postgresql/src/backend/access/transam/commit_ts.c

/*-------------------------------------------------------------------------
 *
 * commit_ts.c
 *		PostgreSQL commit timestamp manager
 *
 * This module is a pg_clog-like system that stores the commit timestamp
 * for each transaction.
 *
 * XLOG interactions: this module generates an XLOG record whenever a new
 * CommitTs page is initialized to zeroes.  Also, one XLOG record is
 * generated for setting of values when the caller requests it; this allows
 * us to support values coming from places other than transaction commit.
 * Other writes of CommitTS come from recording of transaction commit in
 * xact.c, which generates its own XLOG records for these events and will
 * re-perform the status update on redo; so we need make no additional XLOG
 * entry here.
 *
 * Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/commit_ts.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/commit_ts.h"
#include "access/htup_details.h"
#include "access/slru.h"
#include "access/transam.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "utils/builtins.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"

/*
 * Defines for CommitTs page sizes.  A page is the same BLCKSZ as is used
 * everywhere else in Postgres.
 *
 * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
 * CommitTs page numbering also wraps around at
 * 0xFFFFFFFF/COMMIT_TS_XACTS_PER_PAGE, and CommitTs segment numbering at
 * 0xFFFFFFFF/COMMIT_TS_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need take no
 * explicit notice of that fact in this module, except when comparing segment
 * and page numbers in TruncateCommitTs (see CommitTsPagePrecedes).
 */

/*
 * We need 8+4 bytes per xact.  Note that enlarging this struct might mean
 * the largest possible file name is more than 5 chars long; see
 * SlruScanDirectory.
 */
typedef struct CommitTimestampEntry
{
	TimestampTz		time;
	CommitTsNodeId	nodeid;
} CommitTimestampEntry;

#define SizeOfCommitTimestampEntry (offsetof(CommitTimestampEntry, nodeid) + \
									sizeof(CommitTsNodeId))

#define COMMIT_TS_XACTS_PER_PAGE \
	(BLCKSZ / SizeOfCommitTimestampEntry)

#define TransactionIdToCTsPage(xid)	\
	((xid) / (TransactionId) COMMIT_TS_XACTS_PER_PAGE)
#define TransactionIdToCTsEntry(xid)	\
	((xid) % (TransactionId) COMMIT_TS_XACTS_PER_PAGE)

/*
 * Link to shared-memory data structures for CommitTs control
 */
static SlruCtlData CommitTsCtlData;

#define CommitTsCtl (&CommitTsCtlData)

/*
 * We keep a cache of the last value set in shared memory.  This is protected
 * by CommitTsLock.
 */
typedef struct CommitTimestampShared
{
	TransactionId	xidLastCommit;
	CommitTimestampEntry dataLastCommit;
} CommitTimestampShared;

CommitTimestampShared	*commitTsShared;


/* GUC variable */
bool	track_commit_timestamp;

static CommitTsNodeId default_node_id = InvalidCommitTsNodeId;

static void SetXidCommitTsInPage(TransactionId xid, int nsubxids,
					 TransactionId *subxids, TimestampTz ts,
					 CommitTsNodeId nodeid, int pageno);
static void TransactionIdSetCommitTs(TransactionId xid, TimestampTz ts,
						  CommitTsNodeId nodeid, int slotno);
static int	ZeroCommitTsPage(int pageno, bool writeXlog);
static bool CommitTsPagePrecedes(int page1, int page2);
static void WriteZeroPageXlogRec(int pageno);
static void WriteTruncateXlogRec(int pageno);
static void WriteSetTimestampXlogRec(TransactionId mainxid, int nsubxids,
						 TransactionId *subxids, TimestampTz timestamp,
						 CommitTsNodeId nodeid);


/*
 * CommitTsSetDefaultNodeId
 *
 * Set default nodeid for current backend.
 */
void
CommitTsSetDefaultNodeId(CommitTsNodeId nodeid)
{
	default_node_id = nodeid;
}

/*
 * CommitTsGetDefaultNodeId
 *
 * Set default nodeid for current backend.
 */
CommitTsNodeId
CommitTsGetDefaultNodeId(void)
{
	return default_node_id;
}

