Adding upstream version 14.5.upstream/14.5upstream

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

1 files changed, 905 insertions, 0 deletions

diff --git a/contrib/pg_prewarm/autoprewarm.c b/contrib/pg_prewarm/autoprewarm.c
new file mode 100644
index 0000000..b3f73ea
--- /dev/null
+++ b/contrib/pg_prewarm/autoprewarm.c
@@ -0,0 +1,905 @@
+/*-------------------------------------------------------------------------
+ *
+ * autoprewarm.c
+ *		Periodically dump information about the blocks present in
+ *		shared_buffers, and reload them on server restart.
+ *
+ *		Due to locking considerations, we can't actually begin prewarming
+ *		until the server reaches a consistent state.  We need the catalogs
+ *		to be consistent so that we can figure out which relation to lock,
+ *		and we need to lock the relations so that we don't try to prewarm
+ *		pages from a relation that is in the process of being dropped.
+ *
+ *		While prewarming, autoprewarm will use two workers.  There's a
+ *		leader worker that reads and sorts the list of blocks to be
+ *		prewarmed and then launches a per-database worker for each
+ *		relevant database in turn.  The former keeps running after the
+ *		initial prewarm is complete to update the dump file periodically.
+ *
+ *	Copyright (c) 2016-2021, PostgreSQL Global Development Group
+ *
+ *	IDENTIFICATION
+ *		contrib/pg_prewarm/autoprewarm.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include <unistd.h>
+
+#include "access/relation.h"
+#include "access/xact.h"
+#include "catalog/pg_class.h"
+#include "catalog/pg_type.h"
+#include "miscadmin.h"
+#include "pgstat.h"
+#include "postmaster/bgworker.h"
+#include "postmaster/interrupt.h"
+#include "storage/buf_internals.h"
+#include "storage/dsm.h"
+#include "storage/ipc.h"
+#include "storage/latch.h"
+#include "storage/lwlock.h"
+#include "storage/proc.h"
+#include "storage/procsignal.h"
+#include "storage/shmem.h"
+#include "storage/smgr.h"
+#include "tcop/tcopprot.h"
+#include "utils/acl.h"
+#include "utils/datetime.h"
+#include "utils/guc.h"
+#include "utils/memutils.h"
+#include "utils/rel.h"
+#include "utils/relfilenodemap.h"
+#include "utils/resowner.h"
+
+#define AUTOPREWARM_FILE "autoprewarm.blocks"
+
+/* Metadata for each block we dump. */
+typedef struct BlockInfoRecord
+{
+	Oid			database;
+	Oid			tablespace;
+	Oid			filenode;
+	ForkNumber	forknum;
+	BlockNumber blocknum;
+} BlockInfoRecord;
+
+/* Shared state information for autoprewarm bgworker. */
+typedef struct AutoPrewarmSharedState
+{
+	LWLock		lock;			/* mutual exclusion */
+	pid_t		bgworker_pid;	/* for main bgworker */
+	pid_t		pid_using_dumpfile; /* for autoprewarm or block dump */
+
+	/* Following items are for communication with per-database worker */
+	dsm_handle	block_info_handle;
+	Oid			database;
+	int			prewarm_start_idx;
+	int			prewarm_stop_idx;
+	int			prewarmed_blocks;
+} AutoPrewarmSharedState;
+
+void		_PG_init(void);
+void		autoprewarm_main(Datum main_arg);
+void		autoprewarm_database_main(Datum main_arg);
+
+PG_FUNCTION_INFO_V1(autoprewarm_start_worker);
+PG_FUNCTION_INFO_V1(autoprewarm_dump_now);
+
+static void apw_load_buffers(void);
+static int	apw_dump_now(bool is_bgworker, bool dump_unlogged);
+static void apw_start_leader_worker(void);
+static void apw_start_database_worker(void);
+static bool apw_init_shmem(void);
+static void apw_detach_shmem(int code, Datum arg);
+static int	apw_compare_blockinfo(const void *p, const void *q);
+
+/* Pointer to shared-memory state. */
+static AutoPrewarmSharedState *apw_state = NULL;
+
+/* GUC variables. */
+static bool autoprewarm = true; /* start worker? */
+static int	autoprewarm_interval;	/* dump interval */
+
+/*
+ * Module load callback.
