = t_strdup_printf("stat(%s) failed: %m", src_dir);
+		return -1;
+	}
+	src_st.st_mode &= 0666;
+	return fd_copy_stat_permissions(&src_st, dest_fd, dest_path, error_r);
+}
+
+static int file_dict_mkdir(struct file_dict *dict, const char **error_r)
+{
+	const char *path, *p, *root;
+	struct stat st;
+	mode_t mode = 0700;
+
+	p = strrchr(dict->path, '/');
+	if (p == NULL)
+		return 0;
+	path = t_strdup_until(dict->path, p);
+
+	if (stat_first_parent(path, &root, &st) < 0) {
+		if (errno == EACCES)
+			*error_r = eacces_error_get("stat", root);
+		else
+			*error_r = t_strdup_printf("stat(%s) failed: %m", root);
+		return -1;
+	}
+	if ((st.st_mode & S_ISGID) != 0) {
+		/* preserve parent's permissions when it has setgid bit */
+		mode = st.st_mode;
+	}
+
+	if (mkdir_parents(path, mode) < 0 && errno != EEXIST) {
+		if (errno == EACCES)
+			*error_r = eacces_error_get("mkdir_parents", path);
+		else
+			*error_r = t_strdup_printf("mkdir_parents(%s) failed: %m", path);
+		return -1;
+	}
+	return 0;
+}
+
+static int
+file_dict_lock(struct file_dict *dict, struct file_lock **lock_r,
+	       const char **error_r)
+{
+	int ret;
+	const char *error;
+
+	if (file_dict_open_latest(dict, error_r) < 0)
+		return -1;
+
+	if (dict->fd == -1) {
+		/* quota file doesn't exist yet, we need to create it */
+		dict->fd = open(dict->path, O_CREAT | O_RDWR, 0600);
+		if (dict->fd == -1 && errno == ENOENT) {
+			if (file_dict_mkdir(dict, error_r) < 0)
+				return -1;
+			dict->fd = open(dict->path, O_CREAT | O_RDWR, 0600);
+		}
+		if (dict->fd == -1) {
+			if (errno == EACCES)
+				*error_r = eacces_error_get("creat", dict->path);
+			else {
+				*error_r = t_strdup_printf(
+					"creat(%s) failed: %m", dict->path);
+			}
+			return -1;
+		}
+		if (fd_copy_parent_dir_permissions(dict->path, dict->fd,
+						   dict->path, &error) < 0)
+			e_error(dict->dict.event, "%s", error);
+	}
+
+	*lock_r = NULL;
+	struct file_lock_settings lock_set = {
+		.lock_method = dict->lock_method,
+	};
+	do {
+		file_lock_free(lock_r);
+		if (file_wait_lock(dict->fd, dict->path, F_WRLCK, &lock_set,
+				   file_dict_dotlock_settings.timeout,
+				   lock_r, &error) <= 0) {
+			*error_r = t_strdup_printf(
+				"file_wait_lock(%s) failed: %s",
+				dict->path, error);
+			return -1;
+		}
+		/* check again if we need to reopen the file because it was
+		   just replaced */
+	} while ((ret = file_dict_open_latest(dict, error_r)) > 0);
+
+	return ret < 0 ? -1 : 0;
+}
+
+static int
+file_dict_write_changes(struct dict_transaction_memory_context *ctx,
+			bool *atomic_inc_not_found_r, const char **error_r)
+{
+	struct file_dict *dict = (struct file_dict *)ctx->ctx.dict;
+	struct dotlock *dotlock = NULL;
+	struct file_lock *lock = NULL;
+	const char *temp_path = NULL;
+	const char *error;
+	struct hash_iterate_context *iter;
+	struct ostream *output;
+	char *key, *value;
+	string_t *str;
+	int fd = -1;
+
+	*atomic_inc_not_found_r = FALSE;
+
+	if (file_dict_ensure_path_home_dir(dict, ctx->ctx.set.home_dir, error_r) < 0)
+		return -1;
+
+	switch (dict->lock_method) {
+	case FILE_LOCK_METHOD_FCNTL:
+	case FILE_LOCK_METHOD_FLOCK:
+		if (file_dict_lock(dict, &lock, error_r) < 0)
+			return -1;
+		temp_path = t_strdup_printf("%s.tmp", dict->path);
+		fd = creat(temp_path, 0600);
+		if (fd == -1) {
+			*error_r = t_strdup_printf(
+				"dict-file: creat(%s) failed: %m", temp_path);
+			file_unlock(&lock);
+			return -1;
+		}
+		break;
+	case FILE_LOCK_METHOD_DOTLOCK:
+		fd = file_dotlock_open(&file_dict_dotlock_settings, dict->path, 0,
+				       &dotlock);
+		if (fd == -1 && errno == ENOENT) {
+			if (file_dict_mkdir(dict, error_r) < 0)
+				return -1;
+			fd = file_dotlock_open(&file_dict_dotlock_settings,
+					       dict->path, 0, &dotlock);
+		}
+		if (fd == -1) {
+			*error_r = t_strdup_printf(
+				"dict-file: file_dotlock_open(%s) failed: %m",
+				dict->path);
+			return -1;
+		}
+		temp_path = file_dotlock_get_lock_path(dotlock);
+		break;
+	}
+
+	/* refresh once more now that we're locked */
+	if (file_dict_refresh(dict, error_r) < 0) {
+		if (dotlock != NULL)
+			file_dotlock_delete(&dotlock);
+		else {
+			i_close_fd(&fd);
+			file_unlock(&lock);
+		}
+		return -1;
+	}
+	if (dict->fd != -1) {
+		/* preserve the permissions */
+		if (fd_copy_permissions(dict->fd, dict->path, fd, temp_path, &error) < 0)
+			e_error(ctx->ctx.event, "%s", error);
+	} else {
+		/* get initial permissions from parent directory */
+		if (fd_copy_parent_dir_permissions(dict->path, fd, temp_path, &error) < 0)
+			e_error(ctx->ctx.event, "%s", error);
+	}
+	file_dict_apply_changes(ctx, atomic_inc_not_found_r);
+
+	output = o_stream_create_fd(fd, 0);
+	o_stream_cork(output);
+	iter = hash_table_iterate_init(dict->hash);
+	str = t_str_new(256);
+	while (hash_table_iterate(iter, dict->hash, &key, &value)) {
+		str_truncate(str, 0);
+		str_append_tabescaped(str, key);
+		str_append_c(str, '\n');
+		str_append_tabescaped(str, value);
+		str_append_c(str, '\n');
+		o_stream_nsend(output, str_data(str), str_len(str));
+	}
+	hash_table_iterate_deinit(&iter);
+
+	if (o_stream_finish(output) <= 0) {
+		*error_r = t_strdup_printf("write(%s) failed: %s", temp_path,
+					   o_stream_get_error(output));
+		o_stream_destroy(&output);
+		if (dotlock != NULL)
+			file_dotlock_delete(&dotlock);
+		else {
+			i_close_fd(&fd);
+			file_unlock(&lock);
+		}
+		return -1;
+	}
+	o_stream_destroy(&output);
+
+	if (dotlock != NULL) {
+		if (file_dotlock_replace(&dotlock,
+				DOTLOCK_REPLACE_FLAG_DONT_CLOSE_FD) < 0) {
+			*error_r = t_strdup_printf("file_dotlock_replace() failed: %m");
+			i_close_fd(&fd);
+			return -1;
+		}
+	} else {
+		if (rename(temp_path, dict->path) < 0) {
+			*error_r = t_strdup_printf("rename(%s, %s) failed: %m",
+						   temp_path, dict->path);
+			file_unlock(&lock);
+			i_close_fd(&fd);
+			return -1;
+		}
+		/* dict->fd is locked, not the new fd. We're closing dict->fd
+		   so we can just free the lock struct. */
+		file_lock_free(&lock);
+	}
+
+	i_close_fd(&dict->fd);
+	dict->fd = fd;
+	return 0;
+}
+
+static void
+file_dict_transaction_commit(struct dict_transaction_context *_ctx,
+			     bool async ATTR_UNUSED,
+			     dict_transaction_commit_callback_t *callback,
+			     void *context)
+{
+	struct dict_transaction_memory_context *ctx =
+		(struct dict_transaction_memory_context *)_ctx;
+	struct dict_commit_result result;
+	bool atomic_inc_not_found;
+
+	i_zero(&result);
+	if (file_dict_write_changes(ctx, &atomic_inc_not_found, &result.error) < 0)
+		result.ret = DICT_COMMIT_RET_FAILED;
+	else if (atomic_inc_not_found)
+		result.ret = DICT_COMMIT_RET_NOTFOUND;
+	else
+		result.ret = DICT_COMMIT_RET_OK;
+	pool_unref(&ctx->pool);
+
+	callback(&result, context);
+}
+
+struct dict dict_driver_file = {
+	.name = "file",
+	{
+		.init = file_dict_init,
+		.deinit = file_dict_deinit,
+		.lookup = file_dict_lookup,
+		.iterate_init = file_dict_iterate_init,
+		.iterate = file_dict_iterate,
+		.iterate_deinit = file_dict_iterate_deinit,
+		.transaction_init = file_dict_transaction_init,
+		.transaction_commit = file_dict_transaction_commit,
+		.transaction_rollback = dict_transaction_memory_rollback,
+		.set = dict_transaction_memory_set,
+		.unset = dict_transaction_memory_unset,
+		.atomic_inc = dict_transaction_memory_atomic_inc,
+	}
+};
diff --git a/src/lib-dict/dict-iter-lua.c b/src/lib-dict/dict-iter-lua.c
new file mode 100644
index 0000000..780de03
--- /dev/null
+++ b/src/lib-dict/dict-iter-lua.c
@@ -0,0 +1,193 @@
+/* Copyright (c) 2021 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "array.h"
+#include "dict.h"
+#include "dlua-script-private.h"
+#include "dict-lua-private.h"
+#include "dlua-wrapper.h"
+
+struct lua_dict_iter {
+	pool_t pool;
+	struct dict_iterate_context *iter;
+	ARRAY(int) refs;
+	int error_ref;
+
+	lua_State *L;
+	bool yielded:1;
+};
+
+static void lua_dict_iter_unref(struct lua_dict_iter *iter)
+{
+	const char *error;
+
+	/* deinit iteration if it hasn't been done yet */
+	if (dict_iterate_deinit(&iter->iter, &error) < 0) {
+		e_error(dlua_script_from_state(iter->L)->event,
+			"Dict iteration failed: %s", error);
+	}
+
+	pool_unref(&iter->pool);
+}
+
+DLUA_WRAP_C_DATA(dict_iter, struct lua_dict_iter, lua_dict_iter_unref, NULL);
+
+static int lua_dict_iterate_step(lua_State *L);
+
+/* resume after a yield */
+static int lua_dict_iterate_step_continue(lua_State *L,
+					  int status ATTR_UNUSED,
+					  lua_KContext ctx ATTR_UNUSED)
+{
+	return lua_dict_iterate_step(L);
+}
+
+static void lua_dict_iterate_more(struct lua_dict_iter *iter);
+
+/*
+ * Iteration step function
+ *
+ * Takes two args (a userdata state, and previous value) and returns the
+ * next value.
+ */
+static int lua_dict_iterate_step(lua_State *L)
+{
+	struct lua_dict_iter *iter;
+	const int *refs;
+	unsigned nrefs;
+
+	DLUA_REQUIRE_ARGS(L, 2);
+
+	iter = xlua_dict_iter_getptr(L, 1, NULL);
+	iter->yielded = FALSE;
+
+	lua_dict_iterate_more(iter);
+
+	if (iter->iter != NULL) {
+		/* iteration didn't end yet - yield */
+		return lua_dict_iterate_step_continue(L,
+			      lua_yieldk(L, 0, 0, lua_dict_iterate_step_continue), 0);
+	}
+
+	/* dict iteration ended - return first key-value pair */
+	refs = array_get(&iter->refs, &nrefs);
+	i_assert(nrefs % 2 == 0);
+
+	if (nrefs == 0) {
+		if (iter->error_ref != 0) {
+			/* dict iteration generated an error - raise it now */
+			lua_rawgeti(L, LUA_REGISTRYINDEX, iter->error_ref);
+			luaL_unref(L, LUA_REGISTRYINDEX, iter->error_ref);
+			return lua_error(L);
+		}
+
+		return 0; /* return nil */
+	}
+
+	/* get the key & value from the registry */
+	lua_rawgeti(L, LUA_REGISTRYINDEX, refs[0]);
+	lua_rawgeti(L, LUA_REGISTRYINDEX, refs[1]);
+	luaL_unref(L, LUA_REGISTRYINDEX, refs[0]);
+	luaL_unref(L, LUA_REGISTRYINDEX, refs[1]);
+
+	array_delete(&iter->refs, 0, 2);
+
+	return 2;
+}
+
+static void lua_dict_iterate_more(struct lua_dict_iter *iter)
+{
+	const char *key, *const *values;
+	lua_State *L = iter->L;
+	const char *error;
+
+	if (iter->iter == NULL)
+		return; /* done iterating the dict */
+
+	while (dict_iterate_values(iter->iter, &key, &values)) {
+		int ref;
+
+		/* stash key */
+		lua_pushstring(L, key);
+		ref = luaL_ref(L, LUA_REGISTRYINDEX);
+		array_push_back(&iter->refs, &ref);
+
+		/* stash values */
+		lua_newtable(L);
+		for (unsigned int i = 0; values[i] != NULL; i++) {
+			lua_pushstring(L, values[i]);
+			lua_seti(L, -2, i + 1);
+		}
+		ref = luaL_ref(L, LUA_REGISTRYINDEX);
+		array_push_back(&iter->refs, &ref);
+	}
+
+	if (dict_iterate_has_more(iter->iter))
+		return;
+
+	if (dict_iterate_deinit(&iter->iter, &error) < 0) {
+		lua_pushstring(L, error);
+		iter->error_ref = luaL_ref(L, LUA_REGISTRYINDEX);
+	}
+}
+
+/* dict iter callback */
+static void lua_dict_iterate_callback(struct lua_dict_iter *iter)
+{
+	if (iter->yielded)
+		return;
+	iter->yielded = TRUE;
+	dlua_pcall_yieldable_resume(iter->L, 1);
+}
+
+/*
+ * Iterate a dict at key [-(3|4),+2,e]
+ *
+ * Args:
+ *   1) userdata: sturct dict *dict
+ *   2) string: key
+ *   3) integer: flags
+ *   4*) string: username
+ *
+ * Returns:
+ *   Returns a iteration step function and dict iter userdata.
+ *   Username will be NULL if not provided in args.
+ */
+int lua_dict_iterate(lua_State *L)
+{
+	enum dict_iterate_flags flags;
+	struct lua_dict_iter *iter;
+	struct dict *dict;
+	const char *path, *username = NULL;
+	pool_t pool;
+
+	DLUA_REQUIRE_ARGS_IN(L, 3, 4);
+
+	dict = dlua_check_dict(L, 1);
+	path = luaL_checkstring(L, 2);
+	flags = luaL_checkinteger(L, 3);
+	if (lua_gettop(L) >= 4)
+		username = luaL_checkstring(L, 4);
+	lua_dict_check_key_prefix(L, path, username);
+
+	struct dict_op_settings set = {
+		.username = username,
+	};
+
+	/* set up iteration */
+	pool = pool_alloconly_create("lua dict iter", 128);
+	iter = p_new(pool, struct lua_dict_iter, 1);
+	iter->pool = pool;
+	iter->iter = dict_iterate_init(dict, &set, path, flags |
+				      DICT_ITERATE_FLAG_ASYNC);
+	p_array_init(&iter->refs, iter->pool, 32);
+	iter->L = L;
+
+	dict_iterate_set_async_callback(iter->iter,
+					lua_dict_iterate_callback, iter);
+
+	/* push return values: func, state */
+	lua_pushcfunction(L, lua_dict_iterate_step);
+	xlua_pushdict_iter(L, iter, FALSE);
+	return 2;
+}
diff --git a/src/lib-dict/dict-lua-private.h b/src/lib-dict/dict-lua-private.h
new file mode 100644
index 0000000..f9f9943
--- /dev/null
+++ b/src/lib-dict/dict-lua-private.h
@@ -0,0 +1,9 @@
+#ifndef DICT_LUA_PRIVATE_H
+#define DICT_LUA_PRIVATE_H
+
+#include "dict-lua.h"
+
+int lua_dict_iterate(lua_State *l);
+int lua_dict_transaction_begin(lua_State *l);
+
+#endif
diff --git a/src/lib-dict/dict-lua.c b/src/lib-dict/dict-lua.c
new file mode 100644
index 0000000..d5de534
--- /dev/null
+++ b/src/lib-dict/dict-lua.c
@@ -0,0 +1,117 @@
+/* Copyright (c) 2021 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "dict.h"
+#include "dlua-script-private.h"
+#include "dict-lua-private.h"
+#include "dlua-wrapper.h"
+
+static int lua_dict_lookup(lua_State *);
+
+static luaL_Reg lua_dict_methods[] = {
+	{ "lookup", lua_dict_lookup },
+	{ "iterate", lua_dict_iterate },
+	{ "transaction_begin", lua_dict_transaction_begin },
+	{ NULL, NULL },
+};
+
+/* no actual ref counting */
+static void lua_dict_unref(struct dict *dict ATTR_UNUSED)
+{
+}
+
+DLUA_WRAP_C_DATA(dict, struct dict, lua_dict_unref, lua_dict_methods);
+
+static int lua_dict_async_continue(lua_State *L,
+				   int status ATTR_UNUSED,
+				   lua_KContext ctx ATTR_UNUSED)
+{
+	/*
+	 * lua_dict_*_callback() already pushed the result table/nil or error
+	 * string.  We simply need to return/error out.
+	 */
+
+	if (lua_istable(L, -1) || lua_isnil(L, -1))
+		return 1;
+	else
+		return lua_error(L);
+}
+
+static void lua_dict_lookup_callback(const struct dict_lookup_result *result,
+				     lua_State *L)
+{
+	if (result->ret < 0) {
+		lua_pushstring(L, result->error);
+	} else if (result->ret == 0) {
+		lua_pushnil(L);
+	} else {
+		unsigned int i;
+
+		lua_newtable(L);
+
+		for (i = 0; i < str_array_length(result->values); i++) {
+			lua_pushstring(L, result->values[i]);
+			lua_seti(L, -2, i + 1);
+		}
+	}
+
+	dlua_pcall_yieldable_resume(L, 1);
+}
+
+void lua_dict_check_key_prefix(lua_State *L, const char *key,
+			       const char *username)
+{
+	if (str_begins(key, DICT_PATH_SHARED))
+		;
+	else if (str_begins(key, DICT_PATH_PRIVATE)) {
+		if (username == NULL || username[0] == '\0')
+			luaL_error(L, DICT_PATH_PRIVATE" dict key prefix requires username");
+	} else {
+		luaL_error(L, "Invalid dict key prefix");
+	}
+}
+
+/*
+ * Lookup a key in dict [-(2|3),+1,e]
+ *
+ * Args:
+ *   1) userdata: struct dict *dict
+ *   2) string: key
+ *   3*) string: username
+ *
+ * Returns:
+ *   If key is found, returns a table with values.  If key is not found,
+ *   returns nil.