/*
 * TransactionTreeSetCommitTsData
 *
 * Record the final commit timestamp of transaction entries in the commit log
 * for a transaction and its subtransaction tree, as efficiently as possible.
 *
 * xid is the top level transaction id.
 *
 * subxids is an array of xids of length nsubxids, representing subtransactions
 * in the tree of xid. In various cases nsubxids may be zero.
 * The reason why tracking just the parent xid commit timestamp is not enough
 * is that the subtrans SLRU does not stay valid across crashes (it's not
 * permanent) so we need to keep the information about them here. If the
 * subtrans implementation changes in the future, we might want to revisit the
 * decision of storing timestamp info for each subxid.
 *
 * The do_xlog parameter tells us whether to include a XLog record of this
 * or not.  Normal path through RecordTransactionCommit() will be related
 * to a transaction commit XLog record, and so should pass "false" here.
 * Other callers probably want to pass true, so that the given values persist
 * in case of crashes.
 */
void
TransactionTreeSetCommitTsData(TransactionId xid, int nsubxids,
							   TransactionId *subxids, TimestampTz timestamp,
							   CommitTsNodeId nodeid, bool do_xlog)
{
	int			i;
	TransactionId headxid;
	TransactionId newestXact;

	if (!track_commit_timestamp)
		return;

	/*
	 * Comply with the WAL-before-data rule: if caller specified it wants
	 * this value to be recorded in WAL, do so before touching the data.
	 */
	if (do_xlog)
		WriteSetTimestampXlogRec(xid, nsubxids, subxids, timestamp, nodeid);

	/*
	 * Figure out the latest Xid in this batch: either the last subxid if
	 * there's any, otherwise the parent xid.
	 */
	if (nsubxids > 0)
		newestXact = subxids[nsubxids - 1];
	else
		newestXact = xid;

	/*
	 * We split the xids to set the timestamp to in groups belonging to the
	 * same SLRU page; the first element in each such set is its head.  The
	 * first group has the main XID as the head; subsequent sets use the
	 * first subxid not on the previous page as head.  This way, we only have
	 * to lock/modify each SLRU page once.
	 */
	for (i = 0, headxid = xid;;)
	{
		int			pageno = TransactionIdToCTsPage(headxid);
		int			j;

		for (j = i; j < nsubxids; j++)
		{
			if (TransactionIdToCTsPage(subxids[j]) != pageno)
				break;
		}
		/* subxids[i..j] are on the same page as the head */

		SetXidCommitTsInPage(headxid, j - i, subxids + i, timestamp, nodeid,
							 pageno);

		/* if we wrote out all subxids, we're done. */
		if (j + 1 >= nsubxids)
			break;

		/*
		 * Set the new head and skip over it, as well as over the subxids
		 * we just wrote.
		 */
		headxid = subxids[j];
		i += j - i + 1;
	}

	/* update the cached value in shared memory */
	LWLockAcquire(CommitTsLock, LW_EXCLUSIVE);
	commitTsShared->xidLastCommit = xid;
	commitTsShared->dataLastCommit.time = timestamp;
	commitTsShared->dataLastCommit.nodeid = nodeid;

	/* and move forwards our endpoint, if needed */
	if (TransactionIdPrecedes(ShmemVariableCache->newestCommitTs, newestXact))
		ShmemVariableCache->newestCommitTs = newestXact;
	LWLockRelease(CommitTsLock);
}

/*
 * Record the commit timestamp of transaction entries in the commit log for all
 * entries on a single page.  Atomic only on this page.
 */
static void
SetXidCommitTsInPage(TransactionId xid, int nsubxids,
					 TransactionId *subxids, TimestampTz ts,
					 CommitTsNodeId nodeid, int pageno)
{
	int			slotno;
	int			i;

	LWLockAcquire(CommitTsControlLock, LW_EXCLUSIVE);

	slotno = SimpleLruReadPage(CommitTsCtl, pageno, true, xid);

	TransactionIdSetCommitTs(xid, ts, nodeid, slotno);
	for (i = 0; i < nsubxids; i++)
		TransactionIdSetCommitTs(subxids[i], ts, nodeid, slotno);

