Adding upstream version 1:2.43.0.upstream/1%2.43.0

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 3056 insertions, 0 deletions

diff --git a/unpack-trees.c b/unpack-trees.c
new file mode 100644
index 0000000..c2b20b8
--- /dev/null
+++ b/unpack-trees.c
@@ -0,0 +1,3056 @@
+#include "git-compat-util.h"
+#include "advice.h"
+#include "strvec.h"
+#include "repository.h"
+#include "parse.h"
+#include "dir.h"
+#include "environment.h"
+#include "gettext.h"
+#include "hex.h"
+#include "name-hash.h"
+#include "tree.h"
+#include "tree-walk.h"
+#include "cache-tree.h"
+#include "unpack-trees.h"
+#include "progress.h"
+#include "refs.h"
+#include "attr.h"
+#include "read-cache.h"
+#include "split-index.h"
+#include "sparse-index.h"
+#include "submodule.h"
+#include "submodule-config.h"
+#include "symlinks.h"
+#include "trace2.h"
+#include "fsmonitor.h"
+#include "object-store-ll.h"
+#include "promisor-remote.h"
+#include "entry.h"
+#include "parallel-checkout.h"
+#include "setup.h"
+
+/*
+ * Error messages expected by scripts out of plumbing commands such as
+ * read-tree.  Non-scripted Porcelain is not required to use these messages
+ * and in fact are encouraged to reword them to better suit their particular
+ * situation better.  See how "git checkout" and "git merge" replaces
+ * them using setup_unpack_trees_porcelain(), for example.
+ */
+static const char *unpack_plumbing_errors[NB_UNPACK_TREES_WARNING_TYPES] = {
+	/* ERROR_WOULD_OVERWRITE */
+	"Entry '%s' would be overwritten by merge. Cannot merge.",
+
+	/* ERROR_NOT_UPTODATE_FILE */
+	"Entry '%s' not uptodate. Cannot merge.",
+
+	/* ERROR_NOT_UPTODATE_DIR */
+	"Updating '%s' would lose untracked files in it",
+
+	/* ERROR_CWD_IN_THE_WAY */
+	"Refusing to remove '%s' since it is the current working directory.",
+
+	/* ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN */
+	"Untracked working tree file '%s' would be overwritten by merge.",
+
+	/* ERROR_WOULD_LOSE_UNTRACKED_REMOVED */
+	"Untracked working tree file '%s' would be removed by merge.",
+
+	/* ERROR_BIND_OVERLAP */
+	"Entry '%s' overlaps with '%s'.  Cannot bind.",
+
+	/* ERROR_WOULD_LOSE_SUBMODULE */
+	"Submodule '%s' cannot checkout new HEAD.",
+
+	/* NB_UNPACK_TREES_ERROR_TYPES; just a meta value */
+	"",
+
+	/* WARNING_SPARSE_NOT_UPTODATE_FILE */
+	"Path '%s' not uptodate; will not remove from working tree.",
+
+	/* WARNING_SPARSE_UNMERGED_FILE */
+	"Path '%s' unmerged; will not remove from working tree.",
+
+	/* WARNING_SPARSE_ORPHANED_NOT_OVERWRITTEN */
+	"Path '%s' already present; will not overwrite with sparse update.",
+};
+
+#define ERRORMSG(o,type) \
+	( ((o) && (o)->internal.msgs[(type)]) \
+	  ? ((o)->internal.msgs[(type)])      \
+	  : (unpack_plumbing_errors[(type)]) )
+
+static const char *super_prefixed(const char *path, const char *super_prefix)
+{
+	/*
+	 * It is necessary and sufficient to have two static buffers
+	 * here, as the return value of this function is fed to
+	 * error() using the unpack_*_errors[] templates we see above.
+	 */
+	static struct strbuf buf[2] = {STRBUF_INIT, STRBUF_INIT};
+	static int super_prefix_len = -1;
+	static unsigned idx = ARRAY_SIZE(buf) - 1;
+
+	if (super_prefix_len < 0) {
+		if (!super_prefix) {
+			super_prefix_len = 0;
+		} else {
+			int i;
+			for (i = 0; i < ARRAY_SIZE(buf); i++)
+				strbuf_addstr(&buf[i], super_prefix);
+			super_prefix_len = buf[0].len;
+		}
+	}
+
+	if (!super_prefix_len)
+		return path;
+
+	if (++idx >= ARRAY_SIZE(buf))
+		idx = 0;
+
+	strbuf_setlen(&buf[idx], super_prefix_len);
+	strbuf_addstr(&buf[idx], path);
+
+	return buf[idx].buf;
+}
+
+void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
+				  const char *cmd)
+{
+	int i;
+	const char **msgs = opts->internal.msgs;
+	const char *msg;
+
+	strvec_init(&opts->internal.msgs_to_free);
+
+	if (!strcmp(cmd, "checkout"))
+		msg = advice_enabled(ADVICE_COMMIT_BEFORE_MERGE)
+		      ? _("Your local changes to the following files would be overwritten by checkout:\n%%s"
+			  "Please commit your changes or stash them before you switch branches.")
+		      : _("Your local changes to the following files would be overwritten by checkout:\n%%s");
+	else if (!strcmp(cmd, "merge"))
+		msg = advice_enabled(ADVICE_COMMIT_BEFORE_MERGE)
+		      ? _("Your local changes to the following files would be overwritten by merge:\n%%s"
+			  "Please commit your changes or stash them before you merge.")
+		      : _("Your local changes to the following files would be overwritten by merge:\n%%s");
+	else
+		msg = advice_enabled(ADVICE_COMMIT_BEFORE_MERGE)
+		      ? _("Your local changes to the following files would be overwritten by %s:\n%%s"
+			  "Please commit your changes or stash them before you %s.")
+		      : _("Your local changes to the following files would be overwritten by %s:\n%%s");
+	msgs[ERROR_WOULD_OVERWRITE] = msgs[ERROR_NOT_UPTODATE_FILE] =
+		strvec_pushf(&opts->internal.msgs_to_free, msg, cmd, cmd);
+
+	msgs[ERROR_NOT_UPTODATE_DIR] =
+		_("Updating the following directories would lose untracked files in them:\n%s");
+
+	msgs[ERROR_CWD_IN_THE_WAY] =
+		_("Refusing to remove the current working directory:\n%s");
+
+	if (!strcmp(cmd, "checkout"))
+		msg = advice_enabled(ADVICE_COMMIT_BEFORE_MERGE)
+		      ? _("The following untracked working tree files would be removed by checkout:\n%%s"
+			  "Please move or remove them before you switch branches.")
+		      : _("The following untracked working tree files would be removed by checkout:\n%%s");
+	else if (!strcmp(cmd, "merge"))
+		msg = advice_enabled(ADVICE_COMMIT_BEFORE_MERGE)
+		      ? _("The following untracked working tree files would be removed by merge:\n%%s"
+			  "Please move or remove them before you merge.")
+		      : _("The following untracked working tree files would be removed by merge:\n%%s");
+	else
+		msg = advice_enabled(ADVICE_COMMIT_BEFORE_MERGE)
+		      ? _("The following untracked working tree files would be removed by %s:\n%%s"
+			  "Please move or remove them before you %s.")
+		      : _("The following untracked working tree files would be removed by %s:\n%%s");
+	msgs[ERROR_WOULD_LOSE_UNTRACKED_REMOVED] =
+		strvec_pushf(&opts->internal.msgs_to_free, msg, cmd, cmd);
+
+	if (!strcmp(cmd, "checkout"))
+		msg = advice_enabled(ADVICE_COMMIT_BEFORE_MERGE)
+		      ? _("The following untracked working tree files would be overwritten by checkout:\n%%s"
+			  "Please move or remove them before you switch branches.")
+		      : _("The following untracked working tree files would be overwritten by checkout:\n%%s");
+	else if (!strcmp(cmd, "merge"))
+		msg = advice_enabled(ADVICE_COMMIT_BEFORE_MERGE)
+		      ? _("The following untracked working tree files would be overwritten by merge:\n%%s"
+			  "Please move or remove them before you merge.")
+		      : _("The following untracked working tree files would be overwritten by merge:\n%%s");
+	else
+		msg = advice_enabled(ADVICE_COMMIT_BEFORE_MERGE)
+		      ? _("The following untracked working tree files would be overwritten by %s:\n%%s"
+			  "Please move or remove them before you %s.")
+		      : _("The following untracked working tree files would be overwritten by %s:\n%%s");
+	msgs[ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN] =
+		strvec_pushf(&opts->internal.msgs_to_free, msg, cmd, cmd);
+
+	/*
+	 * Special case: ERROR_BIND_OVERLAP refers to a pair of paths, we
+	 * cannot easily display it as a list.
+	 */
+	msgs[ERROR_BIND_OVERLAP] = _("Entry '%s' overlaps with '%s'.  Cannot bind.");
+
+	msgs[ERROR_WOULD_LOSE_SUBMODULE] =
+		_("Cannot update submodule:\n%s");
+
+	msgs[WARNING_SPARSE_NOT_UPTODATE_FILE] =
+		_("The following paths are not up to date and were left despite sparse patterns:\n%s");
+	msgs[WARNING_SPARSE_UNMERGED_FILE] =
+		_("The following paths are unmerged and were left despite sparse patterns:\n%s");
+	msgs[WARNING_SPARSE_ORPHANED_NOT_OVERWRITTEN] =
+		_("The following paths were already present and thus not updated despite sparse patterns:\n%s");
+
+	opts->internal.show_all_errors = 1;
+	/* rejected paths may not have a static buffer */
+	for (i = 0; i < ARRAY_SIZE(opts->internal.unpack_rejects); i++)
+		opts->internal.unpack_rejects[i].strdup_strings = 1;
+}
+
+void clear_unpack_trees_porcelain(struct unpack_trees_options *opts)
+{
+	strvec_clear(&opts->internal.msgs_to_free);
+	memset(opts->internal.msgs, 0, sizeof(opts->internal.msgs));
+}
+
+static int do_add_entry(struct unpack_trees_options *o, struct cache_entry *ce,
+			 unsigned int set, unsigned int clear)
+{
+	clear |= CE_HASHED;
+
+	if (set & CE_REMOVE)
+		set |= CE_WT_REMOVE;
+
+	ce->ce_flags = (ce->ce_flags & ~clear) | set;
+	return add_index_entry(&o->internal.result, ce,
+			       ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
+}
+
+static void add_entry(struct unpack_trees_options *o,
+		      const struct cache_entry *ce,
+		      unsigned int set, unsigned int clear)
+{
+	do_add_entry(o, dup_cache_entry(ce, &o->internal.result), set, clear);
+}
+
+/*
+ * add error messages on path <path>
+ * corresponding to the type <e> with the message <msg>
+ * indicating if it should be display in porcelain or not
+ */
+static int add_rejected_path(struct unpack_trees_options *o,
+			     enum unpack_trees_error_types e,
+			     const char *path)
+{
+	if (o->quiet)
+		return -1;
+
+	if (!o->internal.show_all_errors)
+		return error(ERRORMSG(o, e), super_prefixed(path,
+							    o->super_prefix));
+
+	/*
+	 * Otherwise, insert in a list for future display by
+	 * display_(error|warning)_msgs()
+	 */
+	string_list_append(&o->internal.unpack_rejects[e], path);
+	return -1;
+}
+
+/*
+ * display all the error messages stored in a nice way
+ */
+static void display_error_msgs(struct unpack_trees_options *o)
+{
+	int e;
+	unsigned error_displayed = 0;
+	for (e = 0; e < NB_UNPACK_TREES_ERROR_TYPES; e++) {
+		struct string_list *rejects = &o->internal.unpack_rejects[e];
+
+		if (rejects->nr > 0) {
+			int i;
+			struct strbuf path = STRBUF_INIT;
+
+			error_displayed = 1;
+			for (i = 0; i < rejects->nr; i++)
+				strbuf_addf(&path, "\t%s\n", rejects->items[i].string);
+			error(ERRORMSG(o, e), super_prefixed(path.buf,
+							     o->super_prefix));
+			strbuf_release(&path);
+		}
+		string_list_clear(rejects, 0);
+	}
+	if (error_displayed)
+		fprintf(stderr, _("Aborting\n"));
+}
+
+/*
+ * display all the warning messages stored in a nice way
+ */
+static void display_warning_msgs(struct unpack_trees_options *o)
+{
+	int e;
+	unsigned warning_displayed = 0;
+	for (e = NB_UNPACK_TREES_ERROR_TYPES + 1;
+	     e < NB_UNPACK_TREES_WARNING_TYPES; e++) {
+		struct string_list *rejects = &o->internal.unpack_rejects[e];
+
+		if (rejects->nr > 0) {
+			int i;
+			struct strbuf path = STRBUF_INIT;
+
+			warning_displayed = 1;
+			for (i = 0; i < rejects->nr; i++)
+				strbuf_addf(&path, "\t%s\n", rejects->items[i].string);
+			warning(ERRORMSG(o, e), super_prefixed(path.buf,
+							       o->super_prefix));
+			strbuf_release(&path);
+		}
+		string_list_clear(rejects, 0);
+	}
+	if (warning_displayed)
+		fprintf(stderr, _("After fixing the above paths, you may want to run `git sparse-checkout reapply`.\n"));
+}
+static int check_submodule_move_head(const struct cache_entry *ce,
+				     const char *old_id,
+				     const char *new_id,
+				     struct unpack_trees_options *o)
+{
+	unsigned flags = SUBMODULE_MOVE_HEAD_DRY_RUN;
+	const struct submodule *sub = submodule_from_ce(ce);
+
+	if (!sub)
+		return 0;
+
+	if (o->reset)
+		flags |= SUBMODULE_MOVE_HEAD_FORCE;
+
+	if (submodule_move_head(ce->name, o->super_prefix, old_id, new_id,
+				flags))
+		return add_rejected_path(o, ERROR_WOULD_LOSE_SUBMODULE, ce->name);
+	return 0;
+}
+
+/*
+ * Perform the loading of the repository's gitmodules file.  This function is
+ * used by 'check_update()' to perform loading of the gitmodules file in two
+ * different situations:
+ * (1) before removing entries from the working tree if the gitmodules file has
+ *     been marked for removal.  This situation is specified by 'state' == NULL.