+ *   Username will be NULL if not provided in args.
+ */
+static int lua_dict_lookup(lua_State *L)
+{
+	struct dict *dict;
+	const char *key, *username = NULL;
+
+	DLUA_REQUIRE_ARGS_IN(L, 2, 3);
+
+	dict = xlua_dict_getptr(L, 1, NULL);
+	key = luaL_checkstring(L, 2);
+	if (lua_gettop(L) >= 3)
+		username = luaL_checkstring(L, 3);
+	lua_dict_check_key_prefix(L, key, username);
+
+	struct dict_op_settings set = {
+		.username = username,
+	};
+	dict_lookup_async(dict, &set, key, lua_dict_lookup_callback, L);
+
+	return lua_dict_async_continue(L,
+		lua_yieldk(L, 0, 0, lua_dict_async_continue), 0);
+}
+
+void dlua_push_dict(lua_State *L, struct dict *dict)
+{
+	xlua_pushdict(L, dict, FALSE);
+}
+
+struct dict *dlua_check_dict(lua_State *L, int idx)
+{
+	return xlua_dict_getptr(L, idx, NULL);
+}
diff --git a/src/lib-dict/dict-lua.h b/src/lib-dict/dict-lua.h
new file mode 100644
index 0000000..bf4255c
--- /dev/null
+++ b/src/lib-dict/dict-lua.h
@@ -0,0 +1,18 @@
+#ifndef DICT_LUA_H
+#define DICT_LUA_H
+
+#ifdef DLUA_WITH_YIELDS
+/*
+ * Internally, the dict methods yield via lua_yieldk() as implemented in Lua
+ * 5.3 and newer.
+ */
+
+void lua_dict_check_key_prefix(lua_State *L, const char *key,
+			       const char *username);
+
+void dlua_push_dict(lua_State *L, struct dict *dict);
+struct dict *dlua_check_dict(lua_State *L, int idx);
+
+#endif
+
+#endif
diff --git a/src/lib-dict/dict-memcached-ascii.c b/src/lib-dict/dict-memcached-ascii.c
new file mode 100644
index 0000000..6ae5443
--- /dev/null
+++ b/src/lib-dict/dict-memcached-ascii.c
@@ -0,0 +1,685 @@
+/* Copyright (c) 2008-2018 Dovecot authors, see the included COPYING memcached_ascii */
+
+#include "lib.h"
+#include "array.h"
+#include "str.h"
+#include "istream.h"
+#include "ostream.h"
+#include "connection.h"
+#include "dict-transaction-memory.h"
+#include "dict-private.h"
+
+#define MEMCACHED_DEFAULT_PORT 11211
+#define MEMCACHED_DEFAULT_LOOKUP_TIMEOUT_MSECS (1000*30)
+#define DICT_USERNAME_SEPARATOR '/'
+
+enum memcached_ascii_input_state {
+	/* GET: expecting VALUE or END */
+	MEMCACHED_INPUT_STATE_GET,
+	/* SET: expecting STORED / NOT_STORED */
+	MEMCACHED_INPUT_STATE_STORED,
+	/* DELETE: expecting DELETED */
+	MEMCACHED_INPUT_STATE_DELETED,
+	/* (INCR+ADD)/DECR: expecting number / NOT_FOUND / STORED / NOT_STORED */
+	MEMCACHED_INPUT_STATE_INCRDECR
+};
+
+struct memcached_ascii_connection {
+	struct connection conn;
+	struct memcached_ascii_dict *dict;
+
+	string_t *reply_str;
+	unsigned int reply_bytes_left;
+	bool value_received;
+	bool value_waiting_end;
+};
+
+struct memcached_ascii_dict_reply {
+	unsigned int reply_count;
+	dict_transaction_commit_callback_t *callback;
+	void *context;
+};
+
+struct dict_memcached_ascii_commit_ctx {
+	struct memcached_ascii_dict *dict;
+	struct dict_transaction_memory_context *memctx;
+	string_t *str;
+
+	dict_transaction_commit_callback_t *callback;
+	void *context;
+};
+
+struct memcached_ascii_dict {
+	struct dict dict;
+	struct ip_addr ip;
+	char *key_prefix;
+	in_port_t port;
+	unsigned int timeout_msecs;
+
+	struct timeout *to;
+	struct memcached_ascii_connection conn;
+
+	ARRAY(enum memcached_ascii_input_state) input_states;
+	ARRAY(struct memcached_ascii_dict_reply) replies;
+};
+
+static struct connection_list *memcached_ascii_connections;
+
+static void
+memcached_ascii_callback(struct memcached_ascii_dict *dict,
+			 const struct memcached_ascii_dict_reply *reply,
+			 const struct dict_commit_result *result)
+{
+	if (reply->callback != NULL) {
+		if (dict->dict.prev_ioloop != NULL) {
+			/* Don't let callback see that we've created our
+			   internal ioloop in case it wants to add some ios
+			   or timeouts. */
+			current_ioloop = dict->dict.prev_ioloop;
+		}
+		reply->callback(result, reply->context);
+		if (dict->dict.prev_ioloop != NULL)
+			current_ioloop = dict->dict.ioloop;
+	}
+}
+
+static void
+memcached_ascii_disconnected(struct memcached_ascii_connection *conn,
+			     const char *reason)
+{
+	const struct dict_commit_result result = {
+		DICT_COMMIT_RET_FAILED, reason
+	};
+	const struct memcached_ascii_dict_reply *reply;
+
+	connection_disconnect(&conn->conn);
+	if (conn->dict->dict.ioloop != NULL)
+		io_loop_stop(conn->dict->dict.ioloop);
+
+	array_foreach(&conn->dict->replies, reply)
+		memcached_ascii_callback(conn->dict, reply, &result);
+	array_clear(&conn->dict->replies);
+	array_clear(&conn->dict->input_states);
+	conn->reply_bytes_left = 0;
+}
+
+static void memcached_ascii_conn_destroy(struct connection *_conn)
+{
+	struct memcached_ascii_connection *conn =
+		(struct memcached_ascii_connection *)_conn;
+
+	memcached_ascii_disconnected(conn, connection_disconnect_reason(_conn));
+}
+
+static bool memcached_ascii_input_value(struct memcached_ascii_connection *conn)
+{
+	const unsigned char *data;
+	size_t size;
+
+	data = i_stream_get_data(conn->conn.input, &size);
+	if (size > conn->reply_bytes_left)
+		size = conn->reply_bytes_left;
+	conn->reply_bytes_left -= size;
+
+	str_append_data(conn->reply_str, data, size);
+	i_stream_skip(conn->conn.input, size);
+	if (conn->reply_bytes_left > 0)
+		return FALSE;
+
+	/* finished. drop the trailing CRLF */
+	str_truncate(conn->reply_str, str_len(conn->reply_str)-2);
+	conn->value_received = TRUE;
+	return TRUE;
+}
+
+static int memcached_ascii_input_reply_read(struct memcached_ascii_dict *dict,
+					    const char **error_r)
+{
+	struct memcached_ascii_connection *conn = &dict->conn;
+	const enum memcached_ascii_input_state *states;
+	const char *line, *p;
+	unsigned int count;
+	long long num;
+
+	if (conn->reply_bytes_left > 0) {
+		/* continue reading bulk reply */
+		if (!memcached_ascii_input_value(conn))
+			return 0;
+		conn->value_waiting_end = TRUE;
+	} else if (conn->value_waiting_end) {
+		conn->value_waiting_end = FALSE;
+	} else {
+		str_truncate(conn->reply_str, 0);
+		conn->value_received = FALSE;
+	}
+
+	line = i_stream_next_line(conn->conn.input);
+	if (line == NULL)
+		return 0;
+
+	states = array_get(&dict->input_states, &count);
+	if (count == 0) {
+		*error_r = t_strdup_printf(
+			"memcached_ascii: Unexpected input (expected nothing): %s", line);
+		return -1;
+	}
+	switch (states[0]) {
+	case MEMCACHED_INPUT_STATE_GET:
+		/* VALUE <key> <flags> <bytes>
+		   END */
+		if (str_begins(line, "VALUE ")) {
+			p = strrchr(line, ' ');
+			if (str_to_uint(p+1, &conn->reply_bytes_left) < 0)
+				break;
+			conn->reply_bytes_left += 2; /* CRLF */
+			return memcached_ascii_input_reply_read(dict, error_r);
+		} else if (strcmp(line, "END") == 0)
+			return 1;
+		break;
+	case MEMCACHED_INPUT_STATE_STORED:
+		if (strcmp(line, "STORED") != 0 &&
+		    strcmp(line, "NOT_STORED") != 0)
+			break;
+		return 1;
+	case MEMCACHED_INPUT_STATE_DELETED:
+		if (strcmp(line, "DELETED") != 0)
+			break;
+		return 1;
+	case MEMCACHED_INPUT_STATE_INCRDECR:
+		if (strcmp(line, "NOT_FOUND") != 0 &&
+		    strcmp(line, "STORED") != 0 &&
+		    strcmp(line, "NOT_STORED") != 0 &&
+		    str_to_llong(line, &num) < 0)
+			break;
+		return 1;
+	}
+	*error_r = t_strdup_printf(
+		"memcached_ascii: Unexpected input (state=%d): %s",
+		states[0], line);
+	return -1;
+}
+
+static int memcached_ascii_input_reply(struct memcached_ascii_dict *dict,
+				       const char **error_r)
+{
+	const struct dict_commit_result result = {
+		DICT_COMMIT_RET_OK, NULL
+	};
+	struct memcached_ascii_dict_reply *replies;
+	unsigned int count;
+	int ret;
+
+	if ((ret = memcached_ascii_input_reply_read(dict, error_r)) <= 0)
+		return ret;
+	/* finished a reply */
+	array_pop_front(&dict->input_states);
+
+	replies = array_get_modifiable(&dict->replies, &count);
+	i_assert(count > 0);
+	i_assert(replies[0].reply_count > 0);
+	if (--replies[0].reply_count == 0) {
+		memcached_ascii_callback(dict, &replies[0], &result);
+		array_pop_front(&dict->replies);
+	}
+	return 1;
+}
+
+static void memcached_ascii_conn_input(struct connection *_conn)
+{
+	struct memcached_ascii_connection *conn =
+		(struct memcached_ascii_connection *)_conn;
+	const char *error;
+	int ret;
+
+	switch (i_stream_read(_conn->input)) {
+	case 0:
+		return;
+	case -1:
+		memcached_ascii_disconnected(conn,
+			i_stream_get_disconnect_reason(_conn->input));
+		return;
+	default:
+		break;
+	}
+
+	while ((ret = memcached_ascii_input_reply(conn->dict, &error)) > 0) ;
+	if (ret < 0)
+		memcached_ascii_disconnected(conn, error);
+	io_loop_stop(conn->dict->dict.ioloop);
+}
+
+static int memcached_ascii_input_wait(struct memcached_ascii_dict *dict,
+				      const char **error_r)
+{
+	i_assert(io_loop_is_empty(dict->dict.ioloop));
+	dict->dict.prev_ioloop = current_ioloop;
+	io_loop_set_current(dict->dict.ioloop);
+	if (dict->to != NULL)
+		dict->to = io_loop_move_timeout(&dict->to);
+	connection_switch_ioloop(&dict->conn.conn);
+	io_loop_run(dict->dict.ioloop);
+
+	io_loop_set_current(dict->dict.prev_ioloop);
+	dict->dict.prev_ioloop = NULL;
+
+	if (dict->to != NULL)
+		dict->to = io_loop_move_timeout(&dict->to);
+	connection_switch_ioloop(&dict->conn.conn);
+	i_assert(io_loop_is_empty(dict->dict.ioloop));
+
+	if (dict->conn.conn.fd_in == -1) {
+		*error_r = "memcached_ascii: Communication failure";
+		return -1;
+	}
+	return 0;
+}
+
+static void memcached_ascii_input_timeout(struct memcached_ascii_dict *dict)
+{
+	const char *reason = t_strdup_printf(
+		"memcached_ascii: Request timed out in %u.%03u secs",
+		dict->timeout_msecs/1000, dict->timeout_msecs%1000);
+	memcached_ascii_disconnected(&dict->conn, reason);
+}
+
+static int memcached_ascii_wait_replies(struct memcached_ascii_dict *dict,
+					const char **error_r)
+{
+	int ret = 0;
+
+	dict->to = timeout_add(dict->timeout_msecs,
+			       memcached_ascii_input_timeout, dict);
+	while (array_count(&dict->input_states) > 0) {
+		i_assert(array_count(&dict->replies) > 0);
+
+		if ((ret = memcached_ascii_input_reply(dict, error_r)) != 0) {
+			if (ret < 0)
+				memcached_ascii_disconnected(&dict->conn, *error_r);
+			break;
+		}
+		ret = memcached_ascii_input_wait(dict, error_r);
+		if (ret != 0)
+			break;
+	}
+
+	timeout_remove(&dict->to);
+	return ret < 0 ? -1 : 0;
+}
+
+static int memcached_ascii_wait(struct memcached_ascii_dict *dict,
+				const char **error_r)
+{
+	int ret;
+
+	i_assert(dict->conn.conn.fd_in != -1);
+
+	if (dict->conn.conn.input == NULL) {
+		/* waiting for connection to finish */
+		dict->to = timeout_add(dict->timeout_msecs,
+				       memcached_ascii_input_timeout, dict);
+		ret = memcached_ascii_input_wait(dict, error_r);
+		timeout_remove(&dict->to);
+		if (ret < 0)
+			return -1;
+	}
+	if (memcached_ascii_wait_replies(dict, error_r) < 0)
+		return -1;
+	i_assert(array_count(&dict->input_states) == 0);
+	i_assert(array_count(&dict->replies) == 0);
+	return 0;
+}
+
+static void
+memcached_ascii_conn_connected(struct connection *_conn, bool success)
+{
+	struct memcached_ascii_connection *conn = (struct memcached_ascii_connection *)_conn;
+
+	if (!success) {
+		e_error(conn->conn.event, "connect() failed: %m");
+	}
+	if (conn->dict->dict.ioloop != NULL)
+		io_loop_stop(conn->dict->dict.ioloop);
+}
+
+static const struct connection_settings memcached_ascii_conn_set = {
+	.input_max_size = SIZE_MAX,
+	.output_max_size = SIZE_MAX,
+	.client = TRUE
+};
+
+static const struct connection_vfuncs memcached_ascii_conn_vfuncs = {
+	.destroy = memcached_ascii_conn_destroy,
+	.input = memcached_ascii_conn_input,
+	.client_connected = memcached_ascii_conn_connected
+};
+
+static const char *memcached_ascii_escape_username(const char *username)
+{
+	const char *p;
+	string_t *str = t_str_new(64);
+
+	for (p = username; *p != '\0'; p++) {
+		switch (*p) {
+		case DICT_USERNAME_SEPARATOR:
+			str_append(str, "\\-");
+			break;
+		case '\\':
+			str_append(str, "\\\\");
+			break;
+		default:
+			str_append_c(str, *p);
+		}
+	}
+	return str_c(str);
+}
+
+static int
+memcached_ascii_dict_init(struct dict *driver, const char *uri,
+			  const struct dict_settings *set,
+			  struct dict **dict_r, const char **error_r)
+{
+	struct memcached_ascii_dict *dict;
+	const char *const *args;
+	struct ioloop *old_ioloop = current_ioloop;
+	int ret = 0;
+
+	if (memcached_ascii_connections == NULL) {
+		memcached_ascii_connections =
+			connection_list_init(&memcached_ascii_conn_set,
+					     &memcached_ascii_conn_vfuncs);
+	}
+
+	dict = i_new(struct memcached_ascii_dict, 1);
+	if (net_addr2ip("127.0.0.1", &dict->ip) < 0)
+		i_unreached();
+	dict->port = MEMCACHED_DEFAULT_PORT;
+	dict->timeout_msecs = MEMCACHED_DEFAULT_LOOKUP_TIMEOUT_MSECS;
+	dict->key_prefix = i_strdup("");
+
+	args = t_strsplit(uri, ":");
+	for (; *args != NULL; args++) {
+		if (str_begins(*args, "host=")) {
+			if (net_addr2ip(*args+5, &dict->ip) < 0) {
+				*error_r = t_strdup_printf("Invalid IP: %s",
+							   *args+5);
+				ret = -1;
+			}
+		} else if (str_begins(*args, "port=")) {
+			if (net_str2port(*args+5, &dict->port) < 0) {
+				*error_r = t_strdup_printf("Invalid port: %s",
+							   *args+5);
+				ret = -1;
+			}
+		} else if (str_begins(*args, "prefix=")) {
+			i_free(dict->key_prefix);
+			dict->key_prefix = i_strdup(*args + 7);
+		} else if (str_begins(*args, "timeout_msecs=")) {
+			if (str_to_uint(*args+14, &dict->timeout_msecs) < 0) {
+				*error_r = t_strdup_printf(
+					"Invalid timeout_msecs: %s", *args+14);
+				ret = -1;
+			}
+		} else {
+			*error_r = t_strdup_printf("Unknown parameter: %s",
+						   *args);
+			ret = -1;
+		}
+	}
+	if (ret < 0) {
+		i_free(dict->key_prefix);
+		i_free(dict);
+		return -1;
+	}
+
+	dict->conn.conn.event_parent = set->event_parent;
+	connection_init_client_ip(memcached_ascii_connections, &dict->conn.conn,
+				  NULL, &dict->ip, dict->port);
+	event_set_append_log_prefix(dict->conn.conn.event, "memcached: ");
+	dict->dict = *driver;
+	dict->conn.reply_str = str_new(default_pool, 256);
+	dict->conn.dict = dict;
+
+	i_array_init(&dict->input_states, 4);
+	i_array_init(&dict->replies, 4);
+
+	dict->dict.ioloop = io_loop_create();
+	io_loop_set_current(old_ioloop);
+	*dict_r = &dict->dict;
+	return 0;
+}
+
+static void memcached_ascii_dict_deinit(struct dict *_dict)
+{
+	struct memcached_ascii_dict *dict =
+		(struct memcached_ascii_dict *)_dict;
+	struct ioloop *old_ioloop = current_ioloop;
+	const char *error;
+
+	if (array_count(&dict->input_states) > 0) {
+		if (memcached_ascii_wait(dict, &error) < 0)
+			i_error("%s", error);
+	}
+	connection_deinit(&dict->conn.conn);
+
+	io_loop_set_current(dict->dict.ioloop);
+	io_loop_destroy(&dict->dict.ioloop);
+	io_loop_set_current(old_ioloop);
+
+	str_free(&dict->conn.reply_str);
+	array_free(&dict->replies);
+	array_free(&dict->input_states);
+	i_free(dict->key_prefix);
+	i_free(dict);
+
+	if (memcached_ascii_connections->connections == NULL)
+		connection_list_deinit(&memcached_ascii_connections);
+}
+
+static int memcached_ascii_connect(struct memcached_ascii_dict *dict,
+				   const char **error_r)
+{
+	if (dict->conn.conn.input != NULL)
+		return 0;
+
+	if (dict->conn.conn.fd_in == -1) {
+		if (connection_client_connect(&dict->conn.conn) < 0) {