	CommitTsCtl->shared->page_dirty[slotno] = true;

	LWLockRelease(CommitTsControlLock);
}

/*
 * Sets the commit timestamp of a single transaction.
 *
 * Must be called with CommitTsControlLock held
 */
static void
TransactionIdSetCommitTs(TransactionId xid, TimestampTz ts,
						 CommitTsNodeId nodeid, int slotno)
{
	int			entryno = TransactionIdToCTsEntry(xid);
	CommitTimestampEntry entry;

	Assert(TransactionIdIsNormal(xid));

	entry.time = ts;
	entry.nodeid = nodeid;

	memcpy(CommitTsCtl->shared->page_buffer[slotno] +
		   SizeOfCommitTimestampEntry * entryno,
		   &entry, SizeOfCommitTimestampEntry);
}

/*
 * Interrogate the commit timestamp of a transaction.
 *
 * Return value indicates whether commit timestamp record was found for
 * given xid.
 */
bool
TransactionIdGetCommitTsData(TransactionId xid, TimestampTz *ts,
							 CommitTsNodeId *nodeid)
{
	int			pageno = TransactionIdToCTsPage(xid);
	int			entryno = TransactionIdToCTsEntry(xid);
	int			slotno;
	CommitTimestampEntry entry;
	TransactionId oldestCommitTs;
	TransactionId newestCommitTs;

	/* Error if module not enabled */
	if (!track_commit_timestamp)
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("could not get commit timestamp data"),
				 errhint("Make sure the configuration parameter \"%s\" is set.",
						 "track_commit_timestamp")));

	/* error if the given Xid doesn't normally commit */
	if (!TransactionIdIsNormal(xid))
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
				 errmsg("cannot retrieve commit timestamp for transaction %u", xid)));

	/*
	 * Return empty if the requested value is outside our valid range.
	 */
	LWLockAcquire(CommitTsLock, LW_SHARED);
	oldestCommitTs = ShmemVariableCache->oldestCommitTs;
	newestCommitTs = ShmemVariableCache->newestCommitTs;
	/* neither is invalid, or both are */
	Assert(TransactionIdIsValid(oldestCommitTs) == TransactionIdIsValid(newestCommitTs));
	LWLockRelease(CommitTsLock);

	if (!TransactionIdIsValid(oldestCommitTs) ||
		TransactionIdPrecedes(xid, oldestCommitTs) ||
		TransactionIdPrecedes(newestCommitTs, xid))
	{
		if (ts)
			*ts = 0;
		if (nodeid)
			*nodeid = InvalidCommitTsNodeId;
		return false;
	}

	/*
	 * Use an unlocked atomic read on our cached value in shared memory; if
	 * it's a hit, acquire a lock and read the data, after verifying that it's
	 * still what we initially read.  Otherwise, fall through to read from
	 * SLRU.
	 */
	if (commitTsShared->xidLastCommit == xid)
	{
		LWLockAcquire(CommitTsLock, LW_SHARED);
		if (commitTsShared->xidLastCommit == xid)
		{
			if (ts)
				*ts = commitTsShared->dataLastCommit.time;
			if (nodeid)
				*nodeid = commitTsShared->dataLastCommit.nodeid;

			LWLockRelease(CommitTsLock);
			return *ts != 0;
		}
		LWLockRelease(CommitTsLock);
	}

	/* lock is acquired by SimpleLruReadPage_ReadOnly */
	slotno = SimpleLruReadPage_ReadOnly(CommitTsCtl, pageno, xid);
	memcpy(&entry,
		   CommitTsCtl->shared->page_buffer[slotno] +
		   SizeOfCommitTimestampEntry * entryno,
		   SizeOfCommitTimestampEntry);

	if (ts)
		*ts = entry.time;
	if (nodeid)
		*nodeid = entry.nodeid;

	LWLockRelease(CommitTsControlLock);
	return *ts != 0;
}

/*
 * Return the Xid of the latest committed transaction.  (As far as this module
 * is concerned, anyway; it's up to the caller to ensure the value is useful
 * for its purposes.)
 *
 * ts and extra are filled with the corresponding data; they can be passed
 * as NULL if not wanted.
 */
TransactionId
GetLatestCommitTsData(TimestampTz *ts, CommitTsNodeId *nodeid)
{
	TransactionId	xid;