+ * (2) before checking out entries to the working tree if the gitmodules file
+ *     has been marked for update.  This situation is specified by 'state' != NULL.
+ */
+static void load_gitmodules_file(struct index_state *index,
+				 struct checkout *state)
+{
+	int pos = index_name_pos(index, GITMODULES_FILE, strlen(GITMODULES_FILE));
+
+	if (pos >= 0) {
+		struct cache_entry *ce = index->cache[pos];
+		if (!state && ce->ce_flags & CE_WT_REMOVE) {
+			repo_read_gitmodules(the_repository, 0);
+		} else if (state && (ce->ce_flags & CE_UPDATE)) {
+			submodule_free(the_repository);
+			checkout_entry(ce, state, NULL, NULL);
+			repo_read_gitmodules(the_repository, 0);
+		}
+	}
+}
+
+static struct progress *get_progress(struct unpack_trees_options *o,
+				     struct index_state *index)
+{
+	unsigned cnt = 0, total = 0;
+
+	if (!o->update || !o->verbose_update)
+		return NULL;
+
+	for (; cnt < index->cache_nr; cnt++) {
+		const struct cache_entry *ce = index->cache[cnt];
+		if (ce->ce_flags & (CE_UPDATE | CE_WT_REMOVE))
+			total++;
+	}
+
+	return start_delayed_progress(_("Updating files"), total);
+}
+
+static void setup_collided_checkout_detection(struct checkout *state,
+					      struct index_state *index)
+{
+	int i;
+
+	state->clone = 1;
+	for (i = 0; i < index->cache_nr; i++)
+		index->cache[i]->ce_flags &= ~CE_MATCHED;
+}
+
+static void report_collided_checkout(struct index_state *index)
+{
+	struct string_list list = STRING_LIST_INIT_NODUP;
+	int i;
+
+	for (i = 0; i < index->cache_nr; i++) {
+		struct cache_entry *ce = index->cache[i];
+
+		if (!(ce->ce_flags & CE_MATCHED))
+			continue;
+
+		string_list_append(&list, ce->name);
+		ce->ce_flags &= ~CE_MATCHED;
+	}
+
+	list.cmp = fspathcmp;
+	string_list_sort(&list);
+
+	if (list.nr) {
+		warning(_("the following paths have collided (e.g. case-sensitive paths\n"
+			  "on a case-insensitive filesystem) and only one from the same\n"
+			  "colliding group is in the working tree:\n"));
+
+		for (i = 0; i < list.nr; i++)
+			fprintf(stderr, "  '%s'\n", list.items[i].string);
+	}
+
+	string_list_clear(&list, 0);
+}
+
+static int must_checkout(const struct cache_entry *ce)
+{
+	return ce->ce_flags & CE_UPDATE;
+}
+
+static int check_updates(struct unpack_trees_options *o,
+			 struct index_state *index)
+{
+	unsigned cnt = 0;
+	int errs = 0;
+	struct progress *progress;
+	struct checkout state = CHECKOUT_INIT;
+	int i, pc_workers, pc_threshold;
+
+	trace_performance_enter();
+	state.super_prefix = o->super_prefix;
+	state.force = 1;
+	state.quiet = 1;
+	state.refresh_cache = 1;
+	state.istate = index;
+	clone_checkout_metadata(&state.meta, &o->meta, NULL);
+
+	if (!o->update || o->dry_run) {
+		remove_marked_cache_entries(index, 0);
+		trace_performance_leave("check_updates");
+		return 0;
+	}
+
+	if (o->clone)
+		setup_collided_checkout_detection(&state, index);
+
+	progress = get_progress(o, index);
+
+	/* Start with clean cache to avoid using any possibly outdated info. */
+	invalidate_lstat_cache();
+
+	git_attr_set_direction(GIT_ATTR_CHECKOUT);
+
+	if (should_update_submodules())
+		load_gitmodules_file(index, NULL);
+
+	for (i = 0; i < index->cache_nr; i++) {
+		const struct cache_entry *ce = index->cache[i];
+
+		if (ce->ce_flags & CE_WT_REMOVE) {
+			display_progress(progress, ++cnt);
+			unlink_entry(ce, o->super_prefix);
+		}
+	}
+
+	remove_marked_cache_entries(index, 0);
+	remove_scheduled_dirs();
+
+	if (should_update_submodules())
+		load_gitmodules_file(index, &state);
+
+	if (repo_has_promisor_remote(the_repository))
+		/*
+		 * Prefetch the objects that are to be checked out in the loop
+		 * below.
+		 */
+		prefetch_cache_entries(index, must_checkout);
+
+	get_parallel_checkout_configs(&pc_workers, &pc_threshold);
+
+	enable_delayed_checkout(&state);
+	if (pc_workers > 1)
+		init_parallel_checkout();
+	for (i = 0; i < index->cache_nr; i++) {
+		struct cache_entry *ce = index->cache[i];
+
+		if (must_checkout(ce)) {
+			size_t last_pc_queue_size = pc_queue_size();
+
+			if (ce->ce_flags & CE_WT_REMOVE)
+				BUG("both update and delete flags are set on %s",
+				    ce->name);
+			ce->ce_flags &= ~CE_UPDATE;
+			errs |= checkout_entry(ce, &state, NULL, NULL);
+
+			if (last_pc_queue_size == pc_queue_size())
+				display_progress(progress, ++cnt);
+		}
+	}
+	if (pc_workers > 1)
+		errs |= run_parallel_checkout(&state, pc_workers, pc_threshold,
+					      progress, &cnt);
+	stop_progress(&progress);
+	errs |= finish_delayed_checkout(&state, o->verbose_update);
+	git_attr_set_direction(GIT_ATTR_CHECKIN);
+
+	if (o->clone)
+		report_collided_checkout(index);
+
+	trace_performance_leave("check_updates");
+	return errs != 0;
+}
+
+static int verify_uptodate_sparse(const struct cache_entry *ce,
+				  struct unpack_trees_options *o);
+static int verify_absent_sparse(const struct cache_entry *ce,
+				enum unpack_trees_error_types,
+				struct unpack_trees_options *o);
+
+static int apply_sparse_checkout(struct index_state *istate,
+				 struct cache_entry *ce,
+				 struct unpack_trees_options *o)
+{
+	int was_skip_worktree = ce_skip_worktree(ce);
+
+	if (ce->ce_flags & CE_NEW_SKIP_WORKTREE)
+		ce->ce_flags |= CE_SKIP_WORKTREE;
+	else
+		ce->ce_flags &= ~CE_SKIP_WORKTREE;
+	if (was_skip_worktree != ce_skip_worktree(ce)) {
+		ce->ce_flags |= CE_UPDATE_IN_BASE;
+		mark_fsmonitor_invalid(istate, ce);
+		istate->cache_changed |= CE_ENTRY_CHANGED;
+	}
+
+	/*
+	 * if (!was_skip_worktree && !ce_skip_worktree()) {
+	 *	This is perfectly normal. Move on;
+	 * }
+	 */
+
+	/*
+	 * Merge strategies may set CE_UPDATE|CE_REMOVE outside checkout
+	 * area as a result of ce_skip_worktree() shortcuts in
+	 * verify_absent() and verify_uptodate().
+	 * Make sure they don't modify worktree if they are already
+	 * outside checkout area
+	 */
+	if (was_skip_worktree && ce_skip_worktree(ce)) {
+		ce->ce_flags &= ~CE_UPDATE;
+
+		/*
+		 * By default, when CE_REMOVE is on, CE_WT_REMOVE is also
+		 * on to get that file removed from both index and worktree.
+		 * If that file is already outside worktree area, don't
+		 * bother remove it.
+		 */
+		if (ce->ce_flags & CE_REMOVE)
+			ce->ce_flags &= ~CE_WT_REMOVE;
+	}
+
+	if (!was_skip_worktree && ce_skip_worktree(ce)) {
+		/*
+		 * If CE_UPDATE is set, verify_uptodate() must be called already
+		 * also stat info may have lost after merged_entry() so calling
+		 * verify_uptodate() again may fail
+		 */
+		if (!(ce->ce_flags & CE_UPDATE) &&
+		    verify_uptodate_sparse(ce, o)) {
+			ce->ce_flags &= ~CE_SKIP_WORKTREE;
+			return -1;
+		}
+		ce->ce_flags |= CE_WT_REMOVE;
+		ce->ce_flags &= ~CE_UPDATE;
+	}
+	if (was_skip_worktree && !ce_skip_worktree(ce)) {
+		if (verify_absent_sparse(ce, WARNING_SPARSE_ORPHANED_NOT_OVERWRITTEN, o))
+			return -1;
+		ce->ce_flags |= CE_UPDATE;
+	}
+	return 0;
+}
+
+static int warn_conflicted_path(struct index_state *istate,
+				int i,
+				struct unpack_trees_options *o)
+{
+	char *conflicting_path = istate->cache[i]->name;
+	int count = 0;
+
+	add_rejected_path(o, WARNING_SPARSE_UNMERGED_FILE, conflicting_path);
+
+	/* Find out how many higher stage entries are at same path */
+	while ((++count) + i < istate->cache_nr &&
+	       !strcmp(conflicting_path, istate->cache[count + i]->name))
+		; /* do nothing */
+
+	return count;
+}
+
+static inline int call_unpack_fn(const struct cache_entry * const *src,
+				 struct unpack_trees_options *o)
+{
+	int ret = o->fn(src, o);
+	if (ret > 0)
+		ret = 0;
+	return ret;
+}
+
+static void mark_ce_used(struct cache_entry *ce, struct unpack_trees_options *o)
+{
+	ce->ce_flags |= CE_UNPACKED;
+
+	if (o->internal.cache_bottom < o->src_index->cache_nr &&
+	    o->src_index->cache[o->internal.cache_bottom] == ce) {
+		int bottom = o->internal.cache_bottom;
+
+		while (bottom < o->src_index->cache_nr &&
+		       o->src_index->cache[bottom]->ce_flags & CE_UNPACKED)
+			bottom++;
+		o->internal.cache_bottom = bottom;
+	}
+}
+
+static void mark_all_ce_unused(struct index_state *index)
+{
+	int i;
+	for (i = 0; i < index->cache_nr; i++)
+		index->cache[i]->ce_flags &= ~(CE_UNPACKED | CE_ADDED | CE_NEW_SKIP_WORKTREE);
+}
+
+static int locate_in_src_index(const struct cache_entry *ce,
+			       struct unpack_trees_options *o)
+{
+	struct index_state *index = o->src_index;
+	int len = ce_namelen(ce);
+	int pos = index_name_pos(index, ce->name, len);
+	if (pos < 0)
+		pos = -1 - pos;
+	return pos;
+}
+
+/*
+ * We call unpack_index_entry() with an unmerged cache entry
+ * only in diff-index, and it wants a single callback.  Skip
+ * the other unmerged entry with the same name.