+			*error_r = t_strdup_printf(
+				"memcached_ascii: Couldn't connect to %s:%u",
+				net_ip2addr(&dict->ip), dict->port);
+			return -1;
+		}
+	}
+	return memcached_ascii_wait(dict, error_r);
+}
+
+static const char *
+memcached_ascii_dict_get_full_key(struct memcached_ascii_dict *dict,
+				  const char *username, const char *key)
+{
+	if (str_begins(key, DICT_PATH_SHARED))
+		key += strlen(DICT_PATH_SHARED);
+	else if (str_begins(key, DICT_PATH_PRIVATE)) {
+		if (strchr(username, DICT_USERNAME_SEPARATOR) == NULL) {
+			key = t_strdup_printf("%s%c%s", username,
+					      DICT_USERNAME_SEPARATOR,
+					      key + strlen(DICT_PATH_PRIVATE));
+		} else {
+			/* escape the username */
+			key = t_strdup_printf("%s%c%s", memcached_ascii_escape_username(username),
+					      DICT_USERNAME_SEPARATOR,
+					      key + strlen(DICT_PATH_PRIVATE));
+		}
+	} else {
+		i_unreached();
+	}
+	if (*dict->key_prefix != '\0')
+		key = t_strconcat(dict->key_prefix, key, NULL);
+	return key;
+}
+
+static int
+memcached_ascii_dict_lookup(struct dict *_dict,
+			    const struct dict_op_settings *set,
+			    pool_t pool, const char *key, const char **value_r,
+			    const char **error_r)
+{
+	struct memcached_ascii_dict *dict = (struct memcached_ascii_dict *)_dict;
+	struct memcached_ascii_dict_reply *reply;
+	enum memcached_ascii_input_state state = MEMCACHED_INPUT_STATE_GET;
+
+	if (memcached_ascii_connect(dict, error_r) < 0)
+		return -1;
+
+	key = memcached_ascii_dict_get_full_key(dict, set->username, key);
+	o_stream_nsend_str(dict->conn.conn.output,
+			   t_strdup_printf("get %s\r\n", key));
+	array_push_back(&dict->input_states, &state);
+
+	reply = array_append_space(&dict->replies);
+	reply->reply_count = 1;
+
+	if (memcached_ascii_wait(dict, error_r) < 0)
+		return -1;
+
+	*value_r = p_strdup(pool, str_c(dict->conn.reply_str));
+	return dict->conn.value_received ? 1 : 0;
+}
+
+static struct dict_transaction_context *
+memcached_ascii_transaction_init(struct dict *_dict)
+{
+	struct dict_transaction_memory_context *ctx;
+	pool_t pool;
+
+	pool = pool_alloconly_create("file dict transaction", 2048);
+	ctx = p_new(pool, struct dict_transaction_memory_context, 1);
+	dict_transaction_memory_init(ctx, _dict, pool);
+	return &ctx->ctx;
+}
+
+static void
+memcached_send_change(struct dict_memcached_ascii_commit_ctx *ctx,
+		      const struct dict_op_settings_private *set,
+		      const struct dict_transaction_memory_change *change)
+{
+	enum memcached_ascii_input_state state;
+	const char *key, *value;
+
+	key = memcached_ascii_dict_get_full_key(ctx->dict, set->username,
+						change->key);
+
+	str_truncate(ctx->str, 0);
+	switch (change->type) {
+	case DICT_CHANGE_TYPE_SET:
+		state = MEMCACHED_INPUT_STATE_STORED;
+		str_printfa(ctx->str, "set %s 0 0 %zu\r\n%s\r\n",
+			    key, strlen(change->value.str), change->value.str);
+		break;
+	case DICT_CHANGE_TYPE_UNSET:
+		state = MEMCACHED_INPUT_STATE_DELETED;
+		str_printfa(ctx->str, "delete %s\r\n", key);
+		break;
+	case DICT_CHANGE_TYPE_INC:
+		state = MEMCACHED_INPUT_STATE_INCRDECR;
+		if (change->value.diff > 0) {
+			str_printfa(ctx->str, "incr %s %lld\r\n",
+				    key, change->value.diff);
+			array_push_back(&ctx->dict->input_states, &state);
+			/* same kludge as with append */
+			value = t_strdup_printf("%lld", change->value.diff);
+			str_printfa(ctx->str, "add %s 0 0 %u\r\n%s\r\n",
+				    key, (unsigned int)strlen(value), value);
+		} else {
+			str_printfa(ctx->str, "decr %s %lld\r\n",
+				    key, -change->value.diff);
+		}
+		break;
+	}
+	array_push_back(&ctx->dict->input_states, &state);
+	o_stream_nsend(ctx->dict->conn.conn.output,
+		       str_data(ctx->str), str_len(ctx->str));
+}
+
+static int
+memcached_ascii_transaction_send(struct dict_memcached_ascii_commit_ctx *ctx,
+				 const struct dict_op_settings_private *set,
+				 const char **error_r)
+{
+	struct memcached_ascii_dict *dict = ctx->dict;
+	struct memcached_ascii_dict_reply *reply;
+	const struct dict_transaction_memory_change *changes;
+	unsigned int i, count, old_state_count;
+
+	if (memcached_ascii_connect(dict, error_r) < 0)
+		return -1;
+
+	old_state_count = array_count(&dict->input_states);
+	changes = array_get(&ctx->memctx->changes, &count);
+	i_assert(count > 0);
+
+	o_stream_cork(dict->conn.conn.output);
+	for (i = 0; i < count; i++) T_BEGIN {
+		memcached_send_change(ctx, set, &changes[i]);
+	} T_END;
+	o_stream_uncork(dict->conn.conn.output);
+
+	reply = array_append_space(&dict->replies);
+	reply->callback = ctx->callback;
+	reply->context = ctx->context;
+	reply->reply_count = array_count(&dict->input_states) - old_state_count;
+	return 0;
+}
+
+static void
+memcached_ascii_transaction_commit(struct dict_transaction_context *_ctx,
+				   bool async,
+				   dict_transaction_commit_callback_t *callback,
+				   void *context)
+{
+	struct dict_transaction_memory_context *ctx =
+		(struct dict_transaction_memory_context *)_ctx;
+	struct memcached_ascii_dict *dict =
+		(struct memcached_ascii_dict *)_ctx->dict;
+	struct dict_memcached_ascii_commit_ctx commit_ctx;
+	struct dict_commit_result result = { DICT_COMMIT_RET_OK, NULL };
+	const struct dict_op_settings_private *set = &_ctx->set;
+
+	if (_ctx->changed) {
+		i_zero(&commit_ctx);
+		commit_ctx.dict = dict;
+		commit_ctx.memctx = ctx;
+		commit_ctx.callback = callback;
+		commit_ctx.context = context;
+		commit_ctx.str = str_new(default_pool, 128);
+
+		result.ret = memcached_ascii_transaction_send(&commit_ctx, set, &result.error);
+		str_free(&commit_ctx.str);
+
+		if (async && result.ret == 0) {
+			pool_unref(&ctx->pool);
+			return;
+		}
+
+		if (result.ret == 0) {
+			if (memcached_ascii_wait(dict, &result.error) < 0)
+				result.ret = -1;
+		}
+	}
+	callback(&result, context);
+	pool_unref(&ctx->pool);
+}
+
+struct dict dict_driver_memcached_ascii = {
+	.name = "memcached_ascii",
+	{
+		.init = memcached_ascii_dict_init,
+		.deinit = memcached_ascii_dict_deinit,
+		.lookup = memcached_ascii_dict_lookup,
+		.transaction_init = memcached_ascii_transaction_init,
+		.transaction_commit = memcached_ascii_transaction_commit,
+		.transaction_rollback = dict_transaction_memory_rollback,
+		.set = dict_transaction_memory_set,
+		.unset = dict_transaction_memory_unset,
+		.atomic_inc = dict_transaction_memory_atomic_inc,
+	}
+};
diff --git a/src/lib-dict/dict-memcached.c b/src/lib-dict/dict-memcached.c
new file mode 100644
index 0000000..c5b7ce6
--- /dev/null
+++ b/src/lib-dict/dict-memcached.c
@@ -0,0 +1,373 @@
+/* Copyright (c) 2013-2018 Dovecot authors, see the included COPYING memcached */
+
+#include "lib.h"
+#include "array.h"
+#include "str.h"
+#include "istream.h"
+#include "ostream.h"
+#include "connection.h"
+#include "dict-private.h"
+
+#define MEMCACHED_DEFAULT_PORT 11211
+#define MEMCACHED_DEFAULT_LOOKUP_TIMEOUT_MSECS (1000*30)
+
+/* we need only very limited memcached functionality, so just define the binary
+   protocol ourself instead requiring protocol_binary.h */
+#define MEMCACHED_REQUEST_HDR_MAGIC 0x80
+#define MEMCACHED_REPLY_HDR_MAGIC 0x81
+
+#define MEMCACHED_REQUEST_HDR_LENGTH 24
+#define MEMCACHED_REPLY_HDR_LENGTH 24
+
+#define MEMCACHED_CMD_GET 0x00
+
+#define MEMCACHED_DATA_TYPE_RAW 0x00
+
+enum memcached_response {
+	MEMCACHED_RESPONSE_OK		= 0x0000,
+	MEMCACHED_RESPONSE_NOTFOUND	= 0x0001,
+	MEMCACHED_RESPONSE_INTERNALERROR= 0x0084,
+	MEMCACHED_RESPONSE_BUSY		= 0x0085,
+	MEMCACHED_RESPONSE_TEMPFAILURE	= 0x0086,
+};
+
+struct memcached_connection {
+	struct connection conn;
+	struct memcached_dict *dict;
+
+	buffer_t *cmd;
+	struct {
+		const unsigned char *value;
+		size_t value_len;
+		uint16_t status; /* enum memcached_response */
+		bool reply_received;
+	} reply;
+};
+
+struct memcached_dict {
+	struct dict dict;
+	struct ip_addr ip;
+	char *key_prefix;
+	in_port_t port;
+	unsigned int timeout_msecs;
+
+	struct memcached_connection conn;
+
+	bool connected;
+};
+
+static struct connection_list *memcached_connections;
+
+static void memcached_conn_destroy(struct connection *_conn)
+{
+	struct memcached_connection *conn = (struct memcached_connection *)_conn;
+
+	conn->dict->connected = FALSE;
+	connection_disconnect(_conn);
+
+	if (conn->dict->dict.ioloop != NULL)
+		io_loop_stop(conn->dict->dict.ioloop);
+}
+
+static int memcached_input_get(struct memcached_connection *conn)
+{
+	const unsigned char *data;
+	size_t size;
+	uint32_t body_len, value_pos;
+	uint16_t key_len, key_pos, status;
+	uint8_t extras_len, data_type;
+
+	data = i_stream_get_data(conn->conn.input, &size);
+	if (size < MEMCACHED_REPLY_HDR_LENGTH)
+		return 0;
+
+	if (data[0] != MEMCACHED_REPLY_HDR_MAGIC) {
+		e_error(conn->conn.event, "Invalid reply magic: %u != %u",
+			data[0], MEMCACHED_REPLY_HDR_MAGIC);
+		return -1;
+	}
+	memcpy(&body_len, data+8, 4); body_len = ntohl(body_len);
+	body_len += MEMCACHED_REPLY_HDR_LENGTH;
+	if (size < body_len) {
+		/* we haven't read the whole response yet */
+		return 0;
+	}
+
+	memcpy(&key_len, data+2, 2); key_len = ntohs(key_len);
+	extras_len = data[4];
+	data_type = data[5];
+	memcpy(&status, data+6, 2); status = ntohs(status);
+	if (data_type != MEMCACHED_DATA_TYPE_RAW) {
+		e_error(conn->conn.event, "Unsupported data type: %u != %u",
+			data[0], MEMCACHED_DATA_TYPE_RAW);
+		return -1;
+	}
+
+	key_pos = MEMCACHED_REPLY_HDR_LENGTH + extras_len;
+	value_pos = key_pos + key_len;
+	if (value_pos > body_len) {
+		e_error(conn->conn.event, "Invalid key/extras lengths");
+		return -1;
+	}
+	conn->reply.value = data + value_pos;
+	conn->reply.value_len = body_len - value_pos;
+	conn->reply.status = status;
+
+	i_stream_skip(conn->conn.input, body_len);
+	conn->reply.reply_received = TRUE;
+
+	if (conn->dict->dict.ioloop != NULL)
+		io_loop_stop(conn->dict->dict.ioloop);
+	return 1;
+}
+
+static void memcached_conn_input(struct connection *_conn)
+{
+	struct memcached_connection *conn = (struct memcached_connection *)_conn;
+
+	switch (i_stream_read(_conn->input)) {
+	case 0:
+		return;
+	case -1:
+		memcached_conn_destroy(_conn);
+		return;
+	default:
+		break;
+	}
+
+	if (memcached_input_get(conn) < 0)
+		memcached_conn_destroy(_conn);
+}
+
+static void memcached_conn_connected(struct connection *_conn, bool success)
+{
+	struct memcached_connection *conn =
+		(struct memcached_connection *)_conn;
+
+	if (!success) {
+		e_error(conn->conn.event, "connect() failed: %m");
+	} else {
+		conn->dict->connected = TRUE;
+	}
+	if (conn->dict->dict.ioloop != NULL)
+		io_loop_stop(conn->dict->dict.ioloop);
+}
+
+static const struct connection_settings memcached_conn_set = {
+	.input_max_size = SIZE_MAX,
+	.output_max_size = SIZE_MAX,
+	.client = TRUE
+};
+
+static const struct connection_vfuncs memcached_conn_vfuncs = {
+	.destroy = memcached_conn_destroy,
+	.input = memcached_conn_input,
+	.client_connected = memcached_conn_connected
+};
+
+static int
+memcached_dict_init(struct dict *driver, const char *uri,
+		    const struct dict_settings *set,
+		    struct dict **dict_r, const char **error_r)
+{
+	struct memcached_dict *dict;
+	const char *const *args;
+	int ret = 0;
+
+	if (memcached_connections == NULL) {
+		memcached_connections =
+			connection_list_init(&memcached_conn_set,
+					     &memcached_conn_vfuncs);
+	}
+
+	dict = i_new(struct memcached_dict, 1);
+	if (net_addr2ip("127.0.0.1", &dict->ip) < 0)
+		i_unreached();
+	dict->port = MEMCACHED_DEFAULT_PORT;
+	dict->timeout_msecs = MEMCACHED_DEFAULT_LOOKUP_TIMEOUT_MSECS;
+	dict->key_prefix = i_strdup("");
+
+	args = t_strsplit(uri, ":");
+	for (; *args != NULL; args++) {
+		if (str_begins(*args, "host=")) {
+			if (net_addr2ip(*args+5, &dict->ip) < 0) {
+				*error_r = t_strdup_printf("Invalid IP: %s",
+							   *args+5);
+				ret = -1;
+			}
+		} else if (str_begins(*args, "port=")) {
+			if (net_str2port(*args+5, &dict->port) < 0) {
+				*error_r = t_strdup_printf("Invalid port: %s",
+							   *args+5);
+				ret = -1;
+			}
+		} else if (str_begins(*args, "prefix=")) {
+			i_free(dict->key_prefix);
+			dict->key_prefix = i_strdup(*args + 7);
+		} else if (str_begins(*args, "timeout_msecs=")) {
+			if (str_to_uint(*args+14, &dict->timeout_msecs) < 0) {
+				*error_r = t_strdup_printf(
+					"Invalid timeout_msecs: %s", *args+14);
+				ret = -1;
+			}
+		} else {
+			*error_r = t_strdup_printf("Unknown parameter: %s",
+						   *args);
+			ret = -1;
+		}
+	}
+	if (ret < 0) {
+		i_free(dict->key_prefix);
+		i_free(dict);
+		return -1;
+	}
+
+	dict->conn.conn.event_parent = set->event_parent;
+
+	connection_init_client_ip(memcached_connections, &dict->conn.conn,
+				  NULL, &dict->ip, dict->port);
+	event_set_append_log_prefix(dict->conn.conn.event, "memcached: ");
+	dict->dict = *driver;
+	dict->conn.cmd = buffer_create_dynamic(default_pool, 256);
+	dict->conn.dict = dict;
+	*dict_r = &dict->dict;
+	return 0;
+}
+
+static void memcached_dict_deinit(struct dict *_dict)
+{
+	struct memcached_dict *dict = (struct memcached_dict *)_dict;
+
+	connection_deinit(&dict->conn.conn);
+	buffer_free(&dict->conn.cmd);
+	i_free(dict->key_prefix);
+	i_free(dict);
+
+	if (memcached_connections->connections == NULL)
+		connection_list_deinit(&memcached_connections);
+}
+
+static void memcached_dict_lookup_timeout(struct memcached_dict *dict)
+{
+	e_error(dict->dict.event, "Lookup timed out in %u.%03u secs",
+		dict->timeout_msecs/1000, dict->timeout_msecs%1000);
+	io_loop_stop(dict->dict.ioloop);
+}
+
+static void memcached_add_header(buffer_t *buf, unsigned int key_len)
+{
+	uint32_t body_len = htonl(key_len);
+
+	i_assert(key_len <= 0xffff);
+
+	buffer_append_c(buf, MEMCACHED_REQUEST_HDR_MAGIC);
+	buffer_append_c(buf, MEMCACHED_CMD_GET);
+	buffer_append_c(buf, (key_len >> 8) & 0xff);
+	buffer_append_c(buf, key_len & 0xff);
+	buffer_append_c(buf, 0); /* extras length */
+	buffer_append_c(buf, MEMCACHED_DATA_TYPE_RAW);
+	buffer_append_zero(buf, 2); /* vbucket id - we probably don't care? */
+	buffer_append(buf, &body_len, sizeof(body_len));
+	buffer_append_zero(buf, 4+8); /* opaque + cas */
+	i_assert(buf->used == MEMCACHED_REQUEST_HDR_LENGTH);
+}
+
+static int
+memcached_dict_lookup(struct dict *_dict, const struct dict_op_settings *set ATTR_UNUSED,
+		      pool_t pool, const char *key, const char **value_r,
+		      const char **error_r)
+{
+	struct memcached_dict *dict = (struct memcached_dict *)_dict;
+	struct ioloop *prev_ioloop = current_ioloop;
+	struct timeout *to;
+	size_t key_len;
+
+	if (str_begins(key, DICT_PATH_SHARED))
+		key += strlen(DICT_PATH_SHARED);
+	else {
+		*error_r = t_strdup_printf("memcached: Only shared keys supported currently");
+		return -1;
+	}
+	if (*dict->key_prefix != '\0')
+		key = t_strconcat(dict->key_prefix, key, NULL);
+	key_len = strlen(key);
+	if (key_len > 0xffff) {
+		*error_r = t_strdup_printf(
+			"memcached: Key is too long (%zu bytes): %s", key_len, key);
+		return -1;
+	}
+
+	i_assert(dict->dict.ioloop == NULL);
+
+	dict->dict.ioloop = io_loop_create();
+	connection_switch_ioloop(&dict->conn.conn);
+
+	if (dict->conn.conn.fd_in == -1 &&
+	    connection_client_connect(&dict->conn.conn) < 0) {