+ */
+void
+_PG_init(void)
+{
+	DefineCustomIntVariable("pg_prewarm.autoprewarm_interval",
+							"Sets the interval between dumps of shared buffers",
+							"If set to zero, time-based dumping is disabled.",
+							&autoprewarm_interval,
+							300,
+							0, INT_MAX / 1000,
+							PGC_SIGHUP,
+							GUC_UNIT_S,
+							NULL,
+							NULL,
+							NULL);
+
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	/* can't define PGC_POSTMASTER variable after startup */
+	DefineCustomBoolVariable("pg_prewarm.autoprewarm",
+							 "Starts the autoprewarm worker.",
+							 NULL,
+							 &autoprewarm,
+							 true,
+							 PGC_POSTMASTER,
+							 0,
+							 NULL,
+							 NULL,
+							 NULL);
+
+	EmitWarningsOnPlaceholders("pg_prewarm");
+
+	RequestAddinShmemSpace(MAXALIGN(sizeof(AutoPrewarmSharedState)));
+
+	/* Register autoprewarm worker, if enabled. */
+	if (autoprewarm)
+		apw_start_leader_worker();
+}
+
+/*
+ * Main entry point for the leader autoprewarm process.  Per-database workers
+ * have a separate entry point.
+ */
+void
+autoprewarm_main(Datum main_arg)
+{
+	bool		first_time = true;
+	bool		final_dump_allowed = true;
+	TimestampTz last_dump_time = 0;
+
+	/* Establish signal handlers; once that's done, unblock signals. */
+	pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
+	pqsignal(SIGHUP, SignalHandlerForConfigReload);
+	pqsignal(SIGUSR1, procsignal_sigusr1_handler);
+	BackgroundWorkerUnblockSignals();
+
+	/* Create (if necessary) and attach to our shared memory area. */
+	if (apw_init_shmem())
+		first_time = false;
+
+	/* Set on-detach hook so that our PID will be cleared on exit. */
+	on_shmem_exit(apw_detach_shmem, 0);
+
+	/*
+	 * Store our PID in the shared memory area --- unless there's already
+	 * another worker running, in which case just exit.
+	 */
+	LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
+	if (apw_state->bgworker_pid != InvalidPid)
+	{
+		LWLockRelease(&apw_state->lock);
+		ereport(LOG,
+				(errmsg("autoprewarm worker is already running under PID %lu",
+						(unsigned long) apw_state->bgworker_pid)));
+		return;
+	}
+	apw_state->bgworker_pid = MyProcPid;
+	LWLockRelease(&apw_state->lock);
+
+	/*
+	 * Preload buffers from the dump file only if we just created the shared
+	 * memory region.  Otherwise, it's either already been done or shouldn't
+	 * be done - e.g. because the old dump file has been overwritten since the
+	 * server was started.
+	 *
+	 * There's not much point in performing a dump immediately after we finish
+	 * preloading; so, if we do end up preloading, consider the last dump time
+	 * to be equal to the current time.
+	 *
+	 * If apw_load_buffers() is terminated early by a shutdown request,
+	 * prevent dumping out our state below the loop, because we'd effectively
+	 * just truncate the saved state to however much we'd managed to preload.
+	 */
+	if (first_time)
+	{
+		apw_load_buffers();
+		final_dump_allowed = !ShutdownRequestPending;
+		last_dump_time = GetCurrentTimestamp();
+	}
+
+	/* Periodically dump buffers until terminated. */
+	while (!ShutdownRequestPending)
+	{
+		/* In case of a SIGHUP, just reload the configuration. */
+		if (ConfigReloadPending)
+		{
+			ConfigReloadPending = false;
+			ProcessConfigFile(PGC_SIGHUP);
+		}
+
+		if (autoprewarm_interval <= 0)
+		{
+			/* We're only dumping at shutdown, so just wait forever. */
+			(void) WaitLatch(MyLatch,
+							 WL_LATCH_SET | WL_EXIT_ON_PM_DEATH,
+							 -1L,
+							 PG_WAIT_EXTENSION);
+		}
+		else
+		{
+			TimestampTz next_dump_time;
+			long		delay_in_ms;
+
+			/* Compute the next dump time. */
+			next_dump_time =
+				TimestampTzPlusMilliseconds(last_dump_time,
+											autoprewarm_interval * 1000);
+			delay_in_ms =
+				TimestampDifferenceMilliseconds(GetCurrentTimestamp(),
+												next_dump_time);
+
+			/* Perform a dump if it's time. */
+			if (delay_in_ms <= 0)
+			{
+				last_dump_time = GetCurrentTimestamp();
+				apw_dump_now(true, false);
+				continue;
+			}
+
+			/* Sleep until the next dump time. */
+			(void) WaitLatch(MyLatch,
+							 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
+							 delay_in_ms,
+							 PG_WAIT_EXTENSION);
+		}
+
+		/* Reset the latch, loop. */
+		ResetLatch(MyLatch);
+	}
+
+	/*
+	 * Dump one last time.  We assume this is probably the result of a system
+	 * shutdown, although it's possible that we've merely been terminated.
