Hi, Alexander, Here, I made some improvements according to your
discussion with Heikki.
On 2024-04-11 18:09, Alexander Korotkov wrote:
On Thu, Apr 11, 2024 at 1:46 AM Heikki Linnakangas <hlinn...@iki.fi>
wrote:
In a nutshell, it's possible for the loop in WaitForLSN to exit
without
cleaning up the process from the heap. I was able to hit that by
adding
a delay after the addLSNWaiter() call:
> TRAP: failed Assert("!procInfo->inHeap"), File:
"../src/backend/commands/waitlsn.c", Line: 114, PID: 1936152
> postgres: heikki postgres [local]
CALL(ExceptionalCondition+0xab)[0x55da1f68787b]
> postgres: heikki postgres [local] CALL(+0x331ec8)[0x55da1f204ec8]
> postgres: heikki postgres [local] CALL(WaitForLSN+0x139)[0x55da1f2052cc]
> postgres: heikki postgres [local]
CALL(pg_wal_replay_wait+0x18b)[0x55da1f2056e5]
> postgres: heikki postgres [local] CALL(ExecuteCallStmt+0x46e)[0x55da1f18031a]
> postgres: heikki postgres [local]
CALL(standard_ProcessUtility+0x8cf)[0x55da1f4b26c9]
I think there's a similar race condition if the timeout is reached at
the same time that the startup process wakes up the process.
Thank you for catching this. I think WaitForLSN() just needs to call
deleteLSNWaiter() unconditionally after exit from the loop.
Fix and add injection point test on this race condition.
On Thu, Apr 11, 2024 at 1:46 AM Heikki Linnakangas <hlinn...@iki.fi>
wrote:
The docs could use some-copy-editing, but just to point out one issue:
> There are also procedures to control the progress of recovery.
That's copy-pasted from an earlier sentence at the table that lists
functions like pg_promote(), pg_wal_replay_pause(), and
pg_is_wal_replay_paused(). The pg_wal_replay_wait() doesn't control
the
progress of recovery like those functions do, it only causes the
calling
backend to wait.
Fix documentation and add extra tests on multi-standby replication
and cascade replication.
--
Ivan Kartyshov
Postgres Professional: www.postgrespro.com
commit ddbee0e9fcd1f0a2a770d476acef677486f62367
Author: i.kartyshov <i.kartys...@postrgespro.ru>
Date: Wed Jun 12 01:29:00 2024 +0300
Subject: [PATCH v18] Implement pg_wal_replay_wait() stored procedure
pg_wal_replay_wait() is to be used on standby and specifies waiting for
the specific WAL location to be replayed before starting the transaction.
This option is useful when the user makes some data changes on primary and
needs a guarantee to see these changes on standby.
The queue of waiters is stored in the shared memory array sorted by LSN.
During replay of WAL waiters whose LSNs are already replayed are deleted from
the shared memory array and woken up by setting of their latches.
pg_wal_replay_wait() needs to wait without any snapshot held. Otherwise,
the snapshot could prevent the replay of WAL records implying a kind of
self-deadlock. This is why it is only possible to implement
pg_wal_replay_wait() as a procedure working in a non-atomic context,
not a function.
Catversion is bumped.