+ */
+static void mark_ce_used_same_name(struct cache_entry *ce,
+				   struct unpack_trees_options *o)
+{
+	struct index_state *index = o->src_index;
+	int len = ce_namelen(ce);
+	int pos;
+
+	for (pos = locate_in_src_index(ce, o); pos < index->cache_nr; pos++) {
+		struct cache_entry *next = index->cache[pos];
+		if (len != ce_namelen(next) ||
+		    memcmp(ce->name, next->name, len))
+			break;
+		mark_ce_used(next, o);
+	}
+}
+
+static struct cache_entry *next_cache_entry(struct unpack_trees_options *o)
+{
+	const struct index_state *index = o->src_index;
+	int pos = o->internal.cache_bottom;
+
+	while (pos < index->cache_nr) {
+		struct cache_entry *ce = index->cache[pos];
+		if (!(ce->ce_flags & CE_UNPACKED))
+			return ce;
+		pos++;
+	}
+	return NULL;
+}
+
+static void add_same_unmerged(const struct cache_entry *ce,
+			      struct unpack_trees_options *o)
+{
+	struct index_state *index = o->src_index;
+	int len = ce_namelen(ce);
+	int pos = index_name_pos(index, ce->name, len);
+
+	if (0 <= pos)
+		die("programming error in a caller of mark_ce_used_same_name");
+	for (pos = -pos - 1; pos < index->cache_nr; pos++) {
+		struct cache_entry *next = index->cache[pos];
+		if (len != ce_namelen(next) ||
+		    memcmp(ce->name, next->name, len))
+			break;
+		add_entry(o, next, 0, 0);
+		mark_ce_used(next, o);
+	}
+}
+
+static int unpack_index_entry(struct cache_entry *ce,
+			      struct unpack_trees_options *o)
+{
+	const struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
+	int ret;
+
+	src[0] = ce;
+
+	mark_ce_used(ce, o);
+	if (ce_stage(ce)) {
+		if (o->skip_unmerged) {
+			add_entry(o, ce, 0, 0);
+			return 0;
+		}
+	}
+	ret = call_unpack_fn(src, o);
+	if (ce_stage(ce))
+		mark_ce_used_same_name(ce, o);
+	return ret;
+}
+
+static int find_cache_pos(struct traverse_info *, const char *p, size_t len);
+
+static void restore_cache_bottom(struct traverse_info *info, int bottom)
+{
+	struct unpack_trees_options *o = info->data;
+
+	if (o->diff_index_cached)
+		return;
+	o->internal.cache_bottom = bottom;
+}
+
+static int switch_cache_bottom(struct traverse_info *info)
+{
+	struct unpack_trees_options *o = info->data;
+	int ret, pos;
+
+	if (o->diff_index_cached)
+		return 0;
+	ret = o->internal.cache_bottom;
+	pos = find_cache_pos(info->prev, info->name, info->namelen);
+
+	if (pos < -1)
+		o->internal.cache_bottom = -2 - pos;
+	else if (pos < 0)
+		o->internal.cache_bottom = o->src_index->cache_nr;
+	return ret;
+}
+
+static inline int are_same_oid(struct name_entry *name_j, struct name_entry *name_k)
+{
+	return !is_null_oid(&name_j->oid) && !is_null_oid(&name_k->oid) && oideq(&name_j->oid, &name_k->oid);
+}
+
+static int all_trees_same_as_cache_tree(int n, unsigned long dirmask,
+					struct name_entry *names,
+					struct traverse_info *info)
+{
+	struct unpack_trees_options *o = info->data;
+	int i;
+
+	if (!o->merge || dirmask != ((1 << n) - 1))
+		return 0;
+
+	for (i = 1; i < n; i++)
+		if (!are_same_oid(names, names + i))
+			return 0;
+
+	return cache_tree_matches_traversal(o->src_index->cache_tree, names, info);
+}
+
+static int index_pos_by_traverse_info(struct name_entry *names,
+				      struct traverse_info *info)
+{
+	struct unpack_trees_options *o = info->data;
+	struct strbuf name = STRBUF_INIT;
+	int pos;
+
+	strbuf_make_traverse_path(&name, info, names->path, names->pathlen);
+	strbuf_addch(&name, '/');
+	pos = index_name_pos(o->src_index, name.buf, name.len);
+	if (pos >= 0) {
+		if (!o->src_index->sparse_index ||
+		    !(o->src_index->cache[pos]->ce_flags & CE_SKIP_WORKTREE))
+			BUG("This is a directory and should not exist in index");
+	} else {
+		pos = -pos - 1;
+	}
+	if (pos >= o->src_index->cache_nr ||
+	    !starts_with(o->src_index->cache[pos]->name, name.buf) ||
+	    (pos > 0 && starts_with(o->src_index->cache[pos-1]->name, name.buf)))
+		BUG("pos %d doesn't point to the first entry of %s in index",
+		    pos, name.buf);
+	strbuf_release(&name);
+	return pos;
+}
+
+/*
+ * Fast path if we detect that all trees are the same as cache-tree at this
+ * path. We'll walk these trees in an iterative loop using cache-tree/index
+ * instead of ODB since we already know what these trees contain.
+ */
+static int traverse_by_cache_tree(int pos, int nr_entries, int nr_names,
+				  struct traverse_info *info)
+{
+	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
+	struct unpack_trees_options *o = info->data;
+	struct cache_entry *tree_ce = NULL;
+	int ce_len = 0;
+	int i, d;
+
+	if (!o->merge)
+		BUG("We need cache-tree to do this optimization");
+
+	/*
+	 * Do what unpack_callback() and unpack_single_entry() normally
+	 * do. But we walk all paths in an iterative loop instead.
+	 *
+	 * D/F conflicts and higher stage entries are not a concern
+	 * because cache-tree would be invalidated and we would never
+	 * get here in the first place.
+	 */
+	for (i = 0; i < nr_entries; i++) {
+		int new_ce_len, len, rc;
+
+		src[0] = o->src_index->cache[pos + i];
+
+		len = ce_namelen(src[0]);
+		new_ce_len = cache_entry_size(len);
+
+		if (new_ce_len > ce_len) {
+			new_ce_len <<= 1;
+			tree_ce = xrealloc(tree_ce, new_ce_len);
+			memset(tree_ce, 0, new_ce_len);
+			ce_len = new_ce_len;
+
+			tree_ce->ce_flags = create_ce_flags(0);
+
+			for (d = 1; d <= nr_names; d++)
+				src[d] = tree_ce;
+		}
+
+		tree_ce->ce_mode = src[0]->ce_mode;
+		tree_ce->ce_namelen = len;
+		oidcpy(&tree_ce->oid, &src[0]->oid);
+		memcpy(tree_ce->name, src[0]->name, len + 1);
+
+		rc = call_unpack_fn((const struct cache_entry * const *)src, o);
+		if (rc < 0) {
+			free(tree_ce);
+			return rc;
+		}
+
+		mark_ce_used(src[0], o);
+	}
+	free(tree_ce);
+	if (o->internal.debug_unpack)
+		printf("Unpacked %d entries from %s to %s using cache-tree\n",
+		       nr_entries,
+		       o->src_index->cache[pos]->name,
+		       o->src_index->cache[pos + nr_entries - 1]->name);
+	return 0;
+}
+
+static int traverse_trees_recursive(int n, unsigned long dirmask,
+				    unsigned long df_conflicts,
+				    struct name_entry *names,
+				    struct traverse_info *info)
+{
+	struct unpack_trees_options *o = info->data;
+	int i, ret, bottom;
+	int nr_buf = 0;
+	struct tree_desc *t;
+	void **buf;
+	struct traverse_info newinfo;
+	struct name_entry *p;
+	int nr_entries;
+
+	nr_entries = all_trees_same_as_cache_tree(n, dirmask, names, info);
+	if (nr_entries > 0) {
+		int pos = index_pos_by_traverse_info(names, info);
+
+		if (!o->merge || df_conflicts)
+			BUG("Wrong condition to get here buddy");
+
+		/*
+		 * All entries up to 'pos' must have been processed
+		 * (i.e. marked CE_UNPACKED) at this point. But to be safe,
+		 * save and restore cache_bottom anyway to not miss
+		 * unprocessed entries before 'pos'.
+		 */
+		bottom = o->internal.cache_bottom;
+		ret = traverse_by_cache_tree(pos, nr_entries, n, info);
+		o->internal.cache_bottom = bottom;
+		return ret;
+	}
+
+	p = names;
+	while (!p->mode)
+		p++;
+
+	newinfo = *info;
+	newinfo.prev = info;
+	newinfo.pathspec = info->pathspec;
+	newinfo.name = p->path;
+	newinfo.namelen = p->pathlen;
+	newinfo.mode = p->mode;
+	newinfo.pathlen = st_add3(newinfo.pathlen, tree_entry_len(p), 1);
+	newinfo.df_conflicts |= df_conflicts;
+
+	ALLOC_ARRAY(t, n);
+	ALLOC_ARRAY(buf, n);
+
+	/*
+	 * Fetch the tree from the ODB for each peer directory in the
+	 * n commits.
+	 *
+	 * For 2- and 3-way traversals, we try to avoid hitting the
+	 * ODB twice for the same OID.  This should yield a nice speed
+	 * up in checkouts and merges when the commits are similar.
+	 *
+	 * We don't bother doing the full O(n^2) search for larger n,
+	 * because wider traversals don't happen that often and we
+	 * avoid the search setup.
+	 *
+	 * When 2 peer OIDs are the same, we just copy the tree
+	 * descriptor data.  This implicitly borrows the buffer
+	 * data from the earlier cell.
+	 */
+	for (i = 0; i < n; i++, dirmask >>= 1) {
+		if (i > 0 && are_same_oid(&names[i], &names[i - 1]))
+			t[i] = t[i - 1];
+		else if (i > 1 && are_same_oid(&names[i], &names[i - 2]))
+			t[i] = t[i - 2];
+		else {
+			const struct object_id *oid = NULL;
+			if (dirmask & 1)
+				oid = &names[i].oid;
+			buf[nr_buf++] = fill_tree_descriptor(the_repository, t + i, oid);
+		}
+	}
+
+	bottom = switch_cache_bottom(&newinfo);
+	ret = traverse_trees(o->src_index, n, t, &newinfo);
+	restore_cache_bottom(&newinfo, bottom);
+
+	for (i = 0; i < nr_buf; i++)
+		free(buf[i]);
+	free(buf);
+	free(t);
+
+	return ret;
+}
+
+/*
+ * Compare the traverse-path to the cache entry without actually
+ * having to generate the textual representation of the traverse
+ * path.
+ *
+ * NOTE! This *only* compares up to the size of the traverse path
+ * itself - the caller needs to do the final check for the cache
+ * entry having more data at the end!
+ */
+static int do_compare_entry_piecewise(const struct cache_entry *ce,
+				      const struct traverse_info *info,
+				      const char *name, size_t namelen,
+				      unsigned mode)
+{
+	int pathlen, ce_len;
+	const char *ce_name;
+
+	if (info->prev) {
+		int cmp = do_compare_entry_piecewise(ce, info->prev,
+						     info->name, info->namelen,
+						     info->mode);
+		if (cmp)
+			return cmp;
+	}
+	pathlen = info->pathlen;
+	ce_len = ce_namelen(ce);
+
+	/* If ce_len < pathlen then we must have previously hit "name == directory" entry */
+	if (ce_len < pathlen)
+		return -1;
+
+	ce_len -= pathlen;
+	ce_name = ce->name + pathlen;
+
+	return df_name_compare(ce_name, ce_len, S_IFREG, name, namelen, mode);
+}
+
+static int do_compare_entry(const struct cache_entry *ce,
+			    const struct traverse_info *info,
+			    const char *name, size_t namelen,
+			    unsigned mode)
+{
+	int pathlen, ce_len;
+	const char *ce_name;
+	int cmp;
+	unsigned ce_mode;
+
+	/*
+	 * If we have not precomputed the traverse path, it is quicker
+	 * to avoid doing so.  But if we have precomputed it,
+	 * it is quicker to use the precomputed version.
+	 */
+	if (!info->traverse_path)
+		return do_compare_entry_piecewise(ce, info, name, namelen, mode);
+
+	cmp = strncmp(ce->name, info->traverse_path, info->pathlen);
+	if (cmp)
+		return cmp;
+
+	pathlen = info->pathlen;
+	ce_len = ce_namelen(ce);
+
+	if (ce_len < pathlen)
+		return -1;
+
+	ce_len -= pathlen;
+	ce_name = ce->name + pathlen;
+
+	ce_mode = S_ISSPARSEDIR(ce->ce_mode) ? S_IFDIR : S_IFREG;
+	return df_name_compare(ce_name, ce_len, ce_mode, name, namelen, mode);
+}
+
+static int compare_entry(const struct cache_entry *ce, const struct traverse_info *info, const struct name_entry *n)
+{
+	int cmp = do_compare_entry(ce, info, n->path, n->pathlen, n->mode);
+	if (cmp)
+		return cmp;
+
+	/*
+	 * At this point, we know that we have a prefix match. If ce
+	 * is a sparse directory, then allow an exact match. This only
+	 * works when the input name is a directory, since ce->name
+	 * ends in a directory separator.
+	 */
+	if (S_ISSPARSEDIR(ce->ce_mode) &&
+	    ce->ce_namelen == traverse_path_len(info, tree_entry_len(n)) + 1)
+		return 0;
+
+	/*
+	 * Even if the beginning compared identically, the ce should
+	 * compare as bigger than a directory leading up to it!
+	 */
+	return ce_namelen(ce) > traverse_path_len(info, tree_entry_len(n));
+}
+
+static int ce_in_traverse_path(const struct cache_entry *ce,
+			       const struct traverse_info *info)
+{
+	if (!info->prev)
+		return 1;
+	if (do_compare_entry(ce, info->prev,
+			     info->name, info->namelen, info->mode))
+		return 0;
+	/*
+	 * If ce (blob) is the same name as the path (which is a tree
+	 * we will be descending into), it won't be inside it.
+	 */
+	return (info->pathlen < ce_namelen(ce));
+}
+
+static struct cache_entry *create_ce_entry(const struct traverse_info *info,
+	const struct name_entry *n,
+	int stage,
+	struct index_state *istate,
+	int is_transient,
+	int is_sparse_directory)
+{
+	size_t len = traverse_path_len(info, tree_entry_len(n));
+	size_t alloc_len = is_sparse_directory ? len + 1 : len;
+	struct cache_entry *ce =
+		is_transient ?
+		make_empty_transient_cache_entry(alloc_len, NULL) :
+		make_empty_cache_entry(istate, alloc_len);
+
+	ce->ce_mode = create_ce_mode(n->mode);
+	ce->ce_flags = create_ce_flags(stage);
+	ce->ce_namelen = len;
+	oidcpy(&ce->oid, &n->oid);
+	/* len+1 because the cache_entry allocates space for NUL */
+	make_traverse_path(ce->name, len + 1, info, n->path, n->pathlen);
+
+	if (is_sparse_directory) {
+		ce->name[len] = '/';
+		ce->name[len + 1] = '\0';
+		ce->ce_namelen++;
+		ce->ce_flags |= CE_SKIP_WORKTREE;
+	}
+
+	return ce;
+}
+
+/*
+ * Determine whether the path specified by 'p' should be unpacked as a new
+ * sparse directory in a sparse index. A new sparse directory 'A/':
+ * - must be outside the sparse cone.