	/* Error if module not enabled */
	if (!track_commit_timestamp)
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("could not get commit timestamp data"),
				 errhint("Make sure the configuration parameter \"%s\" is set.",
						 "track_commit_timestamp")));

	LWLockAcquire(CommitTsLock, LW_SHARED);
	xid = commitTsShared->xidLastCommit;
	if (ts)
		*ts = commitTsShared->dataLastCommit.time;
	if (nodeid)
		*nodeid = commitTsShared->dataLastCommit.nodeid;
	LWLockRelease(CommitTsLock);

	return xid;
}

/*
 * SQL-callable wrapper to obtain commit time of a transaction
 */
Datum
pg_xact_commit_timestamp(PG_FUNCTION_ARGS)
{
	TransactionId	xid = PG_GETARG_UINT32(0);
	TimestampTz		ts;
	bool			found;

	found = TransactionIdGetCommitTsData(xid, &ts, NULL);

	if (!found)
		PG_RETURN_NULL();

	PG_RETURN_TIMESTAMPTZ(ts);
}


Datum
pg_last_committed_xact(PG_FUNCTION_ARGS)
{
	TransactionId	xid;
	TimestampTz		ts;
	Datum       values[2];
	bool        nulls[2];
	TupleDesc   tupdesc;
	HeapTuple	htup;

	/* and construct a tuple with our data */
	xid = GetLatestCommitTsData(&ts, NULL);

	/*
	 * Construct a tuple descriptor for the result row.  This must match this
	 * function's pg_proc entry!
	 */
	tupdesc = CreateTemplateTupleDesc(2, false);
	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "xid",
					   XIDOID, -1, 0);
	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "timestamp",
					   TIMESTAMPTZOID, -1, 0);
	tupdesc = BlessTupleDesc(tupdesc);

	if (!TransactionIdIsNormal(xid))
	{
		memset(nulls, true, sizeof(nulls));
	}
	else
	{
		values[0] = TransactionIdGetDatum(xid);
		nulls[0] = false;

		values[1] = TimestampTzGetDatum(ts);
		nulls[1] = false;
	}

	htup = heap_form_tuple(tupdesc, values, nulls);

	PG_RETURN_DATUM(HeapTupleGetDatum(htup));
}


/*
 * Number of shared CommitTS buffers.
 *
 * We use a very similar logic as for the number of CLOG buffers; see comments
 * in CLOGShmemBuffers.
 */
Size
CommitTsShmemBuffers(void)
{
	return Min(16, Max(4, NBuffers / 1024));
}

/*
 * Shared memory sizing for CommitTs
 */
Size
CommitTsShmemSize(void)
{
	return SimpleLruShmemSize(CommitTsShmemBuffers(), 0) +
		sizeof(CommitTimestampShared);
}

/*
 * Initialize CommitTs at system startup (postmaster start or standalone
 * backend)
 */
void
CommitTsShmemInit(void)
{
	bool	found;

	CommitTsCtl->PagePrecedes = CommitTsPagePrecedes;
	SimpleLruInit(CommitTsCtl, "CommitTs Ctl", CommitTsShmemBuffers(), 0,
				  CommitTsControlLock, "pg_commit_ts");

	commitTsShared = ShmemInitStruct("CommitTs shared",
									 sizeof(CommitTimestampShared),
									 &found);

	if (!IsUnderPostmaster)
	{
		Assert(!found);

		commitTsShared->xidLastCommit = InvalidTransactionId;
		TIMESTAMP_NOBEGIN(commitTsShared->dataLastCommit.time);
		commitTsShared->dataLastCommit.nodeid = InvalidCommitTsNodeId;
	}
	else
		Assert(found);
}

/*
 * This function must be called ONCE on system install.
 *
 * (The CommitTs directory is assumed to have been created by initdb, and
 * CommitTsShmemInit must have been called already.)
 */
void
BootStrapCommitTs(void)
{
	/*
	 * Nothing to do here at present, unlike most other SLRU modules; segments
	 * are created when the server is started with this module enabled.
	 * See StartupCommitTs.
	 */
}