+		e_error(dict->conn.conn.event, "Couldn't connect");
+	} else {
+		to = timeout_add(dict->timeout_msecs,
+				 memcached_dict_lookup_timeout, dict);
+		if (!dict->connected) {
+			/* wait for connection */
+			io_loop_run(dict->dict.ioloop);
+		}
+
+		if (dict->connected) {
+			buffer_set_used_size(dict->conn.cmd, 0);
+			memcached_add_header(dict->conn.cmd, key_len);
+			buffer_append(dict->conn.cmd, key, key_len);
+
+			o_stream_nsend(dict->conn.conn.output,
+				       dict->conn.cmd->data,
+				       dict->conn.cmd->used);
+
+			i_zero(&dict->conn.reply);
+			io_loop_run(dict->dict.ioloop);
+		}
+		timeout_remove(&to);
+	}
+
+	io_loop_set_current(prev_ioloop);
+	connection_switch_ioloop(&dict->conn.conn);
+	io_loop_set_current(dict->dict.ioloop);
+	io_loop_destroy(&dict->dict.ioloop);
+
+	if (!dict->conn.reply.reply_received) {
+		/* we failed in some way. make sure we disconnect since the
+		   connection state isn't known anymore */
+		memcached_conn_destroy(&dict->conn.conn);
+		*error_r = "Communication failure";
+		return -1;
+	}
+	switch (dict->conn.reply.status) {
+	case MEMCACHED_RESPONSE_OK:
+		*value_r = p_strndup(pool, dict->conn.reply.value,
+				     dict->conn.reply.value_len);
+		return 1;
+	case MEMCACHED_RESPONSE_NOTFOUND:
+		return 0;
+	case MEMCACHED_RESPONSE_INTERNALERROR:
+		*error_r = "Lookup failed: Internal error";
+		return -1;
+	case MEMCACHED_RESPONSE_BUSY:
+		*error_r = "Lookup failed: Busy";
+		return -1;
+	case MEMCACHED_RESPONSE_TEMPFAILURE:
+		*error_r = "Lookup failed: Temporary failure";
+		return -1;
+	}
+
+	*error_r = t_strdup_printf("Lookup failed: Error code=%u",
+				   dict->conn.reply.status);
+	return -1;
+}
+
+struct dict dict_driver_memcached = {
+	.name = "memcached",
+	{
+		.init = memcached_dict_init,
+		.deinit = memcached_dict_deinit,
+		.lookup = memcached_dict_lookup,
+	}
+};
diff --git a/src/lib-dict/dict-private.h b/src/lib-dict/dict-private.h
new file mode 100644
index 0000000..7354175
--- /dev/null
+++ b/src/lib-dict/dict-private.h
@@ -0,0 +1,127 @@
+#ifndef DICT_PRIVATE_H
+#define DICT_PRIVATE_H
+
+#include <time.h>
+#include "dict.h"
+
+struct ioloop;
+
+struct dict_vfuncs {
+	int (*init)(struct dict *dict_driver, const char *uri,
+		    const struct dict_settings *set,
+		    struct dict **dict_r, const char **error_r);
+	void (*deinit)(struct dict *dict);
+	void (*wait)(struct dict *dict);
+
+	int (*lookup)(struct dict *dict, const struct dict_op_settings *set,
+		      pool_t pool, const char *key, const char **value_r,
+		      const char **error_r);
+
+	struct dict_iterate_context *
+		(*iterate_init)(struct dict *dict,
+				const struct dict_op_settings *set,
+				const char *path,
+				enum dict_iterate_flags flags);
+	bool (*iterate)(struct dict_iterate_context *ctx,
+			const char **key_r, const char *const **values_r);
+	int (*iterate_deinit)(struct dict_iterate_context *ctx,
+			      const char **error_r);
+
+	struct dict_transaction_context *(*transaction_init)(struct dict *dict);
+	/* call the callback before returning if non-async commits */
+	void (*transaction_commit)(struct dict_transaction_context *ctx,
+				   bool async,
+				   dict_transaction_commit_callback_t *callback,
+				   void *context);
+	void (*transaction_rollback)(struct dict_transaction_context *ctx);
+
+	void (*set)(struct dict_transaction_context *ctx,
+		    const char *key, const char *value);
+	void (*unset)(struct dict_transaction_context *ctx,
+		      const char *key);
+	void (*atomic_inc)(struct dict_transaction_context *ctx,
+			   const char *key, long long diff);
+
+	void (*lookup_async)(struct dict *dict, const struct dict_op_settings *set,
+			     const char *key, dict_lookup_callback_t *callback,
+			     void *context);
+	bool (*switch_ioloop)(struct dict *dict);
+	void (*set_timestamp)(struct dict_transaction_context *ctx,
+			      const struct timespec *ts);
+	void (*set_hide_log_values)(struct dict_transaction_context *ctx,
+			            bool hide_log_values);
+};
+
+struct dict_commit_callback_ctx;
+
+struct dict_op_settings_private {
+	char *username;
+	char *home_dir;
+
+	bool hide_log_values;
+};
+
+struct dict {
+	const char *name;
+
+	struct dict_vfuncs v;
+	unsigned int iter_count;
+	unsigned int transaction_count;
+	struct dict_transaction_context *transactions;
+	int refcount;
+	struct event *event;
+	struct ioloop *ioloop, *prev_ioloop;
+	struct dict_commit_callback_ctx *commits;
+};
+
+struct dict_iterate_context {
+	struct dict *dict;
+	struct event *event;
+	struct dict_op_settings_private set;
+	enum dict_iterate_flags flags;
+
+	dict_iterate_callback_t *async_callback;
+	void *async_context;
+
+	uint64_t row_count, max_rows;
+
+	bool has_more:1;
+};
+
+struct dict_transaction_context {
+	struct dict *dict;
+	struct dict_op_settings_private set;
+	struct dict_transaction_context *prev, *next;
+
+	struct event *event;
+	struct timespec timestamp;
+
+	bool changed:1;
+	bool no_slowness_warning:1;
+};
+
+void dict_transaction_commit_async_noop_callback(
+	const struct dict_commit_result *result, void *context);
+
+extern struct dict dict_driver_client;
+extern struct dict dict_driver_file;
+extern struct dict dict_driver_fs;
+extern struct dict dict_driver_memcached;
+extern struct dict dict_driver_memcached_ascii;
+extern struct dict dict_driver_redis;
+extern struct dict dict_driver_cdb;
+extern struct dict dict_driver_fail;
+
+extern struct dict_iterate_context dict_iter_unsupported;
+extern struct dict_transaction_context dict_transaction_unsupported;
+
+void dict_pre_api_callback(struct dict *dict);
+void dict_post_api_callback(struct dict *dict);
+
+/* Duplicate an object of type dict_op_settings. Used for initializing/freeing
+   iterator and transaction contexts. */
+void dict_op_settings_dup(const struct dict_op_settings *source,
+			  struct dict_op_settings_private *dest_r);
+void dict_op_settings_private_free(struct dict_op_settings_private *set);
+
+#endif
diff --git a/src/lib-dict/dict-redis.c b/src/lib-dict/dict-redis.c
new file mode 100644
index 0000000..51eb322
--- /dev/null
+++ b/src/lib-dict/dict-redis.c
@@ -0,0 +1,831 @@
+/* Copyright (c) 2008-2018 Dovecot authors, see the included COPYING redis */
+
+#include "lib.h"
+#include "array.h"
+#include "str.h"
+#include "istream.h"
+#include "ostream.h"
+#include "connection.h"
+#include "dict-private.h"
+
+#define REDIS_DEFAULT_PORT 6379
+#define REDIS_DEFAULT_LOOKUP_TIMEOUT_MSECS (1000*30)
+#define DICT_USERNAME_SEPARATOR '/'
+
+enum redis_input_state {
+	/* expecting +OK reply for AUTH */
+	REDIS_INPUT_STATE_AUTH,
+	/* expecting +OK reply for SELECT */
+	REDIS_INPUT_STATE_SELECT,
+	/* expecting $-1 / $<size> followed by GET reply */
+	REDIS_INPUT_STATE_GET,
+	/* expecting +QUEUED */
+	REDIS_INPUT_STATE_MULTI,
+	/* expecting +OK reply for DISCARD */
+	REDIS_INPUT_STATE_DISCARD,
+	/* expecting *<nreplies> */
+	REDIS_INPUT_STATE_EXEC,
+	/* expecting EXEC reply */
+	REDIS_INPUT_STATE_EXEC_REPLY
+};
+
+struct redis_connection {
+	struct connection conn;
+	struct redis_dict *dict;
+
+	string_t *last_reply;
+	unsigned int bytes_left;
+	bool value_not_found;
+	bool value_received;
+};
+
+struct redis_dict_reply {
+	unsigned int reply_count;
+	dict_transaction_commit_callback_t *callback;
+	void *context;
+};
+
+struct redis_dict {
+	struct dict dict;
+	char *password, *key_prefix, *expire_value;
+	unsigned int timeout_msecs, db_id;
+
+	struct redis_connection conn;
+
+	ARRAY(enum redis_input_state) input_states;
+	ARRAY(struct redis_dict_reply) replies;
+
+	bool connected;
+	bool transaction_open;
+	bool db_id_set;
+};
+
+struct redis_dict_transaction_context {
+	struct dict_transaction_context ctx;
+	unsigned int cmd_count;
+	char *error;
+};
+
+static struct connection_list *redis_connections;
+
+static void
+redis_input_state_add(struct redis_dict *dict, enum redis_input_state state)
+{
+	array_push_back(&dict->input_states, &state);
+}
+
+static void redis_input_state_remove(struct redis_dict *dict)
+{
+	array_pop_front(&dict->input_states);
+}
+
+static void redis_reply_callback(struct redis_connection *conn,
+				 const struct redis_dict_reply *reply,
+				 const struct dict_commit_result *result)
+{
+	i_assert(reply->callback != NULL);
+	if (conn->dict->dict.prev_ioloop != NULL)
+		io_loop_set_current(conn->dict->dict.prev_ioloop);
+	reply->callback(result, reply->context);
+	if (conn->dict->dict.prev_ioloop != NULL)
+		io_loop_set_current(conn->dict->dict.ioloop);
+}
+
+static void
+redis_disconnected(struct redis_connection *conn, const char *reason)
+{
+	const struct dict_commit_result result = {
+		DICT_COMMIT_RET_FAILED, reason
+	};
+	const struct redis_dict_reply *reply;
+
+	conn->dict->db_id_set = FALSE;
+	conn->dict->connected = FALSE;
+	connection_disconnect(&conn->conn);
+
+	array_foreach(&conn->dict->replies, reply)
+		redis_reply_callback(conn, reply, &result);
+	array_clear(&conn->dict->replies);
+	array_clear(&conn->dict->input_states);
+
+	if (conn->dict->dict.ioloop != NULL)
+		io_loop_stop(conn->dict->dict.ioloop);
+}
+
+static void redis_conn_destroy(struct connection *_conn)
+{
+	struct redis_connection *conn = (struct redis_connection *)_conn;
+
+	redis_disconnected(conn, connection_disconnect_reason(_conn));
+}
+
+static void redis_dict_wait_timeout(struct redis_dict *dict)
+{
+	const char *reason = t_strdup_printf(
+		"redis: Commit timed out in %u.%03u secs",
+		dict->timeout_msecs/1000, dict->timeout_msecs%1000);
+	redis_disconnected(&dict->conn, reason);
+}
+
+static void redis_wait(struct redis_dict *dict)
+{
+	struct timeout *to;
+
+	i_assert(dict->dict.ioloop == NULL);
+
+	dict->dict.prev_ioloop = current_ioloop;
+	dict->dict.ioloop = io_loop_create();
+	to = timeout_add(dict->timeout_msecs, redis_dict_wait_timeout, dict);
+	connection_switch_ioloop(&dict->conn.conn);
+
+	do {
+		io_loop_run(dict->dict.ioloop);
+	} while (array_count(&dict->input_states) > 0);
+
+	timeout_remove(&to);
+	io_loop_set_current(dict->dict.prev_ioloop);
+	connection_switch_ioloop(&dict->conn.conn);
+	io_loop_set_current(dict->dict.ioloop);
+	io_loop_destroy(&dict->dict.ioloop);
+	dict->dict.prev_ioloop = NULL;
+}
+
+static int redis_input_get(struct redis_connection *conn, const char **error_r)
+{
+	const unsigned char *data;
+	size_t size;
+	const char *line;
+
+	if (conn->bytes_left == 0) {
+		/* read the size first */
+		line = i_stream_next_line(conn->conn.input);
+		if (line == NULL)
+			return 0;
+		if (strcmp(line, "$-1") == 0) {
+			conn->value_received = TRUE;
+			conn->value_not_found = TRUE;
+			if (conn->dict->dict.ioloop != NULL)
+				io_loop_stop(conn->dict->dict.ioloop);
+			redis_input_state_remove(conn->dict);
+			return 1;
+		}
+		if (line[0] != '$' || str_to_uint(line+1, &conn->bytes_left) < 0) {
+			*error_r = t_strdup_printf(
+				"redis: Unexpected input (wanted $size): %s", line);
+			return -1;
+		}
+		conn->bytes_left += 2; /* include trailing CRLF */
+	}
+
+	data = i_stream_get_data(conn->conn.input, &size);
+	if (size > conn->bytes_left)
+		size = conn->bytes_left;
+	str_append_data(conn->last_reply, data, size);
+
+	conn->bytes_left -= size;
+	i_stream_skip(conn->conn.input, size);
+
+	if (conn->bytes_left > 0)
+		return 0;
+
+	/* reply fully read - drop trailing CRLF */
+	conn->value_received = TRUE;
+	str_truncate(conn->last_reply, str_len(conn->last_reply)-2);
+
+	if (conn->dict->dict.ioloop != NULL)
+		io_loop_stop(conn->dict->dict.ioloop);
+	redis_input_state_remove(conn->dict);
+	return 1;
+}
+
+static int
+redis_conn_input_more(struct redis_connection *conn, const char **error_r)
+{
+	struct redis_dict *dict = conn->dict;
+	struct redis_dict_reply *reply;
+	const enum redis_input_state *states;
+	enum redis_input_state state;
+	unsigned int count, num_replies;
+	const char *line;
+
+	states = array_get(&dict->input_states, &count);
+	if (count == 0) {
+		line = i_stream_next_line(conn->conn.input);
+		if (line == NULL)
+			return 0;
+		*error_r = t_strdup_printf(
+			"redis: Unexpected input (expected nothing): %s", line);
+		return -1;
+	}
+	state = states[0];
+	if (state == REDIS_INPUT_STATE_GET)
+		return redis_input_get(conn, error_r);
+
+	line = i_stream_next_line(conn->conn.input);
+	if (line == NULL)
+		return 0;
+
+	redis_input_state_remove(dict);
+	switch (state) {
+	case REDIS_INPUT_STATE_GET:
+		i_unreached();
+	case REDIS_INPUT_STATE_AUTH:
+	case REDIS_INPUT_STATE_SELECT:
+	case REDIS_INPUT_STATE_MULTI:
+	case REDIS_INPUT_STATE_DISCARD:
+		if (line[0] != '+')
+			break;
+		return 1;
+	case REDIS_INPUT_STATE_EXEC:
+		if (line[0] != '*' || str_to_uint(line+1, &num_replies) < 0)
+			break;
+
+		reply = array_front_modifiable(&dict->replies);
+		i_assert(reply->reply_count > 0);
+		if (reply->reply_count != num_replies) {
+			*error_r = t_strdup_printf(
+				"redis: EXEC expected %u replies, not %u",
+				reply->reply_count, num_replies);
+			return -1;
+		}
+		return 1;
+	case REDIS_INPUT_STATE_EXEC_REPLY:
+		if (*line != '+' && *line != ':')
+			break;
+		/* success, just ignore the actual reply */
+		reply = array_front_modifiable(&dict->replies);
+		i_assert(reply->reply_count > 0);
+		if (--reply->reply_count == 0) {
+			const struct dict_commit_result result = {
+				DICT_COMMIT_RET_OK, NULL
+			};
+			redis_reply_callback(conn, reply, &result);
+			array_pop_front(&dict->replies);
+			/* if we're running in a dict-ioloop, we're handling a
+			   synchronous commit and need to stop now */
+			if (array_count(&dict->replies) == 0 &&
+			    conn->dict->dict.ioloop != NULL)
+				io_loop_stop(conn->dict->dict.ioloop);
+		}
+		return 1;
+	}
+	str_truncate(dict->conn.last_reply, 0);
+	str_append(dict->conn.last_reply, line);
+	*error_r = t_strdup_printf("redis: Unexpected input (state=%d): %s", state, line);
+	return -1;
+}
+
+static void redis_conn_input(struct connection *_conn)
+{
+	struct redis_connection *conn = (struct redis_connection *)_conn;
+	const char *error = NULL;
+	int ret;
+
+	switch (i_stream_read(_conn->input)) {
+	case 0:
+		return;
+	case -1:
+		redis_disconnected(conn, i_stream_get_error(_conn->input));
+		return;
+	default:
+		break;
+	}
+
+	while ((ret = redis_conn_input_more(conn, &error)) > 0) ;
+	if (ret < 0) {
+		i_assert(error != NULL);
+		redis_disconnected(conn, error);
+	}
+}
+
+static void redis_conn_connected(struct connection *_conn, bool success)
+{
+	struct redis_connection *conn = (struct redis_connection *)_conn;
+
+	if (!success) {
+		e_error(conn->conn.event, "connect() failed: %m");
+	} else {
+		conn->dict->connected = TRUE;
+	}
+	if (conn->dict->dict.ioloop != NULL)
+		io_loop_stop(conn->dict->dict.ioloop);
+}
+
+static const struct connection_settings redis_conn_set = {
+	.input_max_size = SIZE_MAX,
+	.output_max_size = SIZE_MAX,
+	.client = TRUE
+};
+
+static const struct connection_vfuncs redis_conn_vfuncs = {
+	.destroy = redis_conn_destroy,
+	.input = redis_conn_input,
+	.client_connected = redis_conn_connected
+};
+
+static const char *redis_escape_username(const char *username)
+{
+	const char *p;
+	string_t *str = t_str_new(64);