+ * - must not already be in the index (i.e., no index entry with name 'A/'
+ *   exists).
+ * - must not have any child entries in the index (i.e., no index entry
+ *   'A/<something>' exists).
+ * If 'p' meets the above requirements, return 1; otherwise, return 0.
+ */
+static int entry_is_new_sparse_dir(const struct traverse_info *info,
+				   const struct name_entry *p)
+{
+	int res, pos;
+	struct strbuf dirpath = STRBUF_INIT;
+	struct unpack_trees_options *o = info->data;
+
+	if (!S_ISDIR(p->mode))
+		return 0;
+
+	/*
+	 * If the path is inside the sparse cone, it can't be a sparse directory.
+	 */
+	strbuf_add(&dirpath, info->traverse_path, info->pathlen);
+	strbuf_add(&dirpath, p->path, p->pathlen);
+	strbuf_addch(&dirpath, '/');
+	if (path_in_cone_mode_sparse_checkout(dirpath.buf, o->src_index)) {
+		res = 0;
+		goto cleanup;
+	}
+
+	pos = index_name_pos_sparse(o->src_index, dirpath.buf, dirpath.len);
+	if (pos >= 0) {
+		/* Path is already in the index, not a new sparse dir */
+		res = 0;
+		goto cleanup;
+	}
+
+	/* Where would this sparse dir be inserted into the index? */
+	pos = -pos - 1;
+	if (pos >= o->src_index->cache_nr) {
+		/*
+		 * Sparse dir would be inserted at the end of the index, so we
+		 * know it has no child entries.
+		 */
+		res = 1;
+		goto cleanup;
+	}
+
+	/*
+	 * If the dir has child entries in the index, the first would be at the
+	 * position the sparse directory would be inserted. If the entry at this
+	 * position is inside the dir, not a new sparse dir.
+	 */
+	res = strncmp(o->src_index->cache[pos]->name, dirpath.buf, dirpath.len);
+
+cleanup:
+	strbuf_release(&dirpath);
+	return res;
+}
+
+/*
+ * Note that traverse_by_cache_tree() duplicates some logic in this function
+ * without actually calling it. If you change the logic here you may need to
+ * check and change there as well.
+ */
+static int unpack_single_entry(int n, unsigned long mask,
+			       unsigned long dirmask,
+			       struct cache_entry **src,
+			       const struct name_entry *names,
+			       const struct traverse_info *info,
+			       int *is_new_sparse_dir)
+{
+	int i;
+	struct unpack_trees_options *o = info->data;
+	unsigned long conflicts = info->df_conflicts | dirmask;
+	const struct name_entry *p = names;
+
+	*is_new_sparse_dir = 0;
+	if (mask == dirmask && !src[0]) {
+		/*
+		 * If we're not in a sparse index, we can't unpack a directory
+		 * without recursing into it, so we return.
+		 */
+		if (!o->src_index->sparse_index)
+			return 0;
+
+		/* Find first entry with a real name (we could use "mask" too) */
+		while (!p->mode)
+			p++;
+
+		/*
+		 * If the directory is completely missing from the index but
+		 * would otherwise be a sparse directory, we should unpack it.
+		 * If not, we'll return and continue recursively traversing the
+		 * tree.
+		 */
+		*is_new_sparse_dir = entry_is_new_sparse_dir(info, p);
+		if (!*is_new_sparse_dir)
+			return 0;
+	}
+
+	/*
+	 * When we are unpacking a sparse directory, then this isn't necessarily
+	 * a directory-file conflict.
+	 */
+	if (mask == dirmask &&
+	    (*is_new_sparse_dir || (src[0] && S_ISSPARSEDIR(src[0]->ce_mode))))
+		conflicts = 0;
+
+	/*
+	 * Ok, we've filled in up to any potential index entry in src[0],
+	 * now do the rest.
+	 */
+	for (i = 0; i < n; i++) {
+		int stage;
+		unsigned int bit = 1ul << i;
+		if (conflicts & bit) {
+			src[i + o->merge] = o->df_conflict_entry;
+			continue;
+		}
+		if (!(mask & bit))
+			continue;
+		if (!o->merge)
+			stage = 0;
+		else if (i + 1 < o->head_idx)
+			stage = 1;
+		else if (i + 1 > o->head_idx)
+			stage = 3;
+		else
+			stage = 2;
+
+		/*
+		 * If the merge bit is set, then the cache entries are
+		 * discarded in the following block.  In this case,
+		 * construct "transient" cache_entries, as they are
+		 * not stored in the index.  otherwise construct the
+		 * cache entry from the index aware logic.
+		 */
+		src[i + o->merge] = create_ce_entry(info, names + i, stage,
+						    &o->internal.result,
+						    o->merge, bit & dirmask);
+	}
+
+	if (o->merge) {
+		int rc = call_unpack_fn((const struct cache_entry * const *)src,
+					o);
+		for (i = 0; i < n; i++) {
+			struct cache_entry *ce = src[i + o->merge];
+			if (ce != o->df_conflict_entry)
+				discard_cache_entry(ce);
+		}
+		return rc;
+	}
+
+	for (i = 0; i < n; i++)
+		if (src[i] && src[i] != o->df_conflict_entry)
+			if (do_add_entry(o, src[i], 0, 0))
+				return -1;
+
+	return 0;
+}
+
+static int unpack_failed(struct unpack_trees_options *o, const char *message)
+{
+	discard_index(&o->internal.result);
+	if (!o->quiet && !o->exiting_early) {
+		if (message)
+			return error("%s", message);
+		return -1;
+	}
+	return -1;
+}
+
+/*
+ * The tree traversal is looking at name p.  If we have a matching entry,
+ * return it.  If name p is a directory in the index, do not return
+ * anything, as we will want to match it when the traversal descends into
+ * the directory.
+ */
+static int find_cache_pos(struct traverse_info *info,
+			  const char *p, size_t p_len)
+{
+	int pos;
+	struct unpack_trees_options *o = info->data;
+	struct index_state *index = o->src_index;
+	int pfxlen = info->pathlen;
+
+	for (pos = o->internal.cache_bottom; pos < index->cache_nr; pos++) {
+		const struct cache_entry *ce = index->cache[pos];
+		const char *ce_name, *ce_slash;
+		int cmp, ce_len;
+
+		if (ce->ce_flags & CE_UNPACKED) {
+			/*
+			 * cache_bottom entry is already unpacked, so
+			 * we can never match it; don't check it
+			 * again.
+			 */
+			if (pos == o->internal.cache_bottom)
+				++o->internal.cache_bottom;
+			continue;
+		}
+		if (!ce_in_traverse_path(ce, info)) {
+			/*
+			 * Check if we can skip future cache checks
+			 * (because we're already past all possible
+			 * entries in the traverse path).
+			 */
+			if (info->traverse_path) {
+				if (strncmp(ce->name, info->traverse_path,
+					    info->pathlen) > 0)
+					break;
+			}
+			continue;
+		}
+		ce_name = ce->name + pfxlen;
+		ce_slash = strchr(ce_name, '/');
+		if (ce_slash)
+			ce_len = ce_slash - ce_name;
+		else
+			ce_len = ce_namelen(ce) - pfxlen;
+		cmp = name_compare(p, p_len, ce_name, ce_len);
+		/*
+		 * Exact match; if we have a directory we need to
+		 * delay returning it.
+		 */
+		if (!cmp)
+			return ce_slash ? -2 - pos : pos;
+		if (0 < cmp)
+			continue; /* keep looking */
+		/*
+		 * ce_name sorts after p->path; could it be that we
+		 * have files under p->path directory in the index?
+		 * E.g.  ce_name == "t-i", and p->path == "t"; we may
+		 * have "t/a" in the index.
+		 */
+		if (p_len < ce_len && !memcmp(ce_name, p, p_len) &&
+		    ce_name[p_len] < '/')
+			continue; /* keep looking */
+		break;
+	}
+	return -1;
+}
+
+/*
+ * Given a sparse directory entry 'ce', compare ce->name to
+ * info->traverse_path + p->path + '/' if info->traverse_path
+ * is non-empty.
+ *
+ * Compare ce->name to p->path + '/' otherwise. Note that
+ * ce->name must end in a trailing '/' because it is a sparse
+ * directory entry.
+ */
+static int sparse_dir_matches_path(const struct cache_entry *ce,
+				   struct traverse_info *info,
+				   const struct name_entry *p)
+{
+	assert(S_ISSPARSEDIR(ce->ce_mode));
+	assert(ce->name[ce->ce_namelen - 1] == '/');
+
+	if (info->pathlen)
+		return ce->ce_namelen == info->pathlen + p->pathlen + 1 &&
+		       ce->name[info->pathlen - 1] == '/' &&
+		       !strncmp(ce->name, info->traverse_path, info->pathlen) &&
+		       !strncmp(ce->name + info->pathlen, p->path, p->pathlen);
+	return ce->ce_namelen == p->pathlen + 1 &&
+	       !strncmp(ce->name, p->path, p->pathlen);
+}
+
+static struct cache_entry *find_cache_entry(struct traverse_info *info,
+					    const struct name_entry *p)
+{
+	const char *path;
+	int pos = find_cache_pos(info, p->path, p->pathlen);
+	struct unpack_trees_options *o = info->data;
+
+	if (0 <= pos)
+		return o->src_index->cache[pos];
+
+	/*
+	 * Check for a sparse-directory entry named "path/".
+	 * Due to the input p->path not having a trailing
+	 * slash, the negative 'pos' value overshoots the
+	 * expected position, hence "-2" instead of "-1".
+	 */
+	pos = -pos - 2;
+
+	if (pos < 0 || pos >= o->src_index->cache_nr)
+		return NULL;
+
+	/*
+	 * Due to lexicographic sorting and sparse directory
+	 * entries ending with a trailing slash, our path as a
+	 * sparse directory (e.g "subdir/") and	our path as a
+	 * file (e.g. "subdir") might be separated by other
+	 * paths (e.g. "subdir-").
+	 */
+	while (pos >= 0) {
+		struct cache_entry *ce = o->src_index->cache[pos];
+
+		if (!skip_prefix(ce->name, info->traverse_path, &path) ||
+		    strncmp(path, p->path, p->pathlen) ||
+		    path[p->pathlen] != '/')
+			return NULL;
+
+		if (S_ISSPARSEDIR(ce->ce_mode) &&
+		    sparse_dir_matches_path(ce, info, p))
+			return ce;
+
+		pos--;
+	}
+
+	return NULL;
+}
+
+static void debug_path(struct traverse_info *info)
+{
+	if (info->prev) {
+		debug_path(info->prev);
+		if (*info->prev->name)
+			putchar('/');
+	}
+	printf("%s", info->name);
+}
+
+static void debug_name_entry(int i, struct name_entry *n)
+{
+	printf("ent#%d %06o %s\n", i,
+	       n->path ? n->mode : 0,
+	       n->path ? n->path : "(missing)");
+}
+
+static void debug_unpack_callback(int n,
+				  unsigned long mask,
+				  unsigned long dirmask,
+				  struct name_entry *names,
+				  struct traverse_info *info)
+{
+	int i;
+	printf("* unpack mask %lu, dirmask %lu, cnt %d ",
+	       mask, dirmask, n);
+	debug_path(info);
+	putchar('\n');
+	for (i = 0; i < n; i++)
+		debug_name_entry(i, names + i);
+}
+
+/*
+ * Returns true if and only if the given cache_entry is a
+ * sparse-directory entry that matches the given name_entry
+ * from the tree walk at the given traverse_info.
+ */
+static int is_sparse_directory_entry(struct cache_entry *ce,
+				     const struct name_entry *name,
+				     struct traverse_info *info)
+{
+	if (!ce || !name || !S_ISSPARSEDIR(ce->ce_mode))
+		return 0;
+
+	return sparse_dir_matches_path(ce, info, name);
+}
+
+static int unpack_sparse_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *names, struct traverse_info *info)
+{
+	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
+	struct unpack_trees_options *o = info->data;
+	int ret, is_new_sparse_dir;
+
+	assert(o->merge);
+
+	/*
+	 * Unlike in 'unpack_callback', where src[0] is derived from the index when
+	 * merging, src[0] is a transient cache entry derived from the first tree
+	 * provided. Create the temporary entry as if it came from a non-sparse index.
+	 */
+	if (!is_null_oid(&names[0].oid)) {
+		src[0] = create_ce_entry(info, &names[0], 0,
+					&o->internal.result, 1,
+					dirmask & (1ul << 0));
+		src[0]->ce_flags |= (CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE);
+	}
+
+	/*
+	 * 'unpack_single_entry' assumes that src[0] is derived directly from
+	 * the index, rather than from an entry in 'names'. This is *not* true when
+	 * merging a sparse directory, in which case names[0] is the "index" source
+	 * entry. To match the expectations of 'unpack_single_entry', shift past the
+	 * "index" tree (i.e., names[0]) and adjust 'names', 'n', 'mask', and
+	 * 'dirmask' accordingly.