/*
 * Initialize (or reinitialize) a page of CommitTs to zeroes.
 * If writeXlog is TRUE, also emit an XLOG record saying we did this.
 *
 * The page is not actually written, just set up in shared memory.
 * The slot number of the new page is returned.
 *
 * Control lock must be held at entry, and will be held at exit.
 */
static int
ZeroCommitTsPage(int pageno, bool writeXlog)
{
	int			slotno;

	slotno = SimpleLruZeroPage(CommitTsCtl, pageno);

	if (writeXlog)
		WriteZeroPageXlogRec(pageno);

	return slotno;
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup,
 * after StartupXLOG has initialized ShmemVariableCache->nextXid.
 */
void
StartupCommitTs(void)
{
	TransactionId xid = ShmemVariableCache->nextXid;
	int			pageno = TransactionIdToCTsPage(xid);

	LWLockAcquire(CommitTsControlLock, LW_EXCLUSIVE);

	/*
	 * Initialize our idea of the latest page number.
	 */
	CommitTsCtl->shared->latest_page_number = pageno;

	LWLockRelease(CommitTsControlLock);
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup,
 * when commit timestamp is enabled.  Must be called after recovery has
 * finished.
 *
 * This is in charge of creating the currently active segment, if it's not
 * already there.  The reason for this is that the server might have been
 * running with this module disabled for a while and thus might have skipped
 * the normal creation point.
 */
void
CompleteCommitTsInitialization(void)
{
	TransactionId xid = ShmemVariableCache->nextXid;
	int			pageno = TransactionIdToCTsPage(xid);

	/*
	 * Re-Initialize our idea of the latest page number.
	 */
	LWLockAcquire(CommitTsControlLock, LW_EXCLUSIVE);
	CommitTsCtl->shared->latest_page_number = pageno;
	LWLockRelease(CommitTsControlLock);

	/*
	 * If this module is not currently enabled, make sure we don't hand back
	 * possibly-invalid data; also remove segments of old data.
	 */
	if (!track_commit_timestamp)
	{
		LWLockAcquire(CommitTsLock, LW_EXCLUSIVE);
		ShmemVariableCache->oldestCommitTs = InvalidTransactionId;
		ShmemVariableCache->newestCommitTs = InvalidTransactionId;
		LWLockRelease(CommitTsLock);

		TruncateCommitTs(ReadNewTransactionId());

		return;
	}

	/*
	 * If CommitTs is enabled, but it wasn't in the previous server run, we
	 * need to set the oldest and newest values to the next Xid; that way, we
	 * will not try to read data that might not have been set.
	 *
	 * XXX does this have a problem if a server is started with commitTs
	 * enabled, then started with commitTs disabled, then restarted with it
	 * enabled again?  It doesn't look like it does, because there should be a
	 * checkpoint that sets the value to InvalidTransactionId at end of
	 * recovery; and so any chance of injecting new transactions without
	 * CommitTs values would occur after the oldestCommitTs has been set to
	 * Invalid temporarily.
	 */
	LWLockAcquire(CommitTsLock, LW_EXCLUSIVE);
	if (ShmemVariableCache->oldestCommitTs == InvalidTransactionId)
	{
		ShmemVariableCache->oldestCommitTs =
			ShmemVariableCache->newestCommitTs = ReadNewTransactionId();
	}
	LWLockRelease(CommitTsLock);

	/* Finally, create the current segment file, if necessary */
	if (!SimpleLruDoesPhysicalPageExist(CommitTsCtl, pageno))
	{
		int		slotno;

		LWLockAcquire(CommitTsControlLock, LW_EXCLUSIVE);
		slotno = ZeroCommitTsPage(pageno, false);
		SimpleLruWritePage(CommitTsCtl, slotno);
		Assert(!CommitTsCtl->shared->page_dirty[slotno]);
		LWLockRelease(CommitTsControlLock);
	}
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownCommitTs(void)
{
	/* Flush dirty CommitTs pages to disk */
	SimpleLruFlush(CommitTsCtl, false);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 */
void
CheckPointCommitTs(void)
{
	/* Flush dirty CommitTs pages to disk */
	SimpleLruFlush(CommitTsCtl, true);
}