+
+	for (p = username; *p != '\0'; p++) {
+		switch (*p) {
+		case DICT_USERNAME_SEPARATOR:
+			str_append(str, "\\-");
+			break;
+		case '\\':
+			str_append(str, "\\\\");
+			break;
+		default:
+			str_append_c(str, *p);
+		}
+	}
+	return str_c(str);
+}
+
+static int
+redis_dict_init(struct dict *driver, const char *uri,
+		const struct dict_settings *set,
+		struct dict **dict_r, const char **error_r)
+{
+	struct redis_dict *dict;
+	struct ip_addr ip;
+	unsigned int secs;
+	in_port_t port = REDIS_DEFAULT_PORT;
+	const char *const *args, *unix_path = NULL;
+	int ret = 0;
+
+	if (redis_connections == NULL) {
+		redis_connections =
+			connection_list_init(&redis_conn_set,
+					     &redis_conn_vfuncs);
+	}
+
+	dict = i_new(struct redis_dict, 1);
+	if (net_addr2ip("127.0.0.1", &ip) < 0)
+		i_unreached();
+	dict->timeout_msecs = REDIS_DEFAULT_LOOKUP_TIMEOUT_MSECS;
+	dict->key_prefix = i_strdup("");
+	dict->password   = i_strdup("");
+
+	args = t_strsplit(uri, ":");
+	for (; *args != NULL; args++) {
+		if (str_begins(*args, "path=")) {
+			unix_path = *args + 5;
+		} else if (str_begins(*args, "host=")) {
+			if (net_addr2ip(*args+5, &ip) < 0) {
+				*error_r = t_strdup_printf("Invalid IP: %s",
+							   *args+5);
+				ret = -1;
+			}
+		} else if (str_begins(*args, "port=")) {
+			if (net_str2port(*args+5, &port) < 0) {
+				*error_r = t_strdup_printf("Invalid port: %s",
+							   *args+5);
+				ret = -1;
+			}
+		} else if (str_begins(*args, "prefix=")) {
+			i_free(dict->key_prefix);
+			dict->key_prefix = i_strdup(*args + 7);
+		} else if (str_begins(*args, "db=")) {
+			if (str_to_uint(*args+3, &dict->db_id) < 0) {
+				*error_r = t_strdup_printf(
+					"Invalid db number: %s", *args+3);
+				ret = -1;
+			}
+		} else if (str_begins(*args, "expire_secs=")) {
+			const char *value = *args + 12;
+
+			if (str_to_uint(value, &secs) < 0 || secs == 0) {
+				*error_r = t_strdup_printf(
+					"Invalid expire_secs: %s", value);
+				ret = -1;
+			}
+			i_free(dict->expire_value);
+			dict->expire_value = i_strdup(value);
+		} else if (str_begins(*args, "timeout_msecs=")) {
+			if (str_to_uint(*args+14, &dict->timeout_msecs) < 0) {
+				*error_r = t_strdup_printf(
+					"Invalid timeout_msecs: %s", *args+14);
+				ret = -1;
+			}
+		} else if (str_begins(*args, "password=")) {
+			i_free(dict->password);
+			dict->password = i_strdup(*args + 9);
+		} else {
+			*error_r = t_strdup_printf("Unknown parameter: %s",
+						   *args);
+			ret = -1;
+		}
+	}
+	if (ret < 0) {
+		i_free(dict->password);
+		i_free(dict->key_prefix);
+		i_free(dict);
+		return -1;
+	}
+
+	dict->conn.conn.event_parent = set->event_parent;
+
+	if (unix_path != NULL) {
+		connection_init_client_unix(redis_connections, &dict->conn.conn,
+					    unix_path);
+	} else {
+		connection_init_client_ip(redis_connections, &dict->conn.conn,
+					  NULL, &ip, port);
+	}
+	event_set_append_log_prefix(dict->conn.conn.event, "redis: ");
+	dict->dict = *driver;
+	dict->conn.last_reply = str_new(default_pool, 256);
+	dict->conn.dict = dict;
+
+	i_array_init(&dict->input_states, 4);
+	i_array_init(&dict->replies, 4);
+
+	*dict_r = &dict->dict;
+	return 0;
+}
+
+static void redis_dict_deinit(struct dict *_dict)
+{
+	struct redis_dict *dict = (struct redis_dict *)_dict;
+
+	if (array_count(&dict->input_states) > 0) {
+		i_assert(dict->connected);
+		redis_wait(dict);
+	}
+	connection_deinit(&dict->conn.conn);
+	str_free(&dict->conn.last_reply);
+	array_free(&dict->replies);
+	array_free(&dict->input_states);
+	i_free(dict->expire_value);
+	i_free(dict->key_prefix);
+	i_free(dict->password);
+	i_free(dict);
+
+	if (redis_connections->connections == NULL)
+		connection_list_deinit(&redis_connections);
+}
+
+static void redis_dict_wait(struct dict *_dict)
+{
+	struct redis_dict *dict = (struct redis_dict *)_dict;
+
+	if (array_count(&dict->input_states) > 0)
+		redis_wait(dict);
+}
+
+static void redis_dict_lookup_timeout(struct redis_dict *dict)
+{
+	const char *reason = t_strdup_printf(
+		"redis: Lookup timed out in %u.%03u secs",
+		dict->timeout_msecs/1000, dict->timeout_msecs%1000);
+	redis_disconnected(&dict->conn, reason);
+}
+
+static const char *
+redis_dict_get_full_key(struct redis_dict *dict, const char *username,
+			const char *key)
+{
+	const char *username_sp = strchr(username, DICT_USERNAME_SEPARATOR);
+
+	if (str_begins(key, DICT_PATH_SHARED))
+		key += strlen(DICT_PATH_SHARED);
+	else if (str_begins(key, DICT_PATH_PRIVATE)) {
+		key = t_strdup_printf("%s%c%s",
+				      username_sp == NULL ? username :
+						redis_escape_username(username),
+				      DICT_USERNAME_SEPARATOR,
+				      key + strlen(DICT_PATH_PRIVATE));
+	} else {
+		i_unreached();
+	}
+	if (*dict->key_prefix != '\0')
+		key = t_strconcat(dict->key_prefix, key, NULL);
+	return key;
+}
+
+static void redis_dict_auth(struct redis_dict *dict)
+{
+	const char *cmd;
+
+	if (*dict->password == '\0')
+		return;
+
+	cmd = t_strdup_printf("*2\r\n$4\r\nAUTH\r\n$%d\r\n%s\r\n",
+	                      (int)strlen(dict->password), dict->password);
+	o_stream_nsend_str(dict->conn.conn.output, cmd);
+	redis_input_state_add(dict, REDIS_INPUT_STATE_AUTH);
+}
+
+static void redis_dict_select_db(struct redis_dict *dict)
+{
+	const char *cmd, *db_str;
+
+	if (dict->db_id_set)
+		return;
+	dict->db_id_set = TRUE;
+	if (dict->db_id == 0) {
+		/* 0 is the default */
+		return;
+	}
+	db_str = dec2str(dict->db_id);
+	cmd = t_strdup_printf("*2\r\n$6\r\nSELECT\r\n$%d\r\n%s\r\n",
+			      (int)strlen(db_str), db_str);
+	o_stream_nsend_str(dict->conn.conn.output, cmd);
+	redis_input_state_add(dict, REDIS_INPUT_STATE_SELECT);
+}
+
+static int redis_dict_lookup(struct dict *_dict,
+			     const struct dict_op_settings *set,
+			     pool_t pool, const char *key,
+			     const char **value_r, const char **error_r)
+{
+	struct redis_dict *dict = (struct redis_dict *)_dict;
+	struct timeout *to;
+	const char *cmd;
+
+	key = redis_dict_get_full_key(dict, set->username, key);
+
+	dict->conn.value_received = FALSE;
+	dict->conn.value_not_found = FALSE;
+
+	i_assert(dict->dict.ioloop == NULL);
+
+	dict->dict.prev_ioloop = current_ioloop;
+	dict->dict.ioloop = io_loop_create();
+	connection_switch_ioloop(&dict->conn.conn);
+
+	if (dict->conn.conn.fd_in == -1 &&
+	    connection_client_connect(&dict->conn.conn) < 0) {
+		e_error(dict->conn.conn.event, "Couldn't connect");
+	} else {
+		to = timeout_add(dict->timeout_msecs,
+				 redis_dict_lookup_timeout, dict);
+		if (!dict->connected) {
+			/* wait for connection */
+			io_loop_run(dict->dict.ioloop);
+			if (dict->connected)
+				redis_dict_auth(dict);
+		}
+
+		if (dict->connected) {
+			redis_dict_select_db(dict);
+			cmd = t_strdup_printf("*2\r\n$3\r\nGET\r\n$%d\r\n%s\r\n",
+					      (int)strlen(key), key);
+			o_stream_nsend_str(dict->conn.conn.output, cmd);
+
+			str_truncate(dict->conn.last_reply, 0);
+			redis_input_state_add(dict, REDIS_INPUT_STATE_GET);
+			do {
+				io_loop_run(dict->dict.ioloop);
+			} while (array_count(&dict->input_states) > 0);
+		}
+		timeout_remove(&to);
+	}
+
+	io_loop_set_current(dict->dict.prev_ioloop);
+	connection_switch_ioloop(&dict->conn.conn);
+	io_loop_set_current(dict->dict.ioloop);
+	io_loop_destroy(&dict->dict.ioloop);
+	dict->dict.prev_ioloop = NULL;
+
+	if (!dict->conn.value_received) {
+		/* we failed in some way. make sure we disconnect since the
+		   connection state isn't known anymore */
+		*error_r = t_strdup_printf("redis: Communication failure (last reply: %s)",
+					   str_c(dict->conn.last_reply));
+		redis_disconnected(&dict->conn, *error_r);
+		return -1;
+	}
+	if (dict->conn.value_not_found)
+		return 0;
+
+	*value_r = p_strdup(pool, str_c(dict->conn.last_reply));
+	return 1;
+}
+
+static struct dict_transaction_context *
+redis_transaction_init(struct dict *_dict)
+{
+	struct redis_dict *dict = (struct redis_dict *)_dict;
+	struct redis_dict_transaction_context *ctx;
+
+	i_assert(!dict->transaction_open);
+	dict->transaction_open = TRUE;
+
+	ctx = i_new(struct redis_dict_transaction_context, 1);
+	ctx->ctx.dict = _dict;
+
+	if (dict->conn.conn.fd_in == -1 &&
+	    connection_client_connect(&dict->conn.conn) < 0) {
+		e_error(dict->conn.conn.event, "Couldn't connect");
+	} else if (!dict->connected) {
+		/* wait for connection */
+		redis_wait(dict);
+		if (dict->connected)
+			redis_dict_auth(dict);
+	}
+	if (dict->connected)
+		redis_dict_select_db(dict);
+	return &ctx->ctx;
+}
+
+static void
+redis_transaction_commit(struct dict_transaction_context *_ctx, bool async,
+			 dict_transaction_commit_callback_t *callback,
+			 void *context)
+{
+	struct redis_dict_transaction_context *ctx =
+		(struct redis_dict_transaction_context *)_ctx;
+	struct redis_dict *dict = (struct redis_dict *)_ctx->dict;
+	struct redis_dict_reply *reply;
+	unsigned int i;
+	struct dict_commit_result result = { .ret = DICT_COMMIT_RET_OK };
+
+	i_assert(dict->transaction_open);
+	dict->transaction_open = FALSE;
+
+	if (ctx->error != NULL) {
+		/* make sure we're disconnected */
+		redis_disconnected(&dict->conn, ctx->error);
+		result.ret = -1;
+		result.error = ctx->error;
+		callback(&result, context);
+	} else if (_ctx->changed) {
+		i_assert(ctx->cmd_count > 0);
+
+		o_stream_nsend_str(dict->conn.conn.output,
+				   "*1\r\n$4\r\nEXEC\r\n");
+		reply = array_append_space(&dict->replies);
+		reply->callback = callback;
+		reply->context = context;
+		reply->reply_count = ctx->cmd_count;
+		redis_input_state_add(dict, REDIS_INPUT_STATE_EXEC);
+		for (i = 0; i < ctx->cmd_count; i++)
+			redis_input_state_add(dict, REDIS_INPUT_STATE_EXEC_REPLY);
+		if (async) {
+			i_free(ctx);
+			return;
+		}
+		redis_wait(dict);
+	} else {
+		callback(&result, context);
+	}
+	i_free(ctx->error);
+	i_free(ctx);
+}
+
+static void redis_transaction_rollback(struct dict_transaction_context *_ctx)
+{
+	struct redis_dict_transaction_context *ctx =
+		(struct redis_dict_transaction_context *)_ctx;
+	struct redis_dict *dict = (struct redis_dict *)_ctx->dict;
+
+	i_assert(dict->transaction_open);
+	dict->transaction_open = FALSE;
+
+	if (ctx->error != NULL) {
+		/* make sure we're disconnected */
+		redis_disconnected(&dict->conn, ctx->error);
+	} else if (_ctx->changed) {
+		o_stream_nsend_str(dict->conn.conn.output,
+				   "*1\r\n$7\r\nDISCARD\r\n");
+		redis_input_state_add(dict, REDIS_INPUT_STATE_DISCARD);
+	}
+	i_free(ctx->error);
+	i_free(ctx);
+}
+
+static int redis_check_transaction(struct redis_dict_transaction_context *ctx)
+{
+	struct redis_dict *dict = (struct redis_dict *)ctx->ctx.dict;
+
+	if (ctx->error != NULL)
+		return -1;
+	if (!dict->connected) {
+		ctx->error = i_strdup("Disconnected during transaction");
+		return -1;
+	}
+	if (ctx->ctx.changed)
+		return 0;
+
+	redis_input_state_add(dict, REDIS_INPUT_STATE_MULTI);
+	if (o_stream_send_str(dict->conn.conn.output,
+			      "*1\r\n$5\r\nMULTI\r\n") < 0) {
+		ctx->error = i_strdup_printf("write() failed: %s",
+			o_stream_get_error(dict->conn.conn.output));
+		return -1;
+	}
+	return 0;
+}
+
+static void
+redis_append_expire(struct redis_dict_transaction_context *ctx,
+		    string_t *cmd, const char *key)
+{
+	struct redis_dict *dict = (struct redis_dict *)ctx->ctx.dict;
+
+	if (dict->expire_value == NULL)
+		return;
+
+	str_printfa(cmd, "*3\r\n$6\r\nEXPIRE\r\n$%u\r\n%s\r\n$%u\r\n%s\r\n",
+		    (unsigned int)strlen(key), key,
+		    (unsigned int)strlen(dict->expire_value),
+		    dict->expire_value);
+	redis_input_state_add(dict, REDIS_INPUT_STATE_MULTI);
+	ctx->cmd_count++;
+}
+
+static void redis_set(struct dict_transaction_context *_ctx,
+		      const char *key, const char *value)
+{
+	struct redis_dict_transaction_context *ctx =
+		(struct redis_dict_transaction_context *)_ctx;
+	struct redis_dict *dict = (struct redis_dict *)_ctx->dict;
+	const struct dict_op_settings_private *set = &_ctx->set;
+	string_t *cmd;
+
+	if (redis_check_transaction(ctx) < 0)
+		return;
+
+	key = redis_dict_get_full_key(dict, set->username, key);
+	cmd = t_str_new(128);
+	str_printfa(cmd, "*3\r\n$3\r\nSET\r\n$%u\r\n%s\r\n$%u\r\n%s\r\n",
+		    (unsigned int)strlen(key), key,
+		    (unsigned int)strlen(value), value);
+	redis_input_state_add(dict, REDIS_INPUT_STATE_MULTI);
+	ctx->cmd_count++;
+	redis_append_expire(ctx, cmd, key);
+	if (o_stream_send(dict->conn.conn.output, str_data(cmd), str_len(cmd)) < 0) {
+		ctx->error = i_strdup_printf("write() failed: %s",
+			o_stream_get_error(dict->conn.conn.output));
+	}
+}
+
+static void redis_unset(struct dict_transaction_context *_ctx,
+			const char *key)
+{
+	struct redis_dict_transaction_context *ctx =
+		(struct redis_dict_transaction_context *)_ctx;
+	struct redis_dict *dict = (struct redis_dict *)_ctx->dict;
+	const struct dict_op_settings_private *set = &_ctx->set;
+	const char *cmd;
+
+	if (redis_check_transaction(ctx) < 0)
+		return;
+
+	key = redis_dict_get_full_key(dict, set->username, key);
+	cmd = t_strdup_printf("*2\r\n$3\r\nDEL\r\n$%u\r\n%s\r\n",
+			      (unsigned int)strlen(key), key);
+	if (o_stream_send_str(dict->conn.conn.output, cmd) < 0) {
+		ctx->error = i_strdup_printf("write() failed: %s",
+			o_stream_get_error(dict->conn.conn.output));
+	}
+	redis_input_state_add(dict, REDIS_INPUT_STATE_MULTI);
+	ctx->cmd_count++;
+}
+
+static void redis_atomic_inc(struct dict_transaction_context *_ctx,
+			     const char *key, long long diff)
+{
+	struct redis_dict_transaction_context *ctx =
+		(struct redis_dict_transaction_context *)_ctx;
+	struct redis_dict *dict = (struct redis_dict *)_ctx->dict;
+	const struct dict_op_settings_private *set = &_ctx->set;
+	const char *diffstr;
+	string_t *cmd;
+
+	if (redis_check_transaction(ctx) < 0)
+		return;
+
+	key = redis_dict_get_full_key(dict, set->username, key);
+	diffstr = t_strdup_printf("%lld", diff);
+	cmd = t_str_new(128);
+	str_printfa(cmd, "*3\r\n$6\r\nINCRBY\r\n$%u\r\n%s\r\n$%u\r\n%s\r\n",
+		    (unsigned int)strlen(key), key,
+		    (unsigned int)strlen(diffstr), diffstr);
+	redis_input_state_add(dict, REDIS_INPUT_STATE_MULTI);
+	ctx->cmd_count++;
+	redis_append_expire(ctx, cmd, key);
+	if (o_stream_send(dict->conn.conn.output, str_data(cmd), str_len(cmd)) < 0) {
+		ctx->error = i_strdup_printf("write() failed: %s",
+			o_stream_get_error(dict->conn.conn.output));
+	}
+}
+
+struct dict dict_driver_redis = {
+	.name = "redis",
+	{
+		.init = redis_dict_init,
+		.deinit = redis_dict_deinit,
+		.wait = redis_dict_wait,
+		.lookup = redis_dict_lookup,
+		.transaction_init = redis_transaction_init,
+		.transaction_commit = redis_transaction_commit,
+		.transaction_rollback = redis_transaction_rollback,
+		.set = redis_set,
+		.unset = redis_unset,
+		.atomic_inc = redis_atomic_inc,
+	}
+};
diff --git a/src/lib-dict/dict-transaction-memory.c b/src/lib-dict/dict-transaction-memory.c
new file mode 100644
index 0000000..4793ad3
--- /dev/null
+++ b/src/lib-dict/dict-transaction-memory.c
@@ -0,0 +1,59 @@
+/* Copyright (c) 2005-2018 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "array.h"
+#include "dict-transaction-memory.h"
+