+	 */
+	ret = unpack_single_entry(n - 1, mask >> 1, dirmask >> 1, src, names + 1, info, &is_new_sparse_dir);
+
+	if (src[0])
+		discard_cache_entry(src[0]);
+
+	return ret >= 0 ? mask : -1;
+}
+
+/*
+ * Note that traverse_by_cache_tree() duplicates some logic in this function
+ * without actually calling it. If you change the logic here you may need to
+ * check and change there as well.
+ */
+static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *names, struct traverse_info *info)
+{
+	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
+	struct unpack_trees_options *o = info->data;
+	const struct name_entry *p = names;
+	int is_new_sparse_dir;
+
+	/* Find first entry with a real name (we could use "mask" too) */
+	while (!p->mode)
+		p++;
+
+	if (o->internal.debug_unpack)
+		debug_unpack_callback(n, mask, dirmask, names, info);
+
+	/* Are we supposed to look at the index too? */
+	if (o->merge) {
+		while (1) {
+			int cmp;
+			struct cache_entry *ce;
+
+			if (o->diff_index_cached)
+				ce = next_cache_entry(o);
+			else
+				ce = find_cache_entry(info, p);
+
+			if (!ce)
+				break;
+			cmp = compare_entry(ce, info, p);
+			if (cmp < 0) {
+				if (unpack_index_entry(ce, o) < 0)
+					return unpack_failed(o, NULL);
+				continue;
+			}
+			if (!cmp) {
+				if (ce_stage(ce)) {
+					/*
+					 * If we skip unmerged index
+					 * entries, we'll skip this
+					 * entry *and* the tree
+					 * entries associated with it!
+					 */
+					if (o->skip_unmerged) {
+						add_same_unmerged(ce, o);
+						return mask;
+					}
+				}
+				src[0] = ce;
+			}
+			break;
+		}
+	}
+
+	if (unpack_single_entry(n, mask, dirmask, src, names, info, &is_new_sparse_dir))
+		return -1;
+
+	if (o->merge && src[0]) {
+		if (ce_stage(src[0]))
+			mark_ce_used_same_name(src[0], o);
+		else
+			mark_ce_used(src[0], o);
+	}
+
+	/* Now handle any directories.. */
+	if (dirmask) {
+		/* special case: "diff-index --cached" looking at a tree */
+		if (o->diff_index_cached &&
+		    n == 1 && dirmask == 1 && S_ISDIR(names->mode)) {
+			int matches;
+			matches = cache_tree_matches_traversal(o->src_index->cache_tree,
+							       names, info);
+			/*
+			 * Everything under the name matches; skip the
+			 * entire hierarchy.  diff_index_cached codepath
+			 * special cases D/F conflicts in such a way that
+			 * it does not do any look-ahead, so this is safe.
+			 */
+			if (matches) {
+				/*
+				 * Only increment the cache_bottom if the
+				 * directory isn't a sparse directory index
+				 * entry (if it is, it was already incremented)
+				 * in 'mark_ce_used()'
+				 */
+				if (!src[0] || !S_ISSPARSEDIR(src[0]->ce_mode))
+					o->internal.cache_bottom += matches;
+				return mask;
+			}
+		}
+
+		if (!is_sparse_directory_entry(src[0], p, info) &&
+		    !is_new_sparse_dir &&
+		    traverse_trees_recursive(n, dirmask, mask & ~dirmask,
+						    names, info) < 0) {
+			return -1;
+		}
+
+		return mask;
+	}
+
+	return mask;
+}
+
+static int clear_ce_flags_1(struct index_state *istate,
+			    struct cache_entry **cache, int nr,
+			    struct strbuf *prefix,
+			    int select_mask, int clear_mask,
+			    struct pattern_list *pl,
+			    enum pattern_match_result default_match,
+			    int progress_nr);
+
+/* Whole directory matching */
+static int clear_ce_flags_dir(struct index_state *istate,
+			      struct cache_entry **cache, int nr,
+			      struct strbuf *prefix,
+			      char *basename,
+			      int select_mask, int clear_mask,
+			      struct pattern_list *pl,
+			      enum pattern_match_result default_match,
+			      int progress_nr)
+{
+	struct cache_entry **cache_end;
+	int dtype = DT_DIR;
+	int rc;
+	enum pattern_match_result ret, orig_ret;
+	orig_ret = path_matches_pattern_list(prefix->buf, prefix->len,
+					     basename, &dtype, pl, istate);
+
+	strbuf_addch(prefix, '/');
+
+	/* If undecided, use matching result of parent dir in defval */
+	if (orig_ret == UNDECIDED)
+		ret = default_match;
+	else
+		ret = orig_ret;
+
+	for (cache_end = cache; cache_end != cache + nr; cache_end++) {
+		struct cache_entry *ce = *cache_end;
+		if (strncmp(ce->name, prefix->buf, prefix->len))
+			break;
+	}
+
+	if (pl->use_cone_patterns && orig_ret == MATCHED_RECURSIVE) {
+		struct cache_entry **ce = cache;
+		rc = cache_end - cache;
+
+		while (ce < cache_end) {
+			(*ce)->ce_flags &= ~clear_mask;
+			ce++;
+		}
+	} else if (pl->use_cone_patterns && orig_ret == NOT_MATCHED) {
+		rc = cache_end - cache;
+	} else {
+		rc = clear_ce_flags_1(istate, cache, cache_end - cache,
+				      prefix,
+				      select_mask, clear_mask,
+				      pl, ret,
+				      progress_nr);
+	}
+
+	strbuf_setlen(prefix, prefix->len - 1);
+	return rc;
+}
+
+/*
+ * Traverse the index, find every entry that matches according to
+ * o->pl. Do "ce_flags &= ~clear_mask" on those entries. Return the
+ * number of traversed entries.
+ *
+ * If select_mask is non-zero, only entries whose ce_flags has on of
+ * those bits enabled are traversed.
+ *
+ * cache	: pointer to an index entry
+ * prefix_len	: an offset to its path
+ *
+ * The current path ("prefix") including the trailing '/' is
+ *   cache[0]->name[0..(prefix_len-1)]
+ * Top level path has prefix_len zero.
+ */
+static int clear_ce_flags_1(struct index_state *istate,
+			    struct cache_entry **cache, int nr,
+			    struct strbuf *prefix,
+			    int select_mask, int clear_mask,
+			    struct pattern_list *pl,
+			    enum pattern_match_result default_match,
+			    int progress_nr)
+{
+	struct cache_entry **cache_end = nr ? cache + nr : cache;
+
+	/*
+	 * Process all entries that have the given prefix and meet
+	 * select_mask condition
+	 */
+	while(cache != cache_end) {
+		struct cache_entry *ce = *cache;
+		const char *name, *slash;
+		int len, dtype;
+		enum pattern_match_result ret;
+
+		display_progress(istate->progress, progress_nr);
+
+		if (select_mask && !(ce->ce_flags & select_mask)) {
+			cache++;
+			progress_nr++;
+			continue;
+		}
+
+		if (prefix->len && strncmp(ce->name, prefix->buf, prefix->len))
+			break;
+
+		name = ce->name + prefix->len;
+		slash = strchr(name, '/');
+
+		/* If it's a directory, try whole directory match first */
+		if (slash) {
+			int processed;
+
+			len = slash - name;
+			strbuf_add(prefix, name, len);
+
+			processed = clear_ce_flags_dir(istate, cache, cache_end - cache,
+						       prefix,
+						       prefix->buf + prefix->len - len,
+						       select_mask, clear_mask,
+						       pl, default_match,
+						       progress_nr);
+
+			/* clear_c_f_dir eats a whole dir already? */
+			if (processed) {
+				cache += processed;
+				progress_nr += processed;
+				strbuf_setlen(prefix, prefix->len - len);
+				continue;
+			}
+
+			strbuf_addch(prefix, '/');
+			processed = clear_ce_flags_1(istate, cache, cache_end - cache,
+						     prefix,
+						     select_mask, clear_mask, pl,
+						     default_match, progress_nr);
+
+			cache += processed;
+			progress_nr += processed;
+
+			strbuf_setlen(prefix, prefix->len - len - 1);
+			continue;
+		}
+
+		/* Non-directory */
+		dtype = ce_to_dtype(ce);
+		ret = path_matches_pattern_list(ce->name,
+						ce_namelen(ce),
+						name, &dtype, pl, istate);
+		if (ret == UNDECIDED)
+			ret = default_match;
+		if (ret == MATCHED || ret == MATCHED_RECURSIVE)
+			ce->ce_flags &= ~clear_mask;
+		cache++;
+		progress_nr++;
+	}
+
+	display_progress(istate->progress, progress_nr);
+	return nr - (cache_end - cache);
+}
+
+static int clear_ce_flags(struct index_state *istate,
+			  int select_mask, int clear_mask,
+			  struct pattern_list *pl,
+			  int show_progress)
+{
+	static struct strbuf prefix = STRBUF_INIT;
+	char label[100];
+	int rval;
+
+	strbuf_reset(&prefix);
+	if (show_progress)
+		istate->progress = start_delayed_progress(
+					_("Updating index flags"),
+					istate->cache_nr);
+
+	xsnprintf(label, sizeof(label), "clear_ce_flags(0x%08lx,0x%08lx)",
+		  (unsigned long)select_mask, (unsigned long)clear_mask);
+	trace2_region_enter("unpack_trees", label, the_repository);
+	rval = clear_ce_flags_1(istate,
+				istate->cache,
+				istate->cache_nr,
+				&prefix,
+				select_mask, clear_mask,
+				pl, 0, 0);
+	trace2_region_leave("unpack_trees", label, the_repository);
+
+	stop_progress(&istate->progress);
+	return rval;
+}
+
+/*
+ * Set/Clear CE_NEW_SKIP_WORKTREE according to $GIT_DIR/info/sparse-checkout
+ */
+static void mark_new_skip_worktree(struct pattern_list *pl,
+				   struct index_state *istate,
+				   int select_flag, int skip_wt_flag,
+				   int show_progress)
+{
+	int i;
+
+	/*
+	 * 1. Pretend the narrowest worktree: only unmerged entries
+	 * are checked out
+	 */
+	for (i = 0; i < istate->cache_nr; i++) {
+		struct cache_entry *ce = istate->cache[i];
+
+		if (select_flag && !(ce->ce_flags & select_flag))
+			continue;
+
+		if (!ce_stage(ce) && !(ce->ce_flags & CE_CONFLICTED))
+			ce->ce_flags |= skip_wt_flag;
+		else
+			ce->ce_flags &= ~skip_wt_flag;
+	}
+
+	/*
+	 * 2. Widen worktree according to sparse-checkout file.
+	 * Matched entries will have skip_wt_flag cleared (i.e. "in")
+	 */
+	clear_ce_flags(istate, select_flag, skip_wt_flag, pl, show_progress);
+}
+
+static void populate_from_existing_patterns(struct unpack_trees_options *o,
+					    struct pattern_list *pl)
+{
+	if (get_sparse_checkout_patterns(pl) < 0)
+		o->skip_sparse_checkout = 1;
+	else
+		o->internal.pl = pl;
+}
+
+static void update_sparsity_for_prefix(const char *prefix,
+				       struct index_state *istate)
+{
+	int prefix_len = strlen(prefix);
+	struct strbuf ce_prefix = STRBUF_INIT;
+
+	if (!istate->sparse_index)
+		return;
+
+	while (prefix_len > 0 && prefix[prefix_len - 1] == '/')
+		prefix_len--;
+
+	if (prefix_len <= 0)
+		BUG("Invalid prefix passed to update_sparsity_for_prefix");
+
+	strbuf_grow(&ce_prefix, prefix_len + 1);
+	strbuf_add(&ce_prefix, prefix, prefix_len);
+	strbuf_addch(&ce_prefix, '/');
+
+	/*
+	 * If the prefix points to a sparse directory or a path inside a sparse
+	 * directory, the index should be expanded. This is accomplished in one
+	 * of two ways:
+	 * - if the prefix is inside a sparse directory, it will be expanded by
+	 *   the 'ensure_full_index(...)' call in 'index_name_pos(...)'.
+	 * - if the prefix matches an existing sparse directory entry,
+	 *   'index_name_pos(...)' will return its index position, triggering
+	 *   the 'ensure_full_index(...)' below.
+	 */
+	if (!path_in_cone_mode_sparse_checkout(ce_prefix.buf, istate) &&
+	    index_name_pos(istate, ce_prefix.buf, ce_prefix.len) >= 0)
+		ensure_full_index(istate);
+
+	strbuf_release(&ce_prefix);
+}
+
+static int verify_absent(const struct cache_entry *,
+			 enum unpack_trees_error_types,
+			 struct unpack_trees_options *);
+/*
+ * N-way merge "len" trees.  Returns 0 on success, -1 on failure to manipulate the
+ * resulting index, -2 on failure to reflect the changes to the work tree.