+	 */
+	if (final_dump_allowed)
+		apw_dump_now(true, true);
+}
+
+/*
+ * Read the dump file and launch per-database workers one at a time to
+ * prewarm the buffers found there.
+ */
+static void
+apw_load_buffers(void)
+{
+	FILE	   *file = NULL;
+	int			num_elements,
+				i;
+	BlockInfoRecord *blkinfo;
+	dsm_segment *seg;
+
+	/*
+	 * Skip the prewarm if the dump file is in use; otherwise, prevent any
+	 * other process from writing it while we're using it.
+	 */
+	LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
+	if (apw_state->pid_using_dumpfile == InvalidPid)
+		apw_state->pid_using_dumpfile = MyProcPid;
+	else
+	{
+		LWLockRelease(&apw_state->lock);
+		ereport(LOG,
+				(errmsg("skipping prewarm because block dump file is being written by PID %lu",
+						(unsigned long) apw_state->pid_using_dumpfile)));
+		return;
+	}
+	LWLockRelease(&apw_state->lock);
+
+	/*
+	 * Open the block dump file.  Exit quietly if it doesn't exist, but report
+	 * any other error.
+	 */
+	file = AllocateFile(AUTOPREWARM_FILE, "r");
+	if (!file)
+	{
+		if (errno == ENOENT)
+		{
+			LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
+			apw_state->pid_using_dumpfile = InvalidPid;
+			LWLockRelease(&apw_state->lock);
+			return;				/* No file to load. */
+		}
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not read file \"%s\": %m",
+						AUTOPREWARM_FILE)));
+	}
+
+	/* First line of the file is a record count. */
+	if (fscanf(file, "<<%d>>\n", &num_elements) != 1)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not read from file \"%s\": %m",
+						AUTOPREWARM_FILE)));
+
+	/* Allocate a dynamic shared memory segment to store the record data. */
+	seg = dsm_create(sizeof(BlockInfoRecord) * num_elements, 0);
+	blkinfo = (BlockInfoRecord *) dsm_segment_address(seg);
+
+	/* Read records, one per line. */
+	for (i = 0; i < num_elements; i++)
+	{
+		unsigned	forknum;
+
+		if (fscanf(file, "%u,%u,%u,%u,%u\n", &blkinfo[i].database,
+				   &blkinfo[i].tablespace, &blkinfo[i].filenode,
+				   &forknum, &blkinfo[i].blocknum) != 5)
+			ereport(ERROR,
+					(errmsg("autoprewarm block dump file is corrupted at line %d",
+							i + 1)));
+		blkinfo[i].forknum = forknum;
+	}
+
+	FreeFile(file);
+
+	/* Sort the blocks to be loaded. */
+	pg_qsort(blkinfo, num_elements, sizeof(BlockInfoRecord),
+			 apw_compare_blockinfo);
+
+	/* Populate shared memory state. */
+	apw_state->block_info_handle = dsm_segment_handle(seg);
+	apw_state->prewarm_start_idx = apw_state->prewarm_stop_idx = 0;
+	apw_state->prewarmed_blocks = 0;
+
+	/* Get the info position of the first block of the next database. */
+	while (apw_state->prewarm_start_idx < num_elements)
+	{
+		int			j = apw_state->prewarm_start_idx;
+		Oid			current_db = blkinfo[j].database;
+
+		/*
+		 * Advance the prewarm_stop_idx to the first BlockInfoRecord that does
+		 * not belong to this database.
+		 */
+		j++;
+		while (j < num_elements)
+		{
+			if (current_db != blkinfo[j].database)
+			{
+				/*
+				 * Combine BlockInfoRecords for global objects with those of
+				 * the database.