+void dict_transaction_memory_init(struct dict_transaction_memory_context *ctx,
+				  struct dict *dict, pool_t pool)
+{
+	ctx->ctx.dict = dict;
+	ctx->pool = pool;
+	p_array_init(&ctx->changes, pool, 32);
+}
+
+void dict_transaction_memory_rollback(struct dict_transaction_context *_ctx)
+{
+	struct dict_transaction_memory_context *ctx =
+		(struct dict_transaction_memory_context *)_ctx;
+
+	pool_unref(&ctx->pool);
+}
+
+void dict_transaction_memory_set(struct dict_transaction_context *_ctx,
+				 const char *key, const char *value)
+{
+	struct dict_transaction_memory_context *ctx =
+		(struct dict_transaction_memory_context *)_ctx;
+	struct dict_transaction_memory_change *change;
+
+	change = array_append_space(&ctx->changes);
+	change->type = DICT_CHANGE_TYPE_SET;
+	change->key = p_strdup(ctx->pool, key);
+	change->value.str = p_strdup(ctx->pool, value);
+}
+
+void dict_transaction_memory_unset(struct dict_transaction_context *_ctx,
+				   const char *key)
+{
+	struct dict_transaction_memory_context *ctx =
+		(struct dict_transaction_memory_context *)_ctx;
+	struct dict_transaction_memory_change *change;
+
+	change = array_append_space(&ctx->changes);
+	change->type = DICT_CHANGE_TYPE_UNSET;
+	change->key = p_strdup(ctx->pool, key);
+}
+
+void dict_transaction_memory_atomic_inc(struct dict_transaction_context *_ctx,
+					const char *key, long long diff)
+{
+	struct dict_transaction_memory_context *ctx =
+		(struct dict_transaction_memory_context *)_ctx;
+	struct dict_transaction_memory_change *change;
+
+	change = array_append_space(&ctx->changes);
+	change->type = DICT_CHANGE_TYPE_INC;
+	change->key = p_strdup(ctx->pool, key);
+	change->value.diff = diff;
+}
diff --git a/src/lib-dict/dict-transaction-memory.h b/src/lib-dict/dict-transaction-memory.h
new file mode 100644
index 0000000..2164c1e
--- /dev/null
+++ b/src/lib-dict/dict-transaction-memory.h
@@ -0,0 +1,38 @@
+#ifndef DICT_TRANSACTION_MEMORY_H
+#define DICT_TRANSACTION_MEMORY_H
+
+#include "dict-private.h"
+
+enum dict_change_type {
+	DICT_CHANGE_TYPE_SET,
+	DICT_CHANGE_TYPE_UNSET,
+	DICT_CHANGE_TYPE_INC
+};
+
+struct dict_transaction_memory_change {
+	enum dict_change_type type;
+	const char *key;
+	union {
+		const char *str;
+		long long diff;
+	} value;
+};
+
+struct dict_transaction_memory_context {
+	struct dict_transaction_context ctx;
+	pool_t pool;
+	ARRAY(struct dict_transaction_memory_change) changes;
+};
+
+void dict_transaction_memory_init(struct dict_transaction_memory_context *ctx,
+				  struct dict *dict, pool_t pool);
+void dict_transaction_memory_rollback(struct dict_transaction_context *ctx);
+
+void dict_transaction_memory_set(struct dict_transaction_context *ctx,
+				 const char *key, const char *value);
+void dict_transaction_memory_unset(struct dict_transaction_context *ctx,
+				   const char *key);
+void dict_transaction_memory_atomic_inc(struct dict_transaction_context *ctx,
+					const char *key, long long diff);
+
+#endif
diff --git a/src/lib-dict/dict-txn-lua.c b/src/lib-dict/dict-txn-lua.c
new file mode 100644
index 0000000..34a475d
--- /dev/null
+++ b/src/lib-dict/dict-txn-lua.c
@@ -0,0 +1,262 @@
+/* Copyright (c) 2021 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "array.h"
+#include "dict.h"
+#include "dlua-script-private.h"
+#include "dict-lua-private.h"
+#include "dlua-wrapper.h"
+
+struct lua_dict_txn {
+	pool_t pool;
+	struct dict_transaction_context *txn;
+	enum {
+		STATE_OPEN,
+		STATE_COMMITTED,
+		STATE_ABORTED,
+	} state;
+
+	lua_State *L;
+	const char *username;
+};
+
+static int lua_dict_transaction_rollback(lua_State *L);
+static int lua_dict_transaction_commit(lua_State *L);
+static int lua_dict_set(lua_State *L);
+static int lua_dict_unset(lua_State *L);
+static int lua_dict_set_timestamp(lua_State *L);
+
+static luaL_Reg lua_dict_txn_methods[] = {
+	{ "rollback", lua_dict_transaction_rollback },
+	{ "commit", lua_dict_transaction_commit },
+	{ "set", lua_dict_set },
+	{ "unset", lua_dict_unset },
+	{ "set_timestamp", lua_dict_set_timestamp },
+	{ NULL, NULL },
+};
+
+static void sanity_check_txn(lua_State *L, struct lua_dict_txn *txn)
+{
+	switch (txn->state) {
+	case STATE_OPEN:
+		return;
+	case STATE_COMMITTED:
+		luaL_error(L, "dict transaction already committed");
+		return;
+	case STATE_ABORTED:
+		luaL_error(L, "dict transaction already aborted");
+		return;
+	}
+
+	i_unreached();
+}
+
+/* no actual ref counting, but we use it for clean up */
+static void lua_dict_txn_unref(struct lua_dict_txn *txn)
+{
+	/* rollback any transactions that were forgotten about */
+	dict_transaction_rollback(&txn->txn);
+
+	pool_unref(&txn->pool);
+}
+
+DLUA_WRAP_C_DATA(dict_txn, struct lua_dict_txn, lua_dict_txn_unref,
+		 lua_dict_txn_methods);
+
+/*
+ * Abort a transaction [-1,+0,e]
+ *
+ * Args:
+ *   1) userdata: struct lua_dict_txn *
+ */
+static int lua_dict_transaction_rollback(lua_State *L)
+{
+	struct lua_dict_txn *txn;
+
+	DLUA_REQUIRE_ARGS(L, 1);
+
+	txn = xlua_dict_txn_getptr(L, 1, NULL);
+	sanity_check_txn(L, txn);
+
+	txn->state = STATE_ABORTED;
+	dict_transaction_rollback(&txn->txn);
+
+	return 0;
+}
+
+static int lua_dict_transaction_commit_continue(lua_State *L,
+						int status ATTR_UNUSED,
+						lua_KContext ctx ATTR_UNUSED)
+{
+	if (!lua_isnil(L, -1))
+		lua_error(L); /* commit failed */
+
+	lua_pop(L, 1); /* pop the nil indicating the lack of error */
+
+	return 0;
+}
+
+static void
+lua_dict_transaction_commit_callback(const struct dict_commit_result *result,
+				     struct lua_dict_txn *txn)
+{
+
+	switch (result->ret) {
+	case DICT_COMMIT_RET_OK:
+		/* push a nil to indicate everything is ok */
+		lua_pushnil(txn->L);
+		break;
+	case DICT_COMMIT_RET_NOTFOUND:
+		/* we don't expose dict_atomic_inc(), so this should never happen */
+		i_unreached();
+	case DICT_COMMIT_RET_FAILED:
+	case DICT_COMMIT_RET_WRITE_UNCERTAIN:
+		/* push the error we'll raise when we resume */
+		i_assert(result->error != NULL);
+		lua_pushfstring(txn->L, "dict transaction commit failed: %s",
+				result->error);
+		break;
+	}
+
+	dlua_pcall_yieldable_resume(txn->L, 1);
+}
+
+/*
+ * Commit a transaction [-1,+0,e]
+ *
+ * Args:
+ *   1) userdata: struct lua_dict_txn *
+ */
+static int lua_dict_transaction_commit(lua_State *L)
+{
+	struct lua_dict_txn *txn;
+
+	DLUA_REQUIRE_ARGS(L, 1);
+
+	txn = xlua_dict_txn_getptr(L, 1, NULL);
+	sanity_check_txn(L, txn);
+
+	txn->state = STATE_COMMITTED;
+	dict_transaction_commit_async(&txn->txn,
+		lua_dict_transaction_commit_callback, txn);
+
+	return lua_dict_transaction_commit_continue(L,
+		lua_yieldk(L, 0, 0, lua_dict_transaction_commit_continue), 0);
+}
+
+/*
+ * Set key to value [-3,+0,e]
+ *
+ * Args:
+ *   1) userdata: struct lua_dict_txn *
+ *   2) string: key
+ *   3) string: value
+ */
+static int lua_dict_set(lua_State *L)
+{
+	struct lua_dict_txn *txn;
+	const char *key, *value;
+
+	DLUA_REQUIRE_ARGS(L, 3);
+
+	txn = xlua_dict_txn_getptr(L, 1, NULL);
+	key = luaL_checkstring(L, 2);
+	value = luaL_checkstring(L, 3);
+	lua_dict_check_key_prefix(L, key, txn->username);
+
+	dict_set(txn->txn, key, value);
+
+	return 0;
+}
+
+/*
+ * Unset key [-2,+0,e]
+ *
+ * Args:
+ *   1) userdata: struct lua_dict_txn *
+ *   2) string: key
+ */
+static int lua_dict_unset(lua_State *L)
+{
+	struct lua_dict_txn *txn;
+	const char *key;
+
+	DLUA_REQUIRE_ARGS(L, 2);
+
+	txn = xlua_dict_txn_getptr(L, 1, NULL);
+	key = luaL_checkstring(L, 2);
+	lua_dict_check_key_prefix(L, key, txn->username);
+
+	dict_unset(txn->txn, key);
+
+	return 0;
+}
+
+/*
+ * Start a dict transaction [-(1|2),+1,e]
+ *
+ * Args:
+ *   1) userdata: struct dict *
+ *   2*) string: username
+ *
+ * Returns:
+ *   Returns a new transaction object.
+ *   Username will be NULL if not provided in args.
+ */
+int lua_dict_transaction_begin(lua_State *L)
+{
+	struct lua_dict_txn *txn;
+	struct dict *dict;
+	const char *username = NULL;
+	pool_t pool;
+
+	DLUA_REQUIRE_ARGS_IN(L, 1, 2);
+
+	dict = dlua_check_dict(L, 1);
+	if (lua_gettop(L) >= 2)
+		username = luaL_checkstring(L, 2);
+
+	pool = pool_alloconly_create("lua dict txn", 128);
+	txn = p_new(pool, struct lua_dict_txn, 1);
+	txn->pool = pool;
+
+	struct dict_op_settings set = {
+		.username = username,
+	};
+	txn->txn = dict_transaction_begin(dict, &set);
+	txn->state = STATE_OPEN;
+	txn->L = L;
+	txn->username = p_strdup(txn->pool, username);
+
+	xlua_pushdict_txn(L, txn, FALSE);
+
+	return 1;
+}
+
+/*
+ * Set timestamp to the transaction [-2,+0,e]
+ *
+ * Args:
+ *   1) userdata: struct lua_dict_txn *
+ *   2) PosixTimespec : { tv_sec, tv_nsec }
+ */
+static int lua_dict_set_timestamp(lua_State *L)
+{
+	struct lua_dict_txn *txn;
+	lua_Number tv_sec, tv_nsec;
+
+	DLUA_REQUIRE_ARGS(L, 2);
+
+	txn = xlua_dict_txn_getptr(L, 1, NULL);
+	if (dlua_table_get_number_by_str(L, 2, "tv_sec", &tv_sec) <= 0)
+		luaL_error(L, "tv_sec missing from table");
+	if (dlua_table_get_number_by_str(L, 2, "tv_nsec", &tv_nsec) <= 0)
+		luaL_error(L, "tv_nsec missing from table");
+
+	struct timespec ts = {
+		.tv_sec = tv_sec,
+		.tv_nsec = tv_nsec
+	};
+	dict_transaction_set_timestamp(txn->txn, &ts);
+	return 0;
+}
diff --git a/src/lib-dict/dict.c b/src/lib-dict/dict.c
new file mode 100644
index 0000000..bda34ae
--- /dev/null
+++ b/src/lib-dict/dict.c
@@ -0,0 +1,769 @@
+/* Copyright (c) 2005-2018 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "array.h"
+#include "guid.h"
+#include "llist.h"
+#include "ioloop.h"
+#include "str.h"
+#include "ioloop.h"
+#include "dict-private.h"
+
+struct dict_commit_callback_ctx {
+	pool_t pool;
+	struct dict_commit_callback_ctx *prev, *next;
+	struct dict *dict;
+	struct event *event;
+	dict_transaction_commit_callback_t *callback;
+	struct dict_op_settings_private set;
+	struct timeout *to;
+	void *context;
+	struct dict_commit_result result;
+	bool delayed_callback:1;
+};
+
+struct dict_lookup_callback_ctx {
+	struct dict *dict;
+	struct event *event;
+	dict_lookup_callback_t *callback;
+	void *context;
+};
+
+static ARRAY(struct dict *) dict_drivers;
+
+static void
+dict_commit_async_timeout(struct dict_commit_callback_ctx *ctx);
+
+static struct event_category event_category_dict = {
+	.name = "dict",
+};
+
+static struct dict *dict_driver_lookup(const char *name)
+{
+	struct dict *dict;
+
+	array_foreach_elem(&dict_drivers, dict) {
+		if (strcmp(dict->name, name) == 0)
+			return dict;
+	}
+	return NULL;
+}
+
+void dict_transaction_commit_async_noop_callback(
+	const struct dict_commit_result *result ATTR_UNUSED,
+	void *context ATTR_UNUSED)
+{
+	/* do nothing */
+}
+
+void dict_driver_register(struct dict *driver)
+{
+	if (!array_is_created(&dict_drivers))
+		i_array_init(&dict_drivers, 8);
+
+	if (dict_driver_lookup(driver->name) != NULL) {
+		i_fatal("dict_driver_register(%s): Already registered",
+			driver->name);
+	}
+	array_push_back(&dict_drivers, &driver);
+}
+
+void dict_driver_unregister(struct dict *driver)
+{
+	struct dict *const *dicts;
+	unsigned int idx = UINT_MAX;
+
+	array_foreach(&dict_drivers, dicts) {
+		if (*dicts == driver) {
+			idx = array_foreach_idx(&dict_drivers, dicts);
+			break;
+		}
+	}
+	i_assert(idx != UINT_MAX);
+	array_delete(&dict_drivers, idx, 1);
+
+	if (array_count(&dict_drivers) == 0)
+		array_free(&dict_drivers);
+}
+
+int dict_init(const char *uri, const struct dict_settings *set,
+	      struct dict **dict_r, const char **error_r)
+{
+	struct dict_settings set_dup = *set;
+	struct dict *dict;
+	const char *p, *name, *error;
+
+	p = strchr(uri, ':');
+	if (p == NULL) {
+		*error_r = t_strdup_printf("Dictionary URI is missing ':': %s",
+					   uri);
+		return -1;
+	}
+
+	name = t_strdup_until(uri, p);
+	dict = dict_driver_lookup(name);
+	if (dict == NULL) {
+		*error_r = t_strdup_printf("Unknown dict module: %s", name);
+		return -1;
+	}
+	struct event *event = event_create(set->event_parent);
+	event_add_category(event, &event_category_dict);
+	event_add_str(event, "driver", dict->name);
+	event_set_append_log_prefix(event, t_strdup_printf("dict(%s): ",
+				    dict->name));
+	set_dup.event_parent = event;
+	if (dict->v.init(dict, p+1, &set_dup, dict_r, &error) < 0) {
+		*error_r = t_strdup_printf("dict %s: %s", name, error);
+		event_unref(&event);
+		return -1;
+	}
+	i_assert(*dict_r != NULL);
+	(*dict_r)->refcount++;
+	(*dict_r)->event = event;
+	e_debug(event_create_passthrough(event)->set_name("dict_created")->event(),
+		"dict created (uri=%s, base_dir=%s)", uri, set->base_dir);
+
+	return 0;
+}
+
+static void dict_ref(struct dict *dict)
+{
+	i_assert(dict->refcount > 0);
+
+	dict->refcount++;
+}
+
+static void dict_unref(struct dict **_dict)
+{
+	struct dict *dict = *_dict;
+	*_dict = NULL;
+	if (dict == NULL)
+		return;
+	struct event *event = dict->event;
+	i_assert(dict->refcount > 0);
+	if (--dict->refcount == 0) {
+		dict->v.deinit(dict);
+		e_debug(event_create_passthrough(event)->
+			set_name("dict_destroyed")->event(), "dict destroyed");
+		event_unref(&event);
+	}
+}
+
+void dict_deinit(struct dict **_dict)
+{
+	struct dict *dict = *_dict;
+
+	*_dict = NULL;
+
+	i_assert(dict->iter_count == 0);
+	i_assert(dict->transaction_count == 0);
+	i_assert(dict->transactions == NULL);
+	i_assert(dict->commits == NULL);
+	dict_unref(&dict);
+}
+
+void dict_wait(struct dict *dict)
+{
+	struct dict_commit_callback_ctx *commit, *next;
+
+	e_debug(dict->event, "Waiting for dict to finish pending operations");
+	if (dict->v.wait != NULL)
+		dict->v.wait(dict);
+	for (commit = dict->commits; commit != NULL; commit = next) {
+		next = commit->next;
+		dict_commit_async_timeout(commit);
+	}
+}
+
+bool dict_switch_ioloop(struct dict *dict)
+{
+	struct dict_commit_callback_ctx *commit;
+	bool ret = FALSE;
+
+	for (commit = dict->commits; commit != NULL; commit = commit->next) {
+		commit->to = io_loop_move_timeout(&commit->to);
+		ret = TRUE;
+	}
+	if (dict->v.switch_ioloop != NULL) {
+		if (dict->v.switch_ioloop(dict))
+			return TRUE;
+	}
+	return ret;
+}
+
+static bool dict_key_prefix_is_valid(const char *key, const char *username)
+{
+	if (str_begins(key, DICT_PATH_SHARED))
+		return TRUE;
+	if (str_begins(key, DICT_PATH_PRIVATE)) {
+		i_assert(username != NULL && username[0] != '\0');
+		return TRUE;
+	}
+	return FALSE;
+
+}
+
+void dict_pre_api_callback(struct dict *dict)
+{
+	if (dict->prev_ioloop != NULL) {
+		/* Don't let callback see that we've created our
+		   internal ioloop in case it wants to add some ios
+		   or timeouts. */
+		io_loop_set_current(dict->prev_ioloop);
+	}
+}
+
+void dict_post_api_callback(struct dict *dict)
+{
+	if (dict->prev_ioloop != NULL) {
+		io_loop_set_current(dict->ioloop);
+		io_loop_stop(dict->ioloop);
+	}
+}
+
+static void dict_lookup_finished(struct event *event, int ret, const char *error)
+{
+	i_assert(ret >= 0 || error != NULL);
+	const char *key = event_find_field_recursive_str(event, "key");
+	if (ret < 0)
+		event_add_str(event, "error", error);
+	else if (ret == 0)
+		event_add_str(event, "key_not_found", "yes");
+	event_set_name(event, "dict_lookup_finished");
+	e_debug(event, "Lookup finished for '%s': %s",  key, ret > 0 ?