/*
 * Make sure that CommitTs has room for a newly-allocated XID.
 *
 * NB: this is called while holding XidGenLock.  We want it to be very fast
 * most of the time; even when it's not so fast, no actual I/O need happen
 * unless we're forced to write out a dirty CommitTs or xlog page to make room
 * in shared memory.
 *
 * NB: the current implementation relies on track_commit_timestamp being
 * PGC_POSTMASTER.
 */
void
ExtendCommitTs(TransactionId newestXact)
{
	int			pageno;

	/* nothing to do if module not enabled */
	if (!track_commit_timestamp)
		return;

	/*
	 * No work except at first XID of a page.  But beware: just after
	 * wraparound, the first XID of page zero is FirstNormalTransactionId.
	 */
	if (TransactionIdToCTsEntry(newestXact) != 0 &&
		!TransactionIdEquals(newestXact, FirstNormalTransactionId))
		return;

	pageno = TransactionIdToCTsPage(newestXact);

	LWLockAcquire(CommitTsControlLock, LW_EXCLUSIVE);

	/* Zero the page and make an XLOG entry about it */
	ZeroCommitTsPage(pageno, !InRecovery);

	LWLockRelease(CommitTsControlLock);
}

/*
 * Remove all CommitTs segments before the one holding the passed
 * transaction ID.
 *
 * Note that we don't need to flush XLOG here.
 */
void
TruncateCommitTs(TransactionId oldestXact)
{
	int			cutoffPage;

	/*
	 * The cutoff point is the start of the segment containing oldestXact. We
	 * pass the *page* containing oldestXact to SimpleLruTruncate.
	 */
	cutoffPage = TransactionIdToCTsPage(oldestXact);

	/* Check to see if there's any files that could be removed */
	if (!SlruScanDirectory(CommitTsCtl, SlruScanDirCbReportPresence,
						   &cutoffPage))
		return;					/* nothing to remove */

	/* Write XLOG record */
	WriteTruncateXlogRec(cutoffPage);

	/* Now we can remove the old CommitTs segment(s) */
	SimpleLruTruncate(CommitTsCtl, cutoffPage);
}

/*
 * Set the limit values between which commit TS can be consulted.
 */
void
SetCommitTsLimit(TransactionId oldestXact, TransactionId newestXact)
{
	/*
	 * Be careful not to overwrite values that are either further into the
	 * "future" or signal a disabled committs.
	 */
	LWLockAcquire(CommitTsLock, LW_EXCLUSIVE);
	if (ShmemVariableCache->oldestCommitTs != InvalidTransactionId)
	{
		if (TransactionIdPrecedes(ShmemVariableCache->oldestCommitTs, oldestXact))
			ShmemVariableCache->oldestCommitTs = oldestXact;
		if (TransactionIdPrecedes(newestXact, ShmemVariableCache->newestCommitTs))
			ShmemVariableCache->newestCommitTs = newestXact;
	}
	else
	{
		Assert(ShmemVariableCache->newestCommitTs == InvalidTransactionId);
	}
	LWLockRelease(CommitTsLock);
}

/*
 * Move forwards the oldest commitTS value that can be consulted
 */
void
AdvanceOldestCommitTs(TransactionId oldestXact)
{
	LWLockAcquire(CommitTsLock, LW_EXCLUSIVE);
	if (ShmemVariableCache->oldestCommitTs != InvalidTransactionId &&
		TransactionIdPrecedes(ShmemVariableCache->oldestCommitTs, oldestXact))
		ShmemVariableCache->oldestCommitTs = oldestXact;
	LWLockRelease(CommitTsLock);
}