Discussion: https://postgr.es/m/eb12f9b03851bb2583adab5df9579b4b%40postgrespro.ru
Author: Kartyshov Ivan, Alexander Korotkov
Reviewed-by: Michael Paquier, Peter Eisentraut, Dilip Kumar, Amit Kapila
Reviewed-by: Alexander Lakhin, Bharath Rupireddy, Euler Taveira
---
doc/src/sgml/func.sgml | 112 ++++++++++++
src/backend/access/transam/xact.c | 6 +
src/backend/access/transam/xlog.c | 7 +
src/backend/access/transam/xlogrecovery.c | 11 ++
src/backend/catalog/system_functions.sql | 3 +
src/backend/commands/Makefile | 3 +-
src/backend/commands/meson.build | 1 +
src/backend/commands/waitlsn.c | 344 +++++++++++++++++++++++++++++++++++++
src/backend/lib/pairingheap.c | 18 +-
src/backend/storage/ipc/ipci.c | 3 +
src/backend/storage/lmgr/proc.c | 6 +
src/backend/utils/activity/wait_event_names.txt | 2 +
src/include/catalog/pg_proc.dat | 5 +
src/include/commands/waitlsn.h | 77 +++++++++
src/include/lib/pairingheap.h | 3 +
src/include/storage/lwlocklist.h | 1 +
src/test/recovery/meson.build | 2 +
src/test/recovery/t/043_wal_replay_wait.pl | 176 +++++++++++++++++++
src/test/recovery/t/044_wal_replay_wait_injection_test.pl | 98 +++++++++++
src/tools/pgindent/typedefs.list | 2 +
20 files changed, 877 insertions(+), 3 deletions(-)
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 17c44bc338..b36c5b2ad8 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -28762,6 +28762,118 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
the pause, the rate of WAL generation and available disk space.
</para>
+ <para>
+ The procedure shown in <xref linkend="procedures-recovery-control-table"/>
+ can be executed only during recovery.
+ </para>
+
+ <table id="procedures-recovery-control-table">
+ <title>Recovery Procedures</title>
+ <tgroup cols="1">
+ <thead>
+ <row>
+ <entry role="func_table_entry"><para role="func_signature">
+ Procedure
+ </para>
+ <para>
+ Description
+ </para></entry>
+ </row>
+ </thead>
+
+ <tbody>
+ <row>
+ <entry role="func_table_entry"><para role="func_signature">
+ <indexterm>
+ <primary>pg_wal_replay_wait</primary>
+ </indexterm>
+ <function>pg_wal_replay_wait</function> (
+ <parameter>target_lsn</parameter> <type>pg_lsn</type>,
+ <parameter>timeout</parameter> <type>bigint</type> <literal>DEFAULT</literal> <literal>0</literal>)
+ <returnvalue>void</returnvalue>
+ </para>
+ <para>
+ If <parameter>timeout</parameter> is not specified or zero, this
+ procedure returns once WAL is replayed upto
+ <literal>target_lsn</literal>.
+ If the <parameter>timeout</parameter> is specified (in
+ milliseconds) and greater than zero, the procedure waits until the
+ server actually replays the WAL upto <literal>target_lsn</literal> or
+ until the given time has passed. On timeout, an error is emitted.
+ </para></entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <para>
+ <function>pg_wal_replay_wait</function> waits till
+ <parameter>target_lsn</parameter> to be replayed on standby.
+ That is, after this function execution, the value returned by
+ <function>pg_last_wal_replay_lsn</function> should be greater or equal
+ to the <parameter>target_lsn</parameter> value. This is useful to achieve
+ read-your-writes-consistency, while using async replica for reads and
+ primary for writes. In that case <acronym>lsn</acronym> of the last
+ modification should be stored on the client application side or the
+ connection pooler side.
+ </para>
+
+ <para>
+ You can use <function>pg_wal_replay_wait</function> to wait for
+ the <type>pg_lsn</type> value. For example, an application could update
+ the <literal>movie</literal> table and get the <acronym>lsn</acronym> after
+ changes just made. This example uses <function>pg_current_wal_insert_lsn</function>
+ on primary server to get the <acronym>lsn</acronym> given that
+ <varname>synchronous_commit</varname> could be set to
+ <literal>off</literal>.
+
+ <programlisting>
+postgres=# UPDATE movie SET genre = 'Dramatic' WHERE genre = 'Drama';
+UPDATE 100
+postgres=# SELECT pg_current_wal_insert_lsn();
+pg_current_wal_insert_lsn
+--------------------
+0/306EE20
+(1 row)
+ </programlisting>
+
+ Then an application could run <function>pg_wal_replay_wait</function>
+ with the <acronym>lsn</acronym> obtained from primary. After that the
+ changes made of primary should be guaranteed to be visible on replica.
+
+ <programlisting>
+postgres=# CALL pg_wal_replay_wait('0/306EE20');
+CALL
+postgres=# SELECT * FROM movie WHERE genre = 'Drama';