+			"found" :
+			"not found");
+}
+
+static void dict_transaction_finished(struct event *event, enum dict_commit_ret ret,
+				      bool rollback, const char *error)
+{
+	i_assert(ret > DICT_COMMIT_RET_FAILED || error != NULL);
+	if (ret == DICT_COMMIT_RET_FAILED || ret == DICT_COMMIT_RET_WRITE_UNCERTAIN) {
+		 if (ret == DICT_COMMIT_RET_WRITE_UNCERTAIN)
+			event_add_str(event, "write_uncertain", "yes");
+		event_add_str(event, "error", error);
+	} else if (rollback) {
+		event_add_str(event, "rollback", "yes");
+	} else if (ret == 0) {
+		event_add_str(event, "key_not_found", "yes");
+	}
+	event_set_name(event, "dict_transaction_finished");
+	e_debug(event, "Dict transaction finished");
+}
+
+static void
+dict_lookup_callback(const struct dict_lookup_result *result,
+		     void *context)
+{
+	struct dict_lookup_callback_ctx *ctx = context;
+
+	dict_pre_api_callback(ctx->dict);
+	ctx->callback(result, ctx->context);
+	dict_post_api_callback(ctx->dict);
+	dict_lookup_finished(ctx->event, result->ret, result->error);
+	event_unref(&ctx->event);
+
+	dict_unref(&ctx->dict);
+	i_free(ctx);
+}
+
+static void
+dict_commit_async_timeout(struct dict_commit_callback_ctx *ctx)
+{
+	DLLIST_REMOVE(&ctx->dict->commits, ctx);
+	timeout_remove(&ctx->to);
+	dict_pre_api_callback(ctx->dict);
+	if (ctx->callback != NULL)
+		ctx->callback(&ctx->result, ctx->context);
+	else if (ctx->result.ret < 0)
+		e_error(ctx->event, "Commit failed: %s", ctx->result.error);
+	dict_post_api_callback(ctx->dict);
+
+	dict_transaction_finished(ctx->event, ctx->result.ret, FALSE, ctx->result.error);
+	dict_op_settings_private_free(&ctx->set);
+	event_unref(&ctx->event);
+	dict_unref(&ctx->dict);
+	pool_unref(&ctx->pool);
+}
+
+static void dict_commit_callback(const struct dict_commit_result *result,
+				 void *context)
+{
+	struct dict_commit_callback_ctx *ctx = context;
+
+	i_assert(result->ret >= 0 || result->error != NULL);
+	ctx->result = *result;
+	if (ctx->delayed_callback) {
+		ctx->result.error = p_strdup(ctx->pool, ctx->result.error);
+		ctx->to = timeout_add_short(0, dict_commit_async_timeout, ctx);
+	} else {
+		dict_commit_async_timeout(ctx);
+	}
+}
+
+static struct event *
+dict_event_create(struct dict *dict, const struct dict_op_settings *set)
+{
+	struct event *event = event_create(dict->event);
+	if (set->username != NULL)
+		event_add_str(event, "user", set->username);
+	return event;
+}
+
+int dict_lookup(struct dict *dict, const struct dict_op_settings *set,
+		pool_t pool, const char *key,
+		const char **value_r, const char **error_r)
+{
+	struct event *event = dict_event_create(dict, set);
+	int ret;
+	i_assert(dict_key_prefix_is_valid(key, set->username));
+
+	e_debug(event, "Looking up '%s'", key);
+	event_add_str(event, "key", key);
+	ret = dict->v.lookup(dict, set, pool, key, value_r, error_r);
+	dict_lookup_finished(event, ret, *error_r);
+	event_unref(&event);
+	return ret;
+}
+
+#undef dict_lookup_async
+void dict_lookup_async(struct dict *dict, const struct dict_op_settings *set,
+		       const char *key, dict_lookup_callback_t *callback,
+		       void *context)
+{
+	i_assert(dict_key_prefix_is_valid(key, set->username));
+	if (dict->v.lookup_async == NULL) {
+		struct dict_lookup_result result;
+
+		i_zero(&result);
+		/* event is going to be sent by dict_lookup */
+		result.ret = dict_lookup(dict, set, pool_datastack_create(),
+					 key, &result.value, &result.error);
+		const char *const values[] = { result.value, NULL };
+		result.values = values;
+		callback(&result, context);
+		return;
+	}
+	struct dict_lookup_callback_ctx *lctx =
+		i_new(struct dict_lookup_callback_ctx, 1);
+	lctx->dict = dict;
+	dict_ref(lctx->dict);
+	lctx->callback = callback;
+	lctx->context = context;
+	lctx->event = dict_event_create(dict, set);
+	event_add_str(lctx->event, "key", key);
+	e_debug(lctx->event, "Looking up (async) '%s'", key);
+	dict->v.lookup_async(dict, set, key, dict_lookup_callback, lctx);
+}
+
+struct dict_iterate_context *
+dict_iterate_init(struct dict *dict, const struct dict_op_settings *set,
+		  const char *path, enum dict_iterate_flags flags)
+{
+	struct dict_iterate_context *ctx;
+
+	i_assert(path != NULL);
+	i_assert(dict_key_prefix_is_valid(path, set->username));
+
+	if (dict->v.iterate_init == NULL) {
+		/* not supported by backend */
+		ctx = &dict_iter_unsupported;
+	} else {
+		ctx = dict->v.iterate_init(dict, set, path, flags);
+	}
+	/* the dict in context can differ from the dict
+	   passed as parameter, e.g. it can be dict-fail when
+	   iteration is not supported. */
+	ctx->event = dict_event_create(dict, set);
+	ctx->flags = flags;
+	dict_op_settings_dup(set, &ctx->set);
+
+	event_add_str(ctx->event, "key", path);
+	event_set_name(ctx->event, "dict_iteration_started");
+	e_debug(ctx->event, "Iterating prefix %s", path);
+	ctx->dict->iter_count++;
+	return ctx;
+}
+
+bool dict_iterate(struct dict_iterate_context *ctx,
+		  const char **key_r, const char **value_r)
+{
+	const char *const *values;
+
+	if (!dict_iterate_values(ctx, key_r, &values))
+		return FALSE;
+	if ((ctx->flags & DICT_ITERATE_FLAG_NO_VALUE) == 0)
+		*value_r = values[0];
+	return TRUE;
+}
+
+bool dict_iterate_values(struct dict_iterate_context *ctx,
+			 const char **key_r, const char *const **values_r)
+{
+
+	if (ctx->max_rows > 0 && ctx->row_count >= ctx->max_rows) {
+		e_debug(ctx->event, "Maximum row count (%"PRIu64") reached",
+			ctx->max_rows);
+		/* row count was limited */
+		ctx->has_more = FALSE;
+		return FALSE;
+	}
+	if (!ctx->dict->v.iterate(ctx, key_r, values_r))
+		return FALSE;
+	if ((ctx->flags & DICT_ITERATE_FLAG_NO_VALUE) != 0) {
+		/* always return value as NULL to be consistent across
+		   drivers */
+		*values_r = NULL;
+	} else {
+		i_assert(values_r[0] != NULL);
+	}
+	ctx->row_count++;
+	return TRUE;
+}
+
+#undef dict_iterate_set_async_callback
+void dict_iterate_set_async_callback(struct dict_iterate_context *ctx,
+				     dict_iterate_callback_t *callback,
+				     void *context)
+{
+	ctx->async_callback = callback;
+	ctx->async_context = context;
+}
+
+void dict_iterate_set_limit(struct dict_iterate_context *ctx,
+			    uint64_t max_rows)
+{
+	ctx->max_rows = max_rows;
+}
+
+bool dict_iterate_has_more(struct dict_iterate_context *ctx)
+{
+	return ctx->has_more;
+}
+
+int dict_iterate_deinit(struct dict_iterate_context **_ctx,
+			const char **error_r)
+{
+	struct dict_iterate_context *ctx = *_ctx;
+
+	if (ctx == NULL)
+		return 0;
+
+	struct event *event = ctx->event;
+	int ret;
+	uint64_t rows;
+
+	i_assert(ctx->dict->iter_count > 0);
+	ctx->dict->iter_count--;
+
+	*_ctx = NULL;
+	rows = ctx->row_count;
+	struct dict_op_settings_private set_copy = ctx->set;
+	ret = ctx->dict->v.iterate_deinit(ctx, error_r);
+	dict_op_settings_private_free(&set_copy);
+
+	event_add_int(event, "rows", rows);
+	event_set_name(event, "dict_iteration_finished");
+
+	if (ret < 0) {
+		event_add_str(event, "error", *error_r);
+		e_debug(event, "Iteration finished: %s", *error_r);
+	} else {
+		if (rows == 0)
+			event_add_str(event, "key_not_found", "yes");
+		e_debug(event, "Iteration finished, got %"PRIu64" rows", rows);
+	}
+
+	event_unref(&event);
+	return ret;
+}
+
+struct dict_transaction_context *
+dict_transaction_begin(struct dict *dict, const struct dict_op_settings *set)
+{
+	struct dict_transaction_context *ctx;
+	guid_128_t guid;
+	if (dict->v.transaction_init == NULL)
+		ctx = &dict_transaction_unsupported;
+	else
+		ctx = dict->v.transaction_init(dict);
+	/* the dict in context can differ from the dict
+	   passed as parameter, e.g. it can be dict-fail when
+	   transactions are not supported. */
+	ctx->dict->transaction_count++;
+	DLLIST_PREPEND(&ctx->dict->transactions, ctx);
+	ctx->event = dict_event_create(dict, set);
+	dict_op_settings_dup(set, &ctx->set);
+	guid_128_generate(guid);
+	event_add_str(ctx->event, "txid", guid_128_to_string(guid));
+	event_set_name(ctx->event, "dict_transaction_started");
+	e_debug(ctx->event, "Starting transaction");
+	return ctx;
+}
+
+void dict_transaction_no_slowness_warning(struct dict_transaction_context *ctx)
+{
+	ctx->no_slowness_warning = TRUE;
+}
+
+void dict_transaction_set_hide_log_values(struct dict_transaction_context *ctx,
+					  bool hide_log_values)
+{
+	/* Apply hide_log_values to the current transactions dict op settings */
+	ctx->set.hide_log_values = hide_log_values;
+	if (ctx->dict->v.set_hide_log_values != NULL)
+		ctx->dict->v.set_hide_log_values(ctx, hide_log_values);
+}
+
+void dict_transaction_set_timestamp(struct dict_transaction_context *ctx,
+				    const struct timespec *ts)
+{
+	/* These asserts are mainly here to guarantee a possibility in future
+	   to change the API to support multiple timestamps within the same
+	   transaction, so this call would apply only to the following
+	   changes. */
+	i_assert(!ctx->changed);
+	i_assert(ctx->timestamp.tv_sec == 0);
+	i_assert(ts->tv_sec > 0);
+
+	ctx->timestamp = *ts;
+	struct event_passthrough *e = event_create_passthrough(ctx->event)->
+		set_name("dict_set_timestamp");
+
+	e_debug(e->event(), "Setting timestamp on transaction to (%"PRIdTIME_T", %ld)",
+		 ts->tv_sec, ts->tv_nsec);
+	if (ctx->dict->v.set_timestamp != NULL)
+		ctx->dict->v.set_timestamp(ctx, ts);
+}
+
+struct dict_commit_sync_result {
+	int ret;
+	char *error;
+};
+
+static void
+dict_transaction_commit_sync_callback(const struct dict_commit_result *result,
+				      void *context)
+{
+	struct dict_commit_sync_result *sync_result = context;
+
+	sync_result->ret = result->ret;
+	sync_result->error = i_strdup(result->error);
+}
+
+int dict_transaction_commit(struct dict_transaction_context **_ctx,
+			    const char **error_r)
+{
+	pool_t pool = pool_alloconly_create("dict_commit_callback_ctx", 64);
+	struct dict_commit_callback_ctx *cctx =
+		p_new(pool, struct dict_commit_callback_ctx, 1);
+	struct dict_transaction_context *ctx = *_ctx;
+	struct dict_commit_sync_result result;
+
+	*_ctx = NULL;
+	cctx->pool = pool;
+	i_zero(&result);
+	i_assert(ctx->dict->transaction_count > 0);
+	ctx->dict->transaction_count--;
+	DLLIST_REMOVE(&ctx->dict->transactions, ctx);
+	DLLIST_PREPEND(&ctx->dict->commits, cctx);
+	cctx->dict = ctx->dict;
+	dict_ref(cctx->dict);
+	cctx->callback = dict_transaction_commit_sync_callback;
+	cctx->context = &result;
+	cctx->event = ctx->event;
+	cctx->set = ctx->set;
+
+	ctx->dict->v.transaction_commit(ctx, FALSE, dict_commit_callback, cctx);
+	*error_r = t_strdup(result.error);
+	i_free(result.error);
+	return result.ret;
+}
+
+#undef dict_transaction_commit_async
+void dict_transaction_commit_async(struct dict_transaction_context **_ctx,
+				   dict_transaction_commit_callback_t *callback,
+				   void *context)
+{
+	pool_t pool = pool_alloconly_create("dict_commit_callback_ctx", 64);
+	struct dict_commit_callback_ctx *cctx =
+		p_new(pool, struct dict_commit_callback_ctx, 1);
+	struct dict_transaction_context *ctx = *_ctx;
+
+	*_ctx = NULL;
+	i_assert(ctx->dict->transaction_count > 0);
+	ctx->dict->transaction_count--;
+	DLLIST_REMOVE(&ctx->dict->transactions, ctx);
+	DLLIST_PREPEND(&ctx->dict->commits, cctx);
+	if (callback == NULL)
+		callback = dict_transaction_commit_async_noop_callback;
+	cctx->pool = pool;
+	cctx->dict = ctx->dict;
+	dict_ref(cctx->dict);
+	cctx->callback = callback;
+	cctx->context = context;
+	cctx->event = ctx->event;
+	cctx->set = ctx->set;
+	cctx->delayed_callback = TRUE;
+	ctx->dict->v.transaction_commit(ctx, TRUE, dict_commit_callback, cctx);
+	cctx->delayed_callback = FALSE;
+}
+
+void dict_transaction_commit_async_nocallback(
+	struct dict_transaction_context **ctx)
+{
+	dict_transaction_commit_async(ctx, NULL, NULL);
+}
+
+void dict_transaction_rollback(struct dict_transaction_context **_ctx)
+{
+	struct dict_transaction_context *ctx = *_ctx;
+
+	if (ctx == NULL)
+		return;
+
+	struct event *event = ctx->event;
+
+	*_ctx = NULL;
+	i_assert(ctx->dict->transaction_count > 0);
+	ctx->dict->transaction_count--;
+	DLLIST_REMOVE(&ctx->dict->transactions, ctx);
+	struct dict_op_settings_private set_copy = ctx->set;
+	ctx->dict->v.transaction_rollback(ctx);
+	dict_transaction_finished(event, DICT_COMMIT_RET_OK, TRUE, NULL);
+	dict_op_settings_private_free(&set_copy);
+	event_unref(&event);
+}
+
+void dict_set(struct dict_transaction_context *ctx,
+	      const char *key, const char *value)
+{
+	i_assert(dict_key_prefix_is_valid(key, ctx->set.username));
+	struct event_passthrough *e = event_create_passthrough(ctx->event)->
+		set_name("dict_set_key")->
+		add_str("key", key);
+
+	e_debug(e->event(), "Setting '%s' to '%s'", key, value);
+
+	T_BEGIN {
+		ctx->dict->v.set(ctx, key, value);
+	} T_END;
+	ctx->changed = TRUE;
+}
+
+void dict_unset(struct dict_transaction_context *ctx,
+		const char *key)
+{
+	i_assert(dict_key_prefix_is_valid(key, ctx->set.username));
+	struct event_passthrough *e = event_create_passthrough(ctx->event)->
+		set_name("dict_unset_key")->
+		add_str("key", key);
+
+	e_debug(e->event(), "Unsetting '%s'", key);
+
+	T_BEGIN {
+		ctx->dict->v.unset(ctx, key);
+	} T_END;
+	ctx->changed = TRUE;
+}
+
+void dict_atomic_inc(struct dict_transaction_context *ctx,
+		     const char *key, long long diff)
+{
+	i_assert(dict_key_prefix_is_valid(key, ctx->set.username));
+	struct event_passthrough *e = event_create_passthrough(ctx->event)->
+		set_name("dict_increment_key")->
+		add_str("key", key);
+
+	e_debug(e->event(), "Incrementing '%s' with %lld", key, diff);
+
+	if (diff != 0) T_BEGIN {
+		ctx->dict->v.atomic_inc(ctx, key, diff);
+		ctx->changed = TRUE;
+	} T_END;
+}
+
+const char *dict_escape_string(const char *str)
+{
+	const char *p;
+	string_t *ret;
+
+	/* see if we need to escape it */
+	for (p = str; *p != '\0'; p++) {
+		if (*p == '/' || *p == '\\')
+			break;
+	}
+
+	if (*p == '\0')
+		return str;
+
+	/* escape */
+	ret = t_str_new((size_t) (p - str) + 128);
+	str_append_data(ret, str, (size_t) (p - str));
+
+	for (; *p != '\0'; p++) {
+		switch (*p) {
+		case '/':
+			str_append_c(ret, '\\');
+			str_append_c(ret, '|');
+			break;
+		case '\\':
+			str_append_c(ret, '\\');
+			str_append_c(ret, '\\');
+			break;
+		default:
+			str_append_c(ret, *p);
+			break;
+		}
+	}
+	return str_c(ret);
+}
+
+const char *dict_unescape_string(const char *str)
+{
+	const char *p;
+	string_t *ret;
+