+ *
+ * CE_ADDED, CE_UNPACKED and CE_NEW_SKIP_WORKTREE are used internally
+ */
+int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options *o)
+{
+	struct repository *repo = the_repository;
+	int i, ret;
+	static struct cache_entry *dfc;
+	struct pattern_list pl;
+	int free_pattern_list = 0;
+	struct dir_struct dir = DIR_INIT;
+
+	if (o->reset == UNPACK_RESET_INVALID)
+		BUG("o->reset had a value of 1; should be UNPACK_TREES_*_UNTRACKED");
+
+	if (len > MAX_UNPACK_TREES)
+		die("unpack_trees takes at most %d trees", MAX_UNPACK_TREES);
+	if (o->internal.dir)
+		BUG("o->internal.dir is for internal use only");
+	if (o->internal.pl)
+		BUG("o->internal.pl is for internal use only");
+	if (o->df_conflict_entry)
+		BUG("o->df_conflict_entry is an output only field");
+
+	trace_performance_enter();
+	trace2_region_enter("unpack_trees", "unpack_trees", the_repository);
+
+	prepare_repo_settings(repo);
+	if (repo->settings.command_requires_full_index) {
+		ensure_full_index(o->src_index);
+		if (o->dst_index)
+			ensure_full_index(o->dst_index);
+	}
+
+	if (o->reset == UNPACK_RESET_OVERWRITE_UNTRACKED &&
+	    o->preserve_ignored)
+		BUG("UNPACK_RESET_OVERWRITE_UNTRACKED incompatible with preserved ignored files");
+
+	if (!o->preserve_ignored) {
+		o->internal.dir = &dir;
+		o->internal.dir->flags |= DIR_SHOW_IGNORED;
+		setup_standard_excludes(o->internal.dir);
+	}
+
+	if (o->prefix)
+		update_sparsity_for_prefix(o->prefix, o->src_index);
+
+	if (!core_apply_sparse_checkout || !o->update)
+		o->skip_sparse_checkout = 1;
+	if (!o->skip_sparse_checkout) {
+		memset(&pl, 0, sizeof(pl));
+		free_pattern_list = 1;
+		populate_from_existing_patterns(o, &pl);
+	}
+
+	index_state_init(&o->internal.result, o->src_index->repo);
+	o->internal.result.initialized = 1;
+	o->internal.result.timestamp.sec = o->src_index->timestamp.sec;
+	o->internal.result.timestamp.nsec = o->src_index->timestamp.nsec;
+	o->internal.result.version = o->src_index->version;
+	if (!o->src_index->split_index) {
+		o->internal.result.split_index = NULL;
+	} else if (o->src_index == o->dst_index) {
+		/*
+		 * o->dst_index (and thus o->src_index) will be discarded
+		 * and overwritten with o->internal.result at the end of
+		 * this function, so just use src_index's split_index to
+		 * avoid having to create a new one.
+		 */
+		o->internal.result.split_index = o->src_index->split_index;
+		if (o->src_index->cache_changed & SPLIT_INDEX_ORDERED)
+			o->internal.result.cache_changed |= SPLIT_INDEX_ORDERED;
+		o->internal.result.split_index->refcount++;
+	} else {
+		o->internal.result.split_index =
+			init_split_index(&o->internal.result);
+	}
+	oidcpy(&o->internal.result.oid, &o->src_index->oid);
+	o->internal.merge_size = len;
+	mark_all_ce_unused(o->src_index);
+
+	o->internal.result.fsmonitor_last_update =
+		xstrdup_or_null(o->src_index->fsmonitor_last_update);
+	o->internal.result.fsmonitor_has_run_once = o->src_index->fsmonitor_has_run_once;
+
+	if (!o->src_index->initialized &&
+	    !repo->settings.command_requires_full_index &&
+	    is_sparse_index_allowed(&o->internal.result, 0))
+		o->internal.result.sparse_index = 1;
+
+	/*
+	 * Sparse checkout loop #1: set NEW_SKIP_WORKTREE on existing entries
+	 */
+	if (!o->skip_sparse_checkout)
+		mark_new_skip_worktree(o->internal.pl, o->src_index, 0,
+				       CE_NEW_SKIP_WORKTREE, o->verbose_update);
+
+	if (!dfc)
+		dfc = xcalloc(1, cache_entry_size(0));
+	o->df_conflict_entry = dfc;
+
+	if (len) {
+		const char *prefix = o->prefix ? o->prefix : "";
+		struct traverse_info info;
+
+		setup_traverse_info(&info, prefix);
+		info.fn = unpack_callback;
+		info.data = o;
+		info.show_all_errors = o->internal.show_all_errors;
+		info.pathspec = o->pathspec;
+
+		if (o->prefix) {
+			/*
+			 * Unpack existing index entries that sort before the
+			 * prefix the tree is spliced into.  Note that o->merge
+			 * is always true in this case.
+			 */
+			while (1) {
+				struct cache_entry *ce = next_cache_entry(o);
+				if (!ce)
+					break;
+				if (ce_in_traverse_path(ce, &info))
+					break;
+				if (unpack_index_entry(ce, o) < 0)
+					goto return_failed;
+			}
+		}
+
+		trace_performance_enter();
+		trace2_region_enter("unpack_trees", "traverse_trees", the_repository);
+		ret = traverse_trees(o->src_index, len, t, &info);
+		trace2_region_leave("unpack_trees", "traverse_trees", the_repository);
+		trace_performance_leave("traverse_trees");
+		if (ret < 0)
+			goto return_failed;
+	}
+
+	/* Any left-over entries in the index? */
+	if (o->merge) {
+		while (1) {
+			struct cache_entry *ce = next_cache_entry(o);
+			if (!ce)
+				break;
+			if (unpack_index_entry(ce, o) < 0)
+				goto return_failed;
+		}
+	}
+	mark_all_ce_unused(o->src_index);
+
+	if (o->trivial_merges_only && o->internal.nontrivial_merge) {
+		ret = unpack_failed(o, "Merge requires file-level merging");
+		goto done;
+	}
+
+	if (!o->skip_sparse_checkout) {
+		/*
+		 * Sparse checkout loop #2: set NEW_SKIP_WORKTREE on entries not in loop #1
+		 * If they will have NEW_SKIP_WORKTREE, also set CE_SKIP_WORKTREE
+		 * so apply_sparse_checkout() won't attempt to remove it from worktree
+		 */
+		mark_new_skip_worktree(o->internal.pl, &o->internal.result,
+				       CE_ADDED, CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE,
+				       o->verbose_update);
+
+		ret = 0;
+		for (i = 0; i < o->internal.result.cache_nr; i++) {
+			struct cache_entry *ce = o->internal.result.cache[i];
+
+			/*
+			 * Entries marked with CE_ADDED in merged_entry() do not have
+			 * verify_absent() check (the check is effectively disabled
+			 * because CE_NEW_SKIP_WORKTREE is set unconditionally).
+			 *
+			 * Do the real check now because we have had
+			 * correct CE_NEW_SKIP_WORKTREE
+			 */
+			if (ce->ce_flags & CE_ADDED &&
+			    verify_absent(ce, WARNING_SPARSE_ORPHANED_NOT_OVERWRITTEN, o))
+				ret = 1;
+
+			if (apply_sparse_checkout(&o->internal.result, ce, o))
+				ret = 1;
+		}
+		if (ret == 1) {
+			/*
+			 * Inability to sparsify or de-sparsify individual
+			 * paths is not an error, but just a warning.
+			 */
+			if (o->internal.show_all_errors)
+				display_warning_msgs(o);
+			ret = 0;
+		}
+	}
+
+	ret = check_updates(o, &o->internal.result) ? (-2) : 0;
+	if (o->dst_index) {
+		move_index_extensions(&o->internal.result, o->src_index);
+		if (!ret) {
+			if (git_env_bool("GIT_TEST_CHECK_CACHE_TREE", 0))
+				cache_tree_verify(the_repository,
+						  &o->internal.result);
+			if (!o->skip_cache_tree_update &&
+			    !cache_tree_fully_valid(o->internal.result.cache_tree))
+				cache_tree_update(&o->internal.result,
+						  WRITE_TREE_SILENT |
+						  WRITE_TREE_REPAIR);
+		}
+
+		o->internal.result.updated_workdir = 1;
+		discard_index(o->dst_index);
+		*o->dst_index = o->internal.result;
+	} else {
+		discard_index(&o->internal.result);
+	}
+	o->src_index = NULL;
+
+done:
+	if (free_pattern_list)
+		clear_pattern_list(&pl);
+	if (o->internal.dir) {
+		dir_clear(o->internal.dir);
+		o->internal.dir = NULL;
+	}
+	trace2_region_leave("unpack_trees", "unpack_trees", the_repository);
+	trace_performance_leave("unpack_trees");
+	return ret;
+
+return_failed:
+	if (o->internal.show_all_errors)
+		display_error_msgs(o);
+	mark_all_ce_unused(o->src_index);
+	ret = unpack_failed(o, NULL);
+	if (o->exiting_early)
+		ret = 0;
+	goto done;
+}
+
+/*
+ * Update SKIP_WORKTREE bits according to sparsity patterns, and update
+ * working directory to match.
+ *
+ * CE_NEW_SKIP_WORKTREE is used internally.
+ */
+enum update_sparsity_result update_sparsity(struct unpack_trees_options *o,
+					    struct pattern_list *pl)
+{
+	enum update_sparsity_result ret = UPDATE_SPARSITY_SUCCESS;
+	int i;
+	unsigned old_show_all_errors;
+	int free_pattern_list = 0;
+
+	old_show_all_errors = o->internal.show_all_errors;
+	o->internal.show_all_errors = 1;
+	index_state_init(&o->internal.result, o->src_index->repo);
+
+	/* Sanity checks */
+	if (!o->update || o->index_only || o->skip_sparse_checkout)
+		BUG("update_sparsity() is for reflecting sparsity patterns in working directory");
+	if (o->src_index != o->dst_index || o->fn)
+		BUG("update_sparsity() called wrong");
+
+	trace_performance_enter();
+
+	/* If we weren't given patterns, use the recorded ones */
+	if (!pl) {
+		free_pattern_list = 1;
+		pl = xcalloc(1, sizeof(*pl));
+		populate_from_existing_patterns(o, pl);
+	}
+	o->internal.pl = pl;
+
+	/* Expand sparse directories as needed */
+	expand_index(o->src_index, o->internal.pl);
+
+	/* Set NEW_SKIP_WORKTREE on existing entries. */
+	mark_all_ce_unused(o->src_index);
+	mark_new_skip_worktree(o->internal.pl, o->src_index, 0,
+			       CE_NEW_SKIP_WORKTREE, o->verbose_update);
+
+	/* Then loop over entries and update/remove as needed */
+	ret = UPDATE_SPARSITY_SUCCESS;
+	for (i = 0; i < o->src_index->cache_nr; i++) {
+		struct cache_entry *ce = o->src_index->cache[i];
+
+
+		if (ce_stage(ce)) {
+			/* -1 because for loop will increment by 1 */
+			i += warn_conflicted_path(o->src_index, i, o) - 1;
+			ret = UPDATE_SPARSITY_WARNINGS;
+			continue;
+		}
+
+		if (apply_sparse_checkout(o->src_index, ce, o))
+			ret = UPDATE_SPARSITY_WARNINGS;
+	}
+
+	if (check_updates(o, o->src_index))
+		ret = UPDATE_SPARSITY_WORKTREE_UPDATE_FAILURES;
+
+	display_warning_msgs(o);
+	o->internal.show_all_errors = old_show_all_errors;
+	if (free_pattern_list) {
+		clear_pattern_list(pl);
+		free(pl);
+		o->internal.pl = NULL;
+	}
+	trace_performance_leave("update_sparsity");
+	return ret;
+}
+
+/* Here come the merge functions */
+
+static int reject_merge(const struct cache_entry *ce,
+			struct unpack_trees_options *o)
+{
+	return add_rejected_path(o, ERROR_WOULD_OVERWRITE, ce->name);
+}
+
+static int same(const struct cache_entry *a, const struct cache_entry *b)
+{
+	if (!!a != !!b)
+		return 0;
+	if (!a && !b)
+		return 1;
+	if ((a->ce_flags | b->ce_flags) & CE_CONFLICTED)
+		return 0;
+	return a->ce_mode == b->ce_mode &&
+	       oideq(&a->oid, &b->oid);
+}
+
+
+/*
+ * When a CE gets turned into an unmerged entry, we
+ * want it to be up-to-date
+ */
+static int verify_uptodate_1(const struct cache_entry *ce,
+			     struct unpack_trees_options *o,
+			     enum unpack_trees_error_types error_type)
+{
+	struct stat st;
+
+	if (o->index_only)
+		return 0;
+
+	/*
+	 * CE_VALID and CE_SKIP_WORKTREE cheat, we better check again
+	 * if this entry is truly up-to-date because this file may be
+	 * overwritten.
+	 */
+	if ((ce->ce_flags & CE_VALID) || ce_skip_worktree(ce))
+		; /* keep checking */
+	else if (o->reset || ce_uptodate(ce))
+		return 0;
+
+	if (!lstat(ce->name, &st)) {
+		int flags = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE;
+		unsigned changed = ie_match_stat(o->src_index, ce, &st, flags);
+
+		if (submodule_from_ce(ce)) {
+			int r = check_submodule_move_head(ce,
+				"HEAD", oid_to_hex(&ce->oid), o);
+			if (r)
+				return add_rejected_path(o, error_type, ce->name);
+			return 0;
+		}
+
+		if (!changed)
+			return 0;
+		/*
+		 * Historic default policy was to allow submodule to be out
+		 * of sync wrt the superproject index. If the submodule was
+		 * not considered interesting above, we don't care here.