+				 */
+				if (current_db != InvalidOid)
+					break;
+				current_db = blkinfo[j].database;
+			}
+
+			j++;
+		}
+
+		/*
+		 * If we reach this point with current_db == InvalidOid, then only
+		 * BlockInfoRecords belonging to global objects exist.  We can't
+		 * prewarm without a database connection, so just bail out.
+		 */
+		if (current_db == InvalidOid)
+			break;
+
+		/* Configure stop point and database for next per-database worker. */
+		apw_state->prewarm_stop_idx = j;
+		apw_state->database = current_db;
+		Assert(apw_state->prewarm_start_idx < apw_state->prewarm_stop_idx);
+
+		/* If we've run out of free buffers, don't launch another worker. */
+		if (!have_free_buffer())
+			break;
+
+		/*
+		 * Likewise, don't launch if we've already been told to shut down.
+		 * (The launch would fail anyway, but we might as well skip it.)
+		 */
+		if (ShutdownRequestPending)
+			break;
+
+		/*
+		 * Start a per-database worker to load blocks for this database; this
+		 * function will return once the per-database worker exits.
+		 */
+		apw_start_database_worker();
+
+		/* Prepare for next database. */
+		apw_state->prewarm_start_idx = apw_state->prewarm_stop_idx;
+	}
+
+	/* Clean up. */
+	dsm_detach(seg);
+	LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
+	apw_state->block_info_handle = DSM_HANDLE_INVALID;
+	apw_state->pid_using_dumpfile = InvalidPid;
+	LWLockRelease(&apw_state->lock);
+
+	/* Report our success, if we were able to finish. */
+	if (!ShutdownRequestPending)
+		ereport(LOG,
+				(errmsg("autoprewarm successfully prewarmed %d of %d previously-loaded blocks",
+						apw_state->prewarmed_blocks, num_elements)));
+}
+
+/*
+ * Prewarm all blocks for one database (and possibly also global objects, if
+ * those got grouped with this database).
+ */
+void
+autoprewarm_database_main(Datum main_arg)
+{
+	int			pos;
+	BlockInfoRecord *block_info;
+	Relation	rel = NULL;
+	BlockNumber nblocks = 0;
+	BlockInfoRecord *old_blk = NULL;
+	dsm_segment *seg;
+
+	/* Establish signal handlers; once that's done, unblock signals. */
+	pqsignal(SIGTERM, die);
+	BackgroundWorkerUnblockSignals();
+
+	/* Connect to correct database and get block information. */
+	apw_init_shmem();
+	seg = dsm_attach(apw_state->block_info_handle);
+	if (seg == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("could not map dynamic shared memory segment")));
+	BackgroundWorkerInitializeConnectionByOid(apw_state->database, InvalidOid, 0);
+	block_info = (BlockInfoRecord *) dsm_segment_address(seg);
+	pos = apw_state->prewarm_start_idx;
+
+	/*
+	 * Loop until we run out of blocks to prewarm or until we run out of free
+	 * buffers.
+	 */
+	while (pos < apw_state->prewarm_stop_idx && have_free_buffer())
+	{
+		BlockInfoRecord *blk = &block_info[pos++];
+		Buffer		buf;
+
+		CHECK_FOR_INTERRUPTS();
+
+		/*
+		 * Quit if we've reached records for another database. If previous
+		 * blocks are of some global objects, then continue pre-warming.
+		 */
+		if (old_blk != NULL && old_blk->database != blk->database &&
+			old_blk->database != 0)
+			break;
+
+		/*
+		 * As soon as we encounter a block of a new relation, close the old
+		 * relation. Note that rel will be NULL if try_relation_open failed
+		 * previously; in that case, there is nothing to close.
+		 */
+		if (old_blk != NULL && old_blk->filenode != blk->filenode &&
+			rel != NULL)
+		{
+			relation_close(rel, AccessShareLock);
+			rel = NULL;
+			CommitTransactionCommand();
+		}
+
+		/*
+		 * Try to open each new relation, but only once, when we first
+		 * encounter it. If it's been dropped, skip the associated blocks.