+	/* see if we need to unescape it */
+	for (p = str; *p != '\0'; p++) {
+		if (*p == '\\')
+			break;
+	}
+
+	if (*p == '\0')
+		return str;
+
+	/* unescape */
+	ret = t_str_new((size_t) (p - str) + strlen(p) + 1);
+	str_append_data(ret, str, (size_t) (p - str));
+
+	for (; *p != '\0'; p++) {
+		if (*p != '\\')
+			str_append_c(ret, *p);
+		else {
+			if (*++p == '|')
+				str_append_c(ret, '/');
+			else if (*p == '\0')
+				break;
+			else
+				str_append_c(ret, *p);
+		}
+	}
+	return str_c(ret);
+}
+
+void dict_op_settings_dup(const struct dict_op_settings *source,
+			  struct dict_op_settings_private *dest_r)
+{
+	i_zero(dest_r);
+	dest_r->username = i_strdup(source->username);
+	dest_r->home_dir = i_strdup(source->home_dir);
+	dest_r->hide_log_values = source->hide_log_values;
+}
+
+void dict_op_settings_private_free(struct dict_op_settings_private *set)
+{
+	i_free(set->username);
+	i_free(set->home_dir);
+}
diff --git a/src/lib-dict/dict.h b/src/lib-dict/dict.h
new file mode 100644
index 0000000..8cac9c7
--- /dev/null
+++ b/src/lib-dict/dict.h
@@ -0,0 +1,208 @@
+#ifndef DICT_H
+#define DICT_H
+
+#define DICT_PATH_PRIVATE "priv/"
+#define DICT_PATH_SHARED "shared/"
+
+struct timespec;
+struct dict;
+struct dict_iterate_context;
+
+enum dict_iterate_flags {
+	/* Recurse to all the sub-hierarchies (e.g. iterating "foo/" will
+	   return "foo/a", but should it return "foo/a/b"?) */
+	DICT_ITERATE_FLAG_RECURSE             = 0x01,
+	/* Sort returned results by key */
+	DICT_ITERATE_FLAG_SORT_BY_KEY         = 0x02,
+	/* Sort returned results by value */
+	DICT_ITERATE_FLAG_SORT_BY_VALUE       = 0x04,
+	/* Don't return values, only keys */
+	DICT_ITERATE_FLAG_NO_VALUE            = 0x08,
+	/* Don't recurse at all. This is basically the same as dict_lookup(),
+	   but it'll return all the rows instead of only the first one. */
+	DICT_ITERATE_FLAG_EXACT_KEY           = 0x10,
+	/* Perform iteration asynchronously. */
+	DICT_ITERATE_FLAG_ASYNC               = 0x20
+};
+
+enum dict_data_type {
+	DICT_DATA_TYPE_STRING = 0,
+	DICT_DATA_TYPE_UINT32,
+	DICT_DATA_TYPE_LAST
+};
+
+struct dict_settings {
+	const char *base_dir;
+	/* set to parent event, if exists */
+	struct event *event_parent;
+};
+
+struct dict_op_settings {
+	const char *username;
+	/* home directory for the user, if known */
+	const char *home_dir;
+
+	/* Hide values when logging about this transaction. */
+	bool hide_log_values;
+};
+
+struct dict_lookup_result {
+	int ret;
+
+	/* First returned value (ret > 0) */
+	const char *value;
+	/* NULL-terminated list of all returned values (ret > 0) */
+	const char *const *values;
+
+	/* Error message for a failed lookup (ret < 0) */
+	const char *error;
+};
+
+enum dict_commit_ret {
+	DICT_COMMIT_RET_OK = 1,
+	DICT_COMMIT_RET_NOTFOUND = 0,
+	DICT_COMMIT_RET_FAILED = -1,
+	/* write may or may not have succeeded (e.g. write timeout or
+	   disconnected from server) */
+	DICT_COMMIT_RET_WRITE_UNCERTAIN = -2,
+};
+
+struct dict_commit_result {
+	enum dict_commit_ret ret;
+	const char *error;
+};
+
+typedef void dict_lookup_callback_t(const struct dict_lookup_result *result,
+				    void *context);
+typedef void dict_iterate_callback_t(void *context);
+typedef void
+dict_transaction_commit_callback_t(const struct dict_commit_result *result,
+				   void *context);
+
+void dict_driver_register(struct dict *driver);
+void dict_driver_unregister(struct dict *driver);
+
+void dict_drivers_register_builtin(void);
+void dict_drivers_unregister_builtin(void);
+
+void dict_drivers_register_all(void);
+void dict_drivers_unregister_all(void);
+
+/* Open dictionary with given URI (type:data).
+   Returns 0 if ok, -1 if URI is invalid. */
+int dict_init(const char *uri, const struct dict_settings *set,
+	      struct dict **dict_r, const char **error_r);
+/* Close dictionary. */
+void dict_deinit(struct dict **dict);
+/* Wait for all pending asynchronous operations to finish. */
+void dict_wait(struct dict *dict);
+/* Switch the dict to the current ioloop. This can be used to do dict_wait()
+   among other IO work. Returns TRUE if there is actually some work that can
+   be waited on. */
+bool dict_switch_ioloop(struct dict *dict) ATTR_NOWARN_UNUSED_RESULT;
+
+/* Lookup value for key. Set it to NULL if it's not found.
+   Returns 1 if found, 0 if not found and -1 if lookup failed. */
+int dict_lookup(struct dict *dict, const struct dict_op_settings *set, pool_t pool,
+		const char *key, const char **value_r, const char **error_r);
+void dict_lookup_async(struct dict *dict, const struct dict_op_settings *set,
+		       const char *key, dict_lookup_callback_t *callback,
+		       void *context);
+#define dict_lookup_async(dict, set, key, callback, context) \
+	dict_lookup_async(dict, set, key, (dict_lookup_callback_t *)(callback), \
+		1 ? (context) : \
+		CALLBACK_TYPECHECK(callback, \
+			void (*)(const struct dict_lookup_result *, typeof(context))))
+
+/* Iterate through all values in a path. flag indicates how iteration
+   is carried out */
+struct dict_iterate_context *
+dict_iterate_init(struct dict *dict, const struct dict_op_settings *set,
+		  const char *path, enum dict_iterate_flags flags);
+/* Set async callback. Note that if dict_iterate_init() already did all the
+   work, this callback may never be called. So after dict_iterate_init() you
+   should call dict_iterate() in any case to see if all the results are
+   already available. */
+void dict_iterate_set_async_callback(struct dict_iterate_context *ctx,
+				     dict_iterate_callback_t *callback,
+				     void *context);
+#define dict_iterate_set_async_callback(ctx, callback, context) \
+	dict_iterate_set_async_callback(ctx, (dict_iterate_callback_t *)(callback), \
+		1 ? (context) : \
+		CALLBACK_TYPECHECK(callback, void (*)(typeof(context))))
+/* Limit how many rows will be returned by the iteration (0 = unlimited).
+   This allows backends to optimize the query (e.g. use LIMIT 1 with SQL). */
+void dict_iterate_set_limit(struct dict_iterate_context *ctx,
+			    uint64_t max_rows);
+/* If dict_iterate() returns FALSE, the iteration may be finished or if this
+   is an async iteration it may be waiting for more data. If this function
+   returns TRUE, the dict callback is called again with more data. If dict
+   supports multiple values, dict_iterate_values() can be used to return all
+   of them. dict_iterate() returns only the first value and ignores the rest. */
+bool dict_iterate_has_more(struct dict_iterate_context *ctx);
+bool dict_iterate(struct dict_iterate_context *ctx,
+		  const char **key_r, const char **value_r);
+bool dict_iterate_values(struct dict_iterate_context *ctx,
+			 const char **key_r, const char *const **values_r);
+/* Returns 0 = ok, -1 = iteration failed */
+int dict_iterate_deinit(struct dict_iterate_context **ctx, const char **error_r);
+
+/* Start a new dictionary transaction. */
+struct dict_transaction_context *
+dict_transaction_begin(struct dict *dict, const struct dict_op_settings *set);
+/* Don't log a warning if the transaction commit took a long time.
+   This is needed if there are no guarantees that an asynchronous commit will
+   finish up anytime soon. Mainly useful for transactions which aren't
+   especially important whether they finish or not. */
+void dict_transaction_no_slowness_warning(struct dict_transaction_context *ctx);
+/* Set write timestamp for the entire transaction. This must be set before
+   any changes are done and can't be changed afterwards. Currently only
+   dict-sql with Cassandra backend does anything with this. */
+void dict_transaction_set_timestamp(struct dict_transaction_context *ctx,
+				    const struct timespec *ts);
+
+/* Set hide_log_values for the transaction. Currently only
+   dict-sql with Cassandra backend does anything with this. */
+void dict_transaction_set_hide_log_values(struct dict_transaction_context *ctx,
+					  bool hide_log_values);
+/* Commit the transaction. Returns 1 if ok, 0 if dict_atomic_inc() was used
+   on a nonexistent key, -1 if failed. */
+int dict_transaction_commit(struct dict_transaction_context **ctx,
+			    const char **error_r);
+/* Commit the transaction, but don't wait to see if it finishes successfully.
+   The callback is called when the transaction is finished. If it's not called
+   by the time you want to deinitialize dict, call dict_flush() to wait for the
+   result. */
+void dict_transaction_commit_async(struct dict_transaction_context **ctx,
+				   dict_transaction_commit_callback_t *callback,
+				   void *context) ATTR_NULL(2, 3);
+#define dict_transaction_commit_async(ctx, callback, context) \
+	dict_transaction_commit_async(ctx, (dict_transaction_commit_callback_t *)(callback), \
+		1 ? (context) : \
+		CALLBACK_TYPECHECK(callback, \
+			void (*)(const struct dict_commit_result *, typeof(context))))
+/* Same as dict_transaction_commit_async(), but don't call a callback. */
+void dict_transaction_commit_async_nocallback(
+	struct dict_transaction_context **ctx);
+/* Rollback all changes made in transaction. */
+void dict_transaction_rollback(struct dict_transaction_context **ctx);
+
+/* Set key=value in dictionary. */
+void dict_set(struct dict_transaction_context *ctx,
+	      const char *key, const char *value);
+/* Unset a record in dictionary, identified by key*/
+void dict_unset(struct dict_transaction_context *ctx,
+		const char *key);
+/* Increase/decrease a numeric value in dictionary. Note that the value is
+   changed when transaction is being committed, so you can't know beforehand
+   what the value will become. The value is updated only if it already exists,
+   otherwise commit() will return 0. */
+void dict_atomic_inc(struct dict_transaction_context *ctx,
+		     const char *key, long long diff);
+
+/* Escape/unescape '/' characters in a string, so that it can be safely added
+   into path components in dict keys. */
+const char *dict_escape_string(const char *str);
+const char *dict_unescape_string(const char *str);
+
+#endif
diff --git a/src/lib-dict/test-dict-client.c b/src/lib-dict/test-dict-client.c
new file mode 100644
index 0000000..4d4ca68
--- /dev/null
+++ b/src/lib-dict/test-dict-client.c
@@ -0,0 +1,117 @@
+/* Copyright (c) 2016-2018 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "lib-signals.h"
+#include "ioloop.h"
+#include "dict-private.h"
+
+#include <stdio.h>
+
+static int pending = 0;
+static volatile bool stop = FALSE;
+
+static void sig_die(const siginfo_t *si ATTR_UNUSED, void *context ATTR_UNUSED)
+{
+	stop = TRUE;
+}
+
+static void lookup_callback(const struct dict_lookup_result *result,
+			    void *context ATTR_UNUSED)
+{
+	if (result->error != NULL)
+		i_error("%s", result->error);
+	/*else if (result->ret == 0)
+		i_info("not found");
+	else
+		i_info("%s", result->value);*/
+	pending--;
+}
+
+static void commit_callback(const struct dict_commit_result *result,
+			    void *context ATTR_UNUSED)
+{
+	if (result->ret < 0)
+		i_error("commit %d", result->ret);
+	pending--;
+}
+
+int main(int argc, char *argv[])
+{
+	const char *prefix, *uri;
+	struct dict *dict;
+	struct dict_settings set;
+	struct dict_op_settings opset;
+	struct ioloop *ioloop;
+	const char *error;
+	char key[1000], value[100];
+
+	lib_init();
+	lib_signals_init();
+	ioloop = io_loop_create();
+	lib_signals_set_handler(SIGINT, LIBSIG_FLAG_RESTART, sig_die, NULL);
+	dict_driver_register(&dict_driver_client);
+
+	if (argc < 3)
+		i_fatal("Usage: <prefix> <uri>");
+	prefix = argv[1];
+	uri = argv[2];
+
+	i_zero(&set);
+	i_zero(&opset);
+	set.base_dir = "/var/run/dovecot";
+	opset.username = "testuser";
+
+	if (dict_init(uri, &set, &dict, &error) < 0)
+		i_fatal("dict_init(%s) failed: %s", argv[1], error);
+
+	while (!stop) {
+		i_snprintf(key, sizeof(key), "%s/%02x", prefix,
+			   i_rand_limit(0xff));
+		i_snprintf(value, sizeof(value), "%04x", i_rand_limit(0xffff));
+		switch (i_rand_limit(4)) {
+		case 0:
+			pending++;
+			dict_lookup_async(dict, &opset, key, lookup_callback, NULL);
+			break;
+		case 1: {
+			struct dict_transaction_context *trans;
+
+			pending++;
+			trans = dict_transaction_begin(dict, &opset);
+			dict_set(trans, key, value);
+			dict_transaction_commit_async(&trans, commit_callback, NULL);
+			break;
+		}
+		case 2: {
+			struct dict_transaction_context *trans;
+
+			pending++;
+			trans = dict_transaction_begin(dict, &opset);
+			dict_unset(trans, key);
+			dict_transaction_commit_async(&trans, commit_callback, NULL);
+			break;
+		}
+		case 3: {
+			struct dict_iterate_context *iter;
+			const char *k, *v;
+
+			iter = dict_iterate_init(dict, &opset, prefix, DICT_ITERATE_FLAG_EXACT_KEY);
+			while (dict_iterate(iter, &k, &v)) ;
+			if (dict_iterate_deinit(&iter, &error) < 0)
+				i_error("iter failed: %s", error);
+			break;
+		}
+		}
+		while (pending > 100) {
+			dict_wait(dict);
+			printf("%d\n", pending); fflush(stdout);
+		}
+	}
+	dict_wait(dict);
+	dict_deinit(&dict);
+	dict_driver_unregister(&dict_driver_client);
+
+	io_loop_destroy(&ioloop);
+	lib_signals_deinit();
+	lib_deinit();
+}
diff --git a/src/lib-dict/test-dict.c b/src/lib-dict/test-dict.c
new file mode 100644
index 0000000..04864ff
--- /dev/null
+++ b/src/lib-dict/test-dict.c
@@ -0,0 +1,46 @@
+/* Copyright (c) 2010-2018 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "dict-private.h"
+#include "test-common.h"
+
+struct dict dict_driver_client;
+struct dict dict_driver_file;
+struct dict dict_driver_memcached;
+struct dict dict_driver_memcached_ascii;
+struct dict dict_driver_redis;
+
+static void test_dict_escape(void)
+{
+	static const char *input[] = {
+		"", "",
+		"foo", "foo",
+		"foo\\", "foo\\\\",
+		"foo\\bar", "foo\\\\bar",
+		"\\bar", "\\\\bar",
+		"foo/", "foo\\|",
+		"foo/bar", "foo\\|bar",
+		"/bar", "\\|bar",
+		"////", "\\|\\|\\|\\|",
+		"/", "\\|"
+	};
+	unsigned int i;
+
+	test_begin("dict escape");
+	for (i = 0; i < N_ELEMENTS(input); i += 2) {
+		test_assert(strcmp(dict_escape_string(input[i]), input[i+1]) == 0);
+		test_assert(strcmp(dict_unescape_string(input[i+1]), input[i]) == 0);
+	}
+	test_assert(strcmp(dict_unescape_string("x\\"), "x") == 0);
+	test_assert(strcmp(dict_unescape_string("\\"), "") == 0);
+	test_end();
+}
+
+int main(void)
+{
+	static void (*const test_functions[])(void) = {
+		test_dict_escape,
+		NULL
+	};
+	return test_run(test_functions);
+}