+		 */
+		if (S_ISGITLINK(ce->ce_mode))
+			return 0;
+
+		errno = 0;
+	}
+	if (errno == ENOENT)
+		return 0;
+	return add_rejected_path(o, error_type, ce->name);
+}
+
+int verify_uptodate(const struct cache_entry *ce,
+		    struct unpack_trees_options *o)
+{
+	if (!o->skip_sparse_checkout &&
+	    (ce->ce_flags & CE_SKIP_WORKTREE) &&
+	    (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
+		return 0;
+	return verify_uptodate_1(ce, o, ERROR_NOT_UPTODATE_FILE);
+}
+
+static int verify_uptodate_sparse(const struct cache_entry *ce,
+				  struct unpack_trees_options *o)
+{
+	return verify_uptodate_1(ce, o, WARNING_SPARSE_NOT_UPTODATE_FILE);
+}
+
+/*
+ * TODO: We should actually invalidate o->internal.result, not src_index [1].
+ * But since cache tree and untracked cache both are not copied to
+ * o->internal.result until unpacking is complete, we invalidate them on
+ * src_index instead with the assumption that they will be copied to
+ * dst_index at the end.
+ *
+ * [1] src_index->cache_tree is also used in unpack_callback() so if
+ * we invalidate o->internal.result, we need to update it to use
+ * o->internal.result.cache_tree as well.
+ */
+static void invalidate_ce_path(const struct cache_entry *ce,
+			       struct unpack_trees_options *o)
+{
+	if (!ce)
+		return;
+	cache_tree_invalidate_path(o->src_index, ce->name);
+	untracked_cache_invalidate_path(o->src_index, ce->name, 1);
+}
+
+/*
+ * Check that checking out ce->sha1 in subdir ce->name is not
+ * going to overwrite any working files.
+ */
+static int verify_clean_submodule(const char *old_sha1,
+				  const struct cache_entry *ce,
+				  struct unpack_trees_options *o)
+{
+	if (!submodule_from_ce(ce))
+		return 0;
+
+	return check_submodule_move_head(ce, old_sha1,
+					 oid_to_hex(&ce->oid), o);
+}
+
+static int verify_clean_subdirectory(const struct cache_entry *ce,
+				     struct unpack_trees_options *o)
+{
+	/*
+	 * we are about to extract "ce->name"; we would not want to lose
+	 * anything in the existing directory there.
+	 */
+	int namelen;
+	int i;
+	struct dir_struct d;
+	char *pathbuf;
+	int cnt = 0;
+
+	if (S_ISGITLINK(ce->ce_mode)) {
+		struct object_id oid;
+		int sub_head = resolve_gitlink_ref(ce->name, "HEAD", &oid);
+		/*
+		 * If we are not going to update the submodule, then
+		 * we don't care.
+		 */
+		if (!sub_head && oideq(&oid, &ce->oid))
+			return 0;
+		return verify_clean_submodule(sub_head ? NULL : oid_to_hex(&oid),
+					      ce, o);
+	}
+
+	/*
+	 * First let's make sure we do not have a local modification
+	 * in that directory.
+	 */
+	namelen = ce_namelen(ce);
+	for (i = locate_in_src_index(ce, o);
+	     i < o->src_index->cache_nr;
+	     i++) {
+		struct cache_entry *ce2 = o->src_index->cache[i];
+		int len = ce_namelen(ce2);
+		if (len < namelen ||
+		    strncmp(ce->name, ce2->name, namelen) ||
+		    ce2->name[namelen] != '/')
+			break;
+		/*
+		 * ce2->name is an entry in the subdirectory to be
+		 * removed.
+		 */
+		if (!ce_stage(ce2)) {
+			if (verify_uptodate(ce2, o))
+				return -1;
+			add_entry(o, ce2, CE_REMOVE, 0);
+			invalidate_ce_path(ce, o);
+			mark_ce_used(ce2, o);
+		}
+		cnt++;
+	}
+
+	/* Do not lose a locally present file that is not ignored. */
+	pathbuf = xstrfmt("%.*s/", namelen, ce->name);
+
+	memset(&d, 0, sizeof(d));
+	if (o->internal.dir)
+		setup_standard_excludes(&d);
+	i = read_directory(&d, o->src_index, pathbuf, namelen+1, NULL);
+	dir_clear(&d);
+	free(pathbuf);
+	if (i)
+		return add_rejected_path(o, ERROR_NOT_UPTODATE_DIR, ce->name);
+
+	/* Do not lose startup_info->original_cwd */
+	if (startup_info->original_cwd &&
+	    !strcmp(startup_info->original_cwd, ce->name))
+		return add_rejected_path(o, ERROR_CWD_IN_THE_WAY, ce->name);
+
+	return cnt;
+}
+
+/*
+ * This gets called when there was no index entry for the tree entry 'dst',
+ * but we found a file in the working tree that 'lstat()' said was fine,
+ * and we're on a case-insensitive filesystem.
+ *
+ * See if we can find a case-insensitive match in the index that also
+ * matches the stat information, and assume it's that other file!
+ */
+static int icase_exists(struct unpack_trees_options *o, const char *name, int len, struct stat *st)
+{
+	const struct cache_entry *src;
+
+	src = index_file_exists(o->src_index, name, len, 1);
+	return src && !ie_match_stat(o->src_index, src, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
+}
+
+enum absent_checking_type {
+	COMPLETELY_ABSENT,
+	ABSENT_ANY_DIRECTORY
+};
+
+static int check_ok_to_remove(const char *name, int len, int dtype,
+			      const struct cache_entry *ce, struct stat *st,
+			      enum unpack_trees_error_types error_type,
+			      enum absent_checking_type absent_type,
+			      struct unpack_trees_options *o)
+{
+	const struct cache_entry *result;
+
+	/*
+	 * It may be that the 'lstat()' succeeded even though
+	 * target 'ce' was absent, because there is an old
+	 * entry that is different only in case..
+	 *
+	 * Ignore that lstat() if it matches.
+	 */
+	if (ignore_case && icase_exists(o, name, len, st))
+		return 0;
+
+	if (o->internal.dir &&
+	    is_excluded(o->internal.dir, o->src_index, name, &dtype))
+		/*
+		 * ce->name is explicitly excluded, so it is Ok to
+		 * overwrite it.
+		 */
+		return 0;
+	if (S_ISDIR(st->st_mode)) {
+		/*
+		 * We are checking out path "foo" and
+		 * found "foo/." in the working tree.
+		 * This is tricky -- if we have modified
+		 * files that are in "foo/" we would lose
+		 * them.
+		 */
+		if (verify_clean_subdirectory(ce, o) < 0)
+			return -1;
+		return 0;
+	}
+
+	/* If we only care about directories, then we can remove */
+	if (absent_type == ABSENT_ANY_DIRECTORY)
+		return 0;
+
+	/*
+	 * The previous round may already have decided to
+	 * delete this path, which is in a subdirectory that
+	 * is being replaced with a blob.
+	 */
+	result = index_file_exists(&o->internal.result, name, len, 0);
+	if (result) {
+		if (result->ce_flags & CE_REMOVE)
+			return 0;
+	}
+
+	return add_rejected_path(o, error_type, name);
+}
+
+/*
+ * We do not want to remove or overwrite a working tree file that
+ * is not tracked, unless it is ignored.
+ */
+static int verify_absent_1(const struct cache_entry *ce,
+			   enum unpack_trees_error_types error_type,
+			   enum absent_checking_type absent_type,
+			   struct unpack_trees_options *o)
+{
+	int len;
+	struct stat st;
+
+	if (o->index_only || !o->update)
+		return 0;
+
+	if (o->reset == UNPACK_RESET_OVERWRITE_UNTRACKED) {
+		/* Avoid nuking startup_info->original_cwd... */
+		if (startup_info->original_cwd &&
+		    !strcmp(startup_info->original_cwd, ce->name))
+			return add_rejected_path(o, ERROR_CWD_IN_THE_WAY,
+						 ce->name);
+		/* ...but nuke anything else. */
+		return 0;
+	}
+
+	len = check_leading_path(ce->name, ce_namelen(ce), 0);
+	if (!len)
+		return 0;
+	else if (len > 0) {
+		char *path;
+		int ret;
+
+		path = xmemdupz(ce->name, len);
+		if (lstat(path, &st))
+			ret = error_errno("cannot stat '%s'", path);
+		else {
+			if (submodule_from_ce(ce))
+				ret = check_submodule_move_head(ce,
+								oid_to_hex(&ce->oid),
+								NULL, o);
+			else
+				ret = check_ok_to_remove(path, len, DT_UNKNOWN, NULL,
+							 &st, error_type,
+							 absent_type, o);
+		}
+		free(path);
+		return ret;
+	} else if (lstat(ce->name, &st)) {
+		if (errno != ENOENT)
+			return error_errno("cannot stat '%s'", ce->name);
+		return 0;
+	} else {
+		if (submodule_from_ce(ce))
+			return check_submodule_move_head(ce, oid_to_hex(&ce->oid),
+							 NULL, o);
+
+		return check_ok_to_remove(ce->name, ce_namelen(ce),
+					  ce_to_dtype(ce), ce, &st,
+					  error_type, absent_type, o);
+	}
+}
+
+static int verify_absent(const struct cache_entry *ce,
+			 enum unpack_trees_error_types error_type,
+			 struct unpack_trees_options *o)
+{
+	if (!o->skip_sparse_checkout && (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
+		return 0;
+	return verify_absent_1(ce, error_type, COMPLETELY_ABSENT, o);
+}
+
+static int verify_absent_if_directory(const struct cache_entry *ce,
+				      enum unpack_trees_error_types error_type,
+				      struct unpack_trees_options *o)
+{
+	if (!o->skip_sparse_checkout && (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
+		return 0;
+	return verify_absent_1(ce, error_type, ABSENT_ANY_DIRECTORY, o);
+}
+
+static int verify_absent_sparse(const struct cache_entry *ce,
+				enum unpack_trees_error_types error_type,
+				struct unpack_trees_options *o)
+{
+	return verify_absent_1(ce, error_type, COMPLETELY_ABSENT, o);
+}
+
+static int merged_entry(const struct cache_entry *ce,
+			const struct cache_entry *old,
+			struct unpack_trees_options *o)
+{
+	int update = CE_UPDATE;
+	struct cache_entry *merge = dup_cache_entry(ce, &o->internal.result);
+
+	if (!old) {
+		/*
+		 * New index entries. In sparse checkout, the following
+		 * verify_absent() will be delayed until after
+		 * traverse_trees() finishes in unpack_trees(), then:
+		 *
+		 *  - CE_NEW_SKIP_WORKTREE will be computed correctly
+		 *  - verify_absent() be called again, this time with
+		 *    correct CE_NEW_SKIP_WORKTREE
+		 *
+		 * verify_absent() call here does nothing in sparse
+		 * checkout (i.e. o->skip_sparse_checkout == 0)
+		 */
+		update |= CE_ADDED;
+		merge->ce_flags |= CE_NEW_SKIP_WORKTREE;
+
+		if (verify_absent(merge,
+				  ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o)) {
+			discard_cache_entry(merge);
+			return -1;
+		}
+		invalidate_ce_path(merge, o);
+
+		if (submodule_from_ce(ce) && file_exists(ce->name)) {
+			int ret = check_submodule_move_head(ce, NULL,
+							    oid_to_hex(&ce->oid),
+							    o);
+			if (ret)
+				return ret;
+		}
+
+	} else if (!(old->ce_flags & CE_CONFLICTED)) {
+		/*
+		 * See if we can re-use the old CE directly?
+		 * That way we get the uptodate stat info.
+		 *
+		 * This also removes the UPDATE flag on a match; otherwise
+		 * we will end up overwriting local changes in the work tree.
+		 */
+		if (same(old, merge)) {
+			copy_cache_entry(merge, old);
+			update = 0;
+		} else {
+			if (verify_uptodate(old, o)) {
+				discard_cache_entry(merge);
+				return -1;
+			}
+			/* Migrate old flags over */
+			update |= old->ce_flags & (CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE);
+			invalidate_ce_path(old, o);
+		}
+
+		if (submodule_from_ce(ce) && file_exists(ce->name)) {
+			int ret = check_submodule_move_head(ce, oid_to_hex(&old->oid),
+							    oid_to_hex(&ce->oid),
+							    o);
+			if (ret)
+				return ret;
+		}
+	} else {
+		/*
+		 * Previously unmerged entry left as an existence
+		 * marker by read_index_unmerged();
+		 */
+		if (verify_absent_if_directory(merge,
+				  ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o)) {
+			discard_cache_entry(merge);
+			return -1;
+		}
+
+		invalidate_ce_path(old, o);
+	}
+
+	if (do_add_entry(o, merge, update, CE_STAGEMASK) < 0)
+		return -1;
+	return 1;
+}
+
+static int merged_sparse_dir(const struct cache_entry * const *src, int n,
+			     struct unpack_trees_options *o)
+{
+	struct tree_desc t[MAX_UNPACK_TREES + 1];
+	void * tree_bufs[MAX_UNPACK_TREES + 1];
+	struct traverse_info info;
+	int i, ret;
+
+	/*
+	 * Create the tree traversal information for traversing into *only* the
+	 * sparse directory.