+		 */
+		if (old_blk == NULL || old_blk->filenode != blk->filenode)
+		{
+			Oid			reloid;
+
+			Assert(rel == NULL);
+			StartTransactionCommand();
+			reloid = RelidByRelfilenode(blk->tablespace, blk->filenode);
+			if (OidIsValid(reloid))
+				rel = try_relation_open(reloid, AccessShareLock);
+
+			if (!rel)
+				CommitTransactionCommand();
+		}
+		if (!rel)
+		{
+			old_blk = blk;
+			continue;
+		}
+
+		/* Once per fork, check for fork existence and size. */
+		if (old_blk == NULL ||
+			old_blk->filenode != blk->filenode ||
+			old_blk->forknum != blk->forknum)
+		{
+			RelationOpenSmgr(rel);
+
+			/*
+			 * smgrexists is not safe for illegal forknum, hence check whether
+			 * the passed forknum is valid before using it in smgrexists.
+			 */
+			if (blk->forknum > InvalidForkNumber &&
+				blk->forknum <= MAX_FORKNUM &&
+				smgrexists(rel->rd_smgr, blk->forknum))
+				nblocks = RelationGetNumberOfBlocksInFork(rel, blk->forknum);
+			else
+				nblocks = 0;
+		}
+
+		/* Check whether blocknum is valid and within fork file size. */
+		if (blk->blocknum >= nblocks)
+		{
+			/* Move to next forknum. */
+			old_blk = blk;
+			continue;
+		}
+
+		/* Prewarm buffer. */
+		buf = ReadBufferExtended(rel, blk->forknum, blk->blocknum, RBM_NORMAL,
+								 NULL);
+		if (BufferIsValid(buf))
+		{
+			apw_state->prewarmed_blocks++;
+			ReleaseBuffer(buf);
+		}
+
+		old_blk = blk;
+	}
+
+	dsm_detach(seg);
+
+	/* Release lock on previous relation. */
+	if (rel)
+	{
+		relation_close(rel, AccessShareLock);
+		CommitTransactionCommand();
+	}
+}
+
+/*
+ * Dump information on blocks in shared buffers.  We use a text format here
+ * so that it's easy to understand and even change the file contents if
+ * necessary.
+ * Returns the number of blocks dumped.
+ */
+static int
+apw_dump_now(bool is_bgworker, bool dump_unlogged)
+{
+	int			num_blocks;
+	int			i;
+	int			ret;
+	BlockInfoRecord *block_info_array;
+	BufferDesc *bufHdr;
+	FILE	   *file;
+	char		transient_dump_file_path[MAXPGPATH];
+	pid_t		pid;
+
+	LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
+	pid = apw_state->pid_using_dumpfile;
+	if (apw_state->pid_using_dumpfile == InvalidPid)
+		apw_state->pid_using_dumpfile = MyProcPid;
+	LWLockRelease(&apw_state->lock);
+
+	if (pid != InvalidPid)
+	{
+		if (!is_bgworker)
+			ereport(ERROR,
+					(errmsg("could not perform block dump because dump file is being used by PID %lu",
+							(unsigned long) apw_state->pid_using_dumpfile)));
+
+		ereport(LOG,
+				(errmsg("skipping block dump because it is already being performed by PID %lu",
+						(unsigned long) apw_state->pid_using_dumpfile)));
+		return 0;
+	}
+
+	block_info_array =
+		(BlockInfoRecord *) palloc(sizeof(BlockInfoRecord) * NBuffers);
+
+	for (num_blocks = 0, i = 0; i < NBuffers; i++)
+	{
+		uint32		buf_state;
+
+		CHECK_FOR_INTERRUPTS();
+
+		bufHdr = GetBufferDescriptor(i);
+
+		/* Lock each buffer header before inspecting. */
+		buf_state = LockBufHdr(bufHdr);
+
+		/*
+		 * Unlogged tables will be automatically truncated after a crash or
+		 * unclean shutdown. In such cases we need not prewarm them. Dump them
+		 * only if requested by caller.