/*
 * Decide which of two CLOG page numbers is "older" for truncation purposes.
 *
 * We need to use comparison of TransactionIds here in order to do the right
 * thing with wraparound XID arithmetic.  However, if we are asked about
 * page number zero, we don't want to hand InvalidTransactionId to
 * TransactionIdPrecedes: it'll get weird about permanent xact IDs.  So,
 * offset both xids by FirstNormalTransactionId to avoid that.
 */
static bool
CommitTsPagePrecedes(int page1, int page2)
{
	TransactionId xid1;
	TransactionId xid2;

	xid1 = ((TransactionId) page1) * COMMIT_TS_XACTS_PER_PAGE;
	xid1 += FirstNormalTransactionId;
	xid2 = ((TransactionId) page2) * COMMIT_TS_XACTS_PER_PAGE;
	xid2 += FirstNormalTransactionId;

	return TransactionIdPrecedes(xid1, xid2);
}


/*
 * Write a ZEROPAGE xlog record
 */
static void
WriteZeroPageXlogRec(int pageno)
{
	XLogBeginInsert();
	XLogRegisterData((char *) (&pageno), sizeof(int));
	(void) XLogInsert(RM_COMMIT_TS_ID, COMMIT_TS_ZEROPAGE);
}

/*
 * Write a TRUNCATE xlog record
 */
static void
WriteTruncateXlogRec(int pageno)
{
	XLogBeginInsert();
	XLogRegisterData((char *) (&pageno), sizeof(int));
	(void) XLogInsert(RM_COMMIT_TS_ID, COMMIT_TS_TRUNCATE);
}

/*
 * Write a SETTS xlog record
 */
static void
WriteSetTimestampXlogRec(TransactionId mainxid, int nsubxids,
						 TransactionId *subxids, TimestampTz timestamp,
						 CommitTsNodeId nodeid)
{
	xl_commit_ts_set	record;

	record.timestamp = timestamp;
	record.nodeid = nodeid;
	record.mainxid = mainxid;

	XLogBeginInsert();
	XLogRegisterData((char *) &record,
					 offsetof(xl_commit_ts_set, mainxid) +
					 sizeof(TransactionId));
	XLogRegisterData((char *) subxids, nsubxids * sizeof(TransactionId));
	XLogInsert(RM_COMMIT_TS_ID, COMMIT_TS_SETTS);
}

/*
 * CommitTS resource manager's routines
 */
void
commit_ts_redo(XLogReaderState *record)
{
	uint8		info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;

	/* Backup blocks are not used in commit_ts records */
	Assert(!XLogRecHasAnyBlockRefs(record));

	if (info == COMMIT_TS_ZEROPAGE)
	{
		int			pageno;
		int			slotno;

		memcpy(&pageno, XLogRecGetData(record), sizeof(int));

		LWLockAcquire(CommitTsControlLock, LW_EXCLUSIVE);

		slotno = ZeroCommitTsPage(pageno, false);
		SimpleLruWritePage(CommitTsCtl, slotno);
		Assert(!CommitTsCtl->shared->page_dirty[slotno]);

		LWLockRelease(CommitTsControlLock);
	}
	else if (info == COMMIT_TS_TRUNCATE)
	{
		int			pageno;

		memcpy(&pageno, XLogRecGetData(record), sizeof(int));

		/*
		 * During XLOG replay, latest_page_number isn't set up yet; insert a
		 * suitable value to bypass the sanity test in SimpleLruTruncate.
		 */
		CommitTsCtl->shared->latest_page_number = pageno;

		SimpleLruTruncate(CommitTsCtl, pageno);
	}
	else if (info == COMMIT_TS_SETTS)
	{
		xl_commit_ts_set *setts = (xl_commit_ts_set *) XLogRecGetData(record);
		int			nsubxids;
		TransactionId *subxids;

		nsubxids = ((XLogRecGetDataLen(record) - SizeOfCommitTsSet) /
					sizeof(TransactionId));
		if (nsubxids > 0)
		{
			subxids = palloc(sizeof(TransactionId) * nsubxids);
			memcpy(subxids,
				   XLogRecGetData(record) + SizeOfCommitTsSet,
				   sizeof(TransactionId) * nsubxids);
		}
		else
			subxids = NULL;

		TransactionTreeSetCommitTsData(setts->mainxid, nsubxids, subxids,
									   setts->timestamp, setts->nodeid, false);
		if (subxids)
			pfree(subxids);
	}
	else
		elog(PANIC, "commit_ts_redo: unknown op code %u", info);
}