+	 */
+	setup_traverse_info(&info, src[0]->name);
+	info.fn = unpack_sparse_callback;
+	info.data = o;
+	info.show_all_errors = o->internal.show_all_errors;
+	info.pathspec = o->pathspec;
+
+	/* Get the tree descriptors of the sparse directory in each of the merging trees */
+	for (i = 0; i < n; i++)
+		tree_bufs[i] = fill_tree_descriptor(o->src_index->repo, &t[i],
+						    src[i] && !is_null_oid(&src[i]->oid) ? &src[i]->oid : NULL);
+
+	ret = traverse_trees(o->src_index, n, t, &info);
+
+	for (i = 0; i < n; i++)
+		free(tree_bufs[i]);
+
+	return ret;
+}
+
+static int deleted_entry(const struct cache_entry *ce,
+			 const struct cache_entry *old,
+			 struct unpack_trees_options *o)
+{
+	/* Did it exist in the index? */
+	if (!old) {
+		if (verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_REMOVED, o))
+			return -1;
+		return 0;
+	} else if (verify_absent_if_directory(ce, ERROR_WOULD_LOSE_UNTRACKED_REMOVED, o)) {
+		return -1;
+	}
+
+	if (!(old->ce_flags & CE_CONFLICTED) && verify_uptodate(old, o))
+		return -1;
+	add_entry(o, ce, CE_REMOVE, 0);
+	invalidate_ce_path(ce, o);
+	return 1;
+}
+
+static int keep_entry(const struct cache_entry *ce,
+		      struct unpack_trees_options *o)
+{
+	add_entry(o, ce, 0, 0);
+	if (ce_stage(ce))
+		invalidate_ce_path(ce, o);
+	return 1;
+}
+
+#if DBRT_DEBUG
+static void show_stage_entry(FILE *o,
+			     const char *label, const struct cache_entry *ce)
+{
+	if (!ce)
+		fprintf(o, "%s (missing)\n", label);
+	else
+		fprintf(o, "%s%06o %s %d\t%s\n",
+			label,
+			ce->ce_mode,
+			oid_to_hex(&ce->oid),
+			ce_stage(ce),
+			ce->name);
+}
+#endif
+
+int threeway_merge(const struct cache_entry * const *stages,
+		   struct unpack_trees_options *o)
+{
+	const struct cache_entry *index;
+	const struct cache_entry *head;
+	const struct cache_entry *remote = stages[o->head_idx + 1];
+	int count;
+	int head_match = 0;
+	int remote_match = 0;
+
+	int df_conflict_head = 0;
+	int df_conflict_remote = 0;
+
+	int any_anc_missing = 0;
+	int no_anc_exists = 1;
+	int i;
+
+	for (i = 1; i < o->head_idx; i++) {
+		if (!stages[i] || stages[i] == o->df_conflict_entry)
+			any_anc_missing = 1;
+		else
+			no_anc_exists = 0;
+	}
+
+	index = stages[0];
+	head = stages[o->head_idx];
+
+	if (head == o->df_conflict_entry) {
+		df_conflict_head = 1;
+		head = NULL;
+	}
+
+	if (remote == o->df_conflict_entry) {
+		df_conflict_remote = 1;
+		remote = NULL;
+	}
+
+	/*
+	 * First, if there's a #16 situation, note that to prevent #13
+	 * and #14.
+	 */
+	if (!same(remote, head)) {
+		for (i = 1; i < o->head_idx; i++) {
+			if (same(stages[i], head)) {
+				head_match = i;
+			}
+			if (same(stages[i], remote)) {
+				remote_match = i;
+			}
+		}
+	}
+
+	/*
+	 * We start with cases where the index is allowed to match
+	 * something other than the head: #14(ALT) and #2ALT, where it
+	 * is permitted to match the result instead.
+	 */
+	/* #14, #14ALT, #2ALT */
+	if (remote && !df_conflict_head && head_match && !remote_match) {
+		if (index && !same(index, remote) && !same(index, head)) {
+			if (S_ISSPARSEDIR(index->ce_mode))
+				return merged_sparse_dir(stages, 4, o);
+			else
+				return reject_merge(index, o);
+		}
+		return merged_entry(remote, index, o);
+	}
+	/*
+	 * If we have an entry in the index cache, then we want to
+	 * make sure that it matches head.
+	 */
+	if (index && !same(index, head)) {
+		if (S_ISSPARSEDIR(index->ce_mode))
+			return merged_sparse_dir(stages, 4, o);
+		else
+			return reject_merge(index, o);
+	}
+
+	if (head) {
+		/* #5ALT, #15 */
+		if (same(head, remote))
+			return merged_entry(head, index, o);
+		/* #13, #3ALT */
+		if (!df_conflict_remote && remote_match && !head_match)
+			return merged_entry(head, index, o);
+	}
+
+	/* #1 */
+	if (!head && !remote && any_anc_missing)
+		return 0;
+
+	/*
+	 * Under the "aggressive" rule, we resolve mostly trivial
+	 * cases that we historically had git-merge-one-file resolve.
+	 */
+	if (o->aggressive) {
+		int head_deleted = !head;
+		int remote_deleted = !remote;
+		const struct cache_entry *ce = NULL;
+
+		if (index)
+			ce = index;
+		else if (head)
+			ce = head;
+		else if (remote)
+			ce = remote;
+		else {
+			for (i = 1; i < o->head_idx; i++) {
+				if (stages[i] && stages[i] != o->df_conflict_entry) {
+					ce = stages[i];
+					break;
+				}
+			}
+		}
+
+		/*
+		 * Deleted in both.
+		 * Deleted in one and unchanged in the other.
+		 */
+		if ((head_deleted && remote_deleted) ||
+		    (head_deleted && remote && remote_match) ||
+		    (remote_deleted && head && head_match)) {
+			if (index)
+				return deleted_entry(index, index, o);
+			if (ce && !head_deleted) {
+				if (verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_REMOVED, o))
+					return -1;
+			}
+			return 0;
+		}
+		/*
+		 * Added in both, identically.
+		 */
+		if (no_anc_exists && head && remote && same(head, remote))
+			return merged_entry(head, index, o);
+
+	}
+
+	/* Handle "no merge" cases (see t/t1000-read-tree-m-3way.sh) */
+	if (index) {
+		/*
+		 * If we've reached the "no merge" cases and we're merging
+		 * a sparse directory, we may have an "edit/edit" conflict that
+		 * can be resolved by individually merging directory contents.
+		 */
+		if (S_ISSPARSEDIR(index->ce_mode))
+			return merged_sparse_dir(stages, 4, o);
+
+		/*
+		 * If we're not merging a sparse directory, ensure the index is
+		 * up-to-date to avoid files getting overwritten with conflict
+		 * resolution files
+		 */
+		if (verify_uptodate(index, o))
+			return -1;
+	}
+
+	o->internal.nontrivial_merge = 1;
+
+	/* #2, #3, #4, #6, #7, #9, #10, #11. */
+	count = 0;
+	if (!head_match || !remote_match) {
+		for (i = 1; i < o->head_idx; i++) {
+			if (stages[i] && stages[i] != o->df_conflict_entry) {
+				keep_entry(stages[i], o);
+				count++;
+				break;
+			}
+		}
+	}
+#if DBRT_DEBUG
+	else {
+		fprintf(stderr, "read-tree: warning #16 detected\n");
+		show_stage_entry(stderr, "head   ", stages[head_match]);
+		show_stage_entry(stderr, "remote ", stages[remote_match]);
+	}
+#endif
+	if (head) { count += keep_entry(head, o); }
+	if (remote) { count += keep_entry(remote, o); }
+	return count;
+}
+
+/*
+ * Two-way merge.
+ *
+ * The rule is to "carry forward" what is in the index without losing
+ * information across a "fast-forward", favoring a successful merge
+ * over a merge failure when it makes sense.  For details of the
+ * "carry forward" rule, please see <Documentation/git-read-tree.txt>.
+ *
+ */
+int twoway_merge(const struct cache_entry * const *src,
+		 struct unpack_trees_options *o)
+{
+	const struct cache_entry *current = src[0];
+	const struct cache_entry *oldtree = src[1];
+	const struct cache_entry *newtree = src[2];
+
+	if (o->internal.merge_size != 2)
+		return error("Cannot do a twoway merge of %d trees",
+			     o->internal.merge_size);
+
+	if (oldtree == o->df_conflict_entry)
+		oldtree = NULL;
+	if (newtree == o->df_conflict_entry)
+		newtree = NULL;
+
+	if (current) {
+		if (current->ce_flags & CE_CONFLICTED) {
+			if (same(oldtree, newtree) || o->reset) {
+				if (!newtree)
+					return deleted_entry(current, current, o);
+				else
+					return merged_entry(newtree, current, o);
+			}
+			return reject_merge(current, o);
+		} else if ((!oldtree && !newtree) || /* 4 and 5 */
+			 (!oldtree && newtree &&
+			  same(current, newtree)) || /* 6 and 7 */
+			 (oldtree && newtree &&
+			  same(oldtree, newtree)) || /* 14 and 15 */
+			 (oldtree && newtree &&
+			  !same(oldtree, newtree) && /* 18 and 19 */
+			  same(current, newtree))) {
+			return keep_entry(current, o);
+		} else if (oldtree && !newtree && same(current, oldtree)) {
+			/* 10 or 11 */
+			return deleted_entry(oldtree, current, o);
+		} else if (oldtree && newtree &&
+			 same(current, oldtree) && !same(current, newtree)) {
+			/* 20 or 21 */
+			return merged_entry(newtree, current, o);
+		} else if (current && !oldtree && newtree &&
+			   S_ISSPARSEDIR(current->ce_mode) != S_ISSPARSEDIR(newtree->ce_mode) &&
+			   ce_stage(current) == 0) {
+			/*
+			 * This case is a directory/file conflict across the sparse-index
+			 * boundary. When we are changing from one path to another via
+			 * 'git checkout', then we want to replace one entry with another
+			 * via merged_entry(). If there are staged changes, then we should
+			 * reject the merge instead.
+			 */
+			return merged_entry(newtree, current, o);
+		} else if (S_ISSPARSEDIR(current->ce_mode)) {
+			/*
+			 * The sparse directories differ, but we don't know whether that's
+			 * because of two different files in the directory being modified
+			 * (can be trivially merged) or if there is a real file conflict.
+			 * Merge the sparse directory by OID to compare file-by-file.
+			 */
+			return merged_sparse_dir(src, 3, o);
+		} else
+			return reject_merge(current, o);
+	}
+	else if (newtree) {
+		if (oldtree && !o->initial_checkout) {
+			/*
+			 * deletion of the path was staged;
+			 */
+			if (same(oldtree, newtree))
+				return 1;
+			return reject_merge(oldtree, o);
+		}
+		return merged_entry(newtree, current, o);
+	}
+	return deleted_entry(oldtree, current, o);
+}
+
+/*
+ * Bind merge.
+ *
+ * Keep the index entries at stage0, collapse stage1 but make sure
+ * stage0 does not have anything there.
+ */
+int bind_merge(const struct cache_entry * const *src,
+	       struct unpack_trees_options *o)
+{
+	const struct cache_entry *old = src[0];
+	const struct cache_entry *a = src[1];
+
+	if (o->internal.merge_size != 1)
+		return error("Cannot do a bind merge of %d trees",
+			     o->internal.merge_size);
+	if (a && old)
+		return o->quiet ? -1 :
+			error(ERRORMSG(o, ERROR_BIND_OVERLAP),
+			      super_prefixed(a->name, o->super_prefix),
+			      super_prefixed(old->name, o->super_prefix));
+	if (!a)
+		return keep_entry(old, o);
+	else
+		return merged_entry(a, NULL, o);
+}
+
+/*
+ * One-way merge.
+ *
+ * The rule is:
+ * - take the stat information from stage0, take the data from stage1
+ */
+int oneway_merge(const struct cache_entry * const *src,
+		 struct unpack_trees_options *o)
+{
+	const struct cache_entry *old = src[0];
+	const struct cache_entry *a = src[1];
+
+	if (o->internal.merge_size != 1)
+		return error("Cannot do a oneway merge of %d trees",
+			     o->internal.merge_size);
+
+	if (!a || a == o->df_conflict_entry)
+		return deleted_entry(old, old, o);
+
+	if (old && same(old, a)) {
+		int update = 0;
+		if (o->reset && o->update && !ce_uptodate(old) && !ce_skip_worktree(old) &&
+			!(old->ce_flags & CE_FSMONITOR_VALID)) {
+			struct stat st;
+			if (lstat(old->name, &st) ||
+			    ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE))
+				update |= CE_UPDATE;
+		}
+		if (o->update && S_ISGITLINK(old->ce_mode) &&
+		    should_update_submodules() && !verify_uptodate(old, o))
+			update |= CE_UPDATE;
+		add_entry(o, old, update, CE_STAGEMASK);
+		return 0;
+	}
+	return merged_entry(a, old, o);
+}
+
+/*
+ * Merge worktree and untracked entries in a stash entry.
+ *
+ * Ignore all index entries. Collapse remaining trees but make sure that they
+ * don't have any conflicting files.
+ */
+int stash_worktree_untracked_merge(const struct cache_entry * const *src,
+				   struct unpack_trees_options *o)
+{
+	const struct cache_entry *worktree = src[1];
+	const struct cache_entry *untracked = src[2];
+
+	if (o->internal.merge_size != 2)
+		BUG("invalid merge_size: %d", o->internal.merge_size);
+
+	if (worktree && untracked)
+		return error(_("worktree and untracked commit have duplicate entries: %s"),
+			     super_prefixed(worktree->name, o->super_prefix));
+
+	return merged_entry(worktree ? worktree : untracked, NULL, o);
+}