+		 */
+		if (buf_state & BM_TAG_VALID &&
+			((buf_state & BM_PERMANENT) || dump_unlogged))
+		{
+			block_info_array[num_blocks].database = bufHdr->tag.rnode.dbNode;
+			block_info_array[num_blocks].tablespace = bufHdr->tag.rnode.spcNode;
+			block_info_array[num_blocks].filenode = bufHdr->tag.rnode.relNode;
+			block_info_array[num_blocks].forknum = bufHdr->tag.forkNum;
+			block_info_array[num_blocks].blocknum = bufHdr->tag.blockNum;
+			++num_blocks;
+		}
+
+		UnlockBufHdr(bufHdr, buf_state);
+	}
+
+	snprintf(transient_dump_file_path, MAXPGPATH, "%s.tmp", AUTOPREWARM_FILE);
+	file = AllocateFile(transient_dump_file_path, "w");
+	if (!file)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\": %m",
+						transient_dump_file_path)));
+
+	ret = fprintf(file, "<<%d>>\n", num_blocks);
+	if (ret < 0)
+	{
+		int			save_errno = errno;
+
+		FreeFile(file);
+		unlink(transient_dump_file_path);
+		errno = save_errno;
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write to file \"%s\": %m",
+						transient_dump_file_path)));
+	}
+
+	for (i = 0; i < num_blocks; i++)
+	{
+		CHECK_FOR_INTERRUPTS();
+
+		ret = fprintf(file, "%u,%u,%u,%u,%u\n",
+					  block_info_array[i].database,
+					  block_info_array[i].tablespace,
+					  block_info_array[i].filenode,
+					  (uint32) block_info_array[i].forknum,
+					  block_info_array[i].blocknum);
+		if (ret < 0)
+		{
+			int			save_errno = errno;
+
+			FreeFile(file);
+			unlink(transient_dump_file_path);
+			errno = save_errno;
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not write to file \"%s\": %m",
+							transient_dump_file_path)));
+		}
+	}
+
+	pfree(block_info_array);
+
+	/*
+	 * Rename transient_dump_file_path to AUTOPREWARM_FILE to make things
+	 * permanent.
+	 */
+	ret = FreeFile(file);
+	if (ret != 0)
+	{
+		int			save_errno = errno;
+
+		unlink(transient_dump_file_path);
+		errno = save_errno;
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not close file \"%s\": %m",
+						transient_dump_file_path)));
+	}
+
+	(void) durable_rename(transient_dump_file_path, AUTOPREWARM_FILE, ERROR);
+	apw_state->pid_using_dumpfile = InvalidPid;
+
+	ereport(DEBUG1,
+			(errmsg_internal("wrote block details for %d blocks", num_blocks)));
+	return num_blocks;
+}
+
+/*
+ * SQL-callable function to launch autoprewarm.
+ */
+Datum
+autoprewarm_start_worker(PG_FUNCTION_ARGS)
+{
+	pid_t		pid;
+
+	if (!autoprewarm)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("autoprewarm is disabled")));
+
+	apw_init_shmem();
+	LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
+	pid = apw_state->bgworker_pid;
+	LWLockRelease(&apw_state->lock);
+
+	if (pid != InvalidPid)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("autoprewarm worker is already running under PID %lu",
+						(unsigned long) pid)));
+
+	apw_start_leader_worker();
+
+	PG_RETURN_VOID();
+}
+
+/*
+ * SQL-callable function to perform an immediate block dump.
+ *
+ * Note: this is declared to return int8, as insurance against some
+ * very distant day when we might make NBuffers wider than int.
+ */
+Datum
+autoprewarm_dump_now(PG_FUNCTION_ARGS)
+{
+	int			num_blocks;
+
+	apw_init_shmem();
+
+	PG_ENSURE_ERROR_CLEANUP(apw_detach_shmem, 0);
+	{
+		num_blocks = apw_dump_now(false, true);
+	}
+	PG_END_ENSURE_ERROR_CLEANUP(apw_detach_shmem, 0);
+
+	PG_RETURN_INT64((int64) num_blocks);
+}
+
+/*
+ * Allocate and initialize autoprewarm related shared memory, if not already
+ * done, and set up backend-local pointer to that state.  Returns true if an
+ * existing shared memory segment was found.
+ */
+static bool
+apw_init_shmem(void)
+{
+	bool		found;
+
+	LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
+	apw_state = ShmemInitStruct("autoprewarm",
+								sizeof(AutoPrewarmSharedState),
+								&found);
+	if (!found)
+	{
+		/* First time through ... */
+		LWLockInitialize(&apw_state->lock, LWLockNewTrancheId());
+		apw_state->bgworker_pid = InvalidPid;
+		apw_state->pid_using_dumpfile = InvalidPid;
+	}
+	LWLockRelease(AddinShmemInitLock);
+
+	LWLockRegisterTranche(apw_state->lock.tranche, "autoprewarm");
+
+	return found;
+}
+
+/*
+ * Clear our PID from autoprewarm shared state.
+ */
+static void
+apw_detach_shmem(int code, Datum arg)
+{
+	LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
+	if (apw_state->pid_using_dumpfile == MyProcPid)
+		apw_state->pid_using_dumpfile = InvalidPid;
+	if (apw_state->bgworker_pid == MyProcPid)
+		apw_state->bgworker_pid = InvalidPid;
+	LWLockRelease(&apw_state->lock);
+}
+
+/*
+ * Start autoprewarm leader worker process.
+ */
+static void
+apw_start_leader_worker(void)
+{
+	BackgroundWorker worker;
+	BackgroundWorkerHandle *handle;
+	BgwHandleStatus status;
+	pid_t		pid;
+
+	MemSet(&worker, 0, sizeof(BackgroundWorker));
+	worker.bgw_flags = BGWORKER_SHMEM_ACCESS;
+	worker.bgw_start_time = BgWorkerStart_ConsistentState;
+	strcpy(worker.bgw_library_name, "pg_prewarm");
+	strcpy(worker.bgw_function_name, "autoprewarm_main");
+	strcpy(worker.bgw_name, "autoprewarm leader");
+	strcpy(worker.bgw_type, "autoprewarm leader");
+
+	if (process_shared_preload_libraries_in_progress)
+	{
+		RegisterBackgroundWorker(&worker);
+		return;
+	}
+
+	/* must set notify PID to wait for startup */
+	worker.bgw_notify_pid = MyProcPid;
+
+	if (!RegisterDynamicBackgroundWorker(&worker, &handle))
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
+				 errmsg("could not register background process"),
+				 errhint("You may need to increase max_worker_processes.")));
+
+	status = WaitForBackgroundWorkerStartup(handle, &pid);
+	if (status != BGWH_STARTED)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
+				 errmsg("could not start background process"),
+				 errhint("More details may be available in the server log.")));
+}
+
+/*
+ * Start autoprewarm per-database worker process.
+ */
+static void
+apw_start_database_worker(void)
+{
+	BackgroundWorker worker;
+	BackgroundWorkerHandle *handle;
+
+	MemSet(&worker, 0, sizeof(BackgroundWorker));
+	worker.bgw_flags =
+		BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION;
+	worker.bgw_start_time = BgWorkerStart_ConsistentState;
+	worker.bgw_restart_time = BGW_NEVER_RESTART;
+	strcpy(worker.bgw_library_name, "pg_prewarm");
+	strcpy(worker.bgw_function_name, "autoprewarm_database_main");
+	strcpy(worker.bgw_name, "autoprewarm worker");
+	strcpy(worker.bgw_type, "autoprewarm worker");
+
+	/* must set notify PID to wait for shutdown */
+	worker.bgw_notify_pid = MyProcPid;
+
+	if (!RegisterDynamicBackgroundWorker(&worker, &handle))
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
+				 errmsg("registering dynamic bgworker autoprewarm failed"),
+				 errhint("Consider increasing configuration parameter \"max_worker_processes\".")));
+
+	/*
+	 * Ignore return value; if it fails, postmaster has died, but we have
+	 * checks for that elsewhere.
+	 */
+	WaitForBackgroundWorkerShutdown(handle);
+}
+
+/* Compare member elements to check whether they are not equal. */
+#define cmp_member_elem(fld)	\
+do { \
+	if (a->fld < b->fld)		\
+		return -1;				\
+	else if (a->fld > b->fld)	\
+		return 1;				\
+} while(0)
+
+/*
+ * apw_compare_blockinfo
+ *
+ * We depend on all records for a particular database being consecutive
+ * in the dump file; each per-database worker will preload blocks until
+ * it sees a block for some other database.  Sorting by tablespace,
+ * filenode, forknum, and blocknum isn't critical for correctness, but
+ * helps us get a sequential I/O pattern.
+ */
+static int
+apw_compare_blockinfo(const void *p, const void *q)
+{
+	const BlockInfoRecord *a = (const BlockInfoRecord *) p;
+	const BlockInfoRecord *b = (const BlockInfoRecord *) q;
+
+	cmp_member_elem(database);
+	cmp_member_elem(tablespace);
+	cmp_member_elem(filenode);
+	cmp_member_elem(forknum);
+	cmp_member_elem(blocknum);
+
+	return 0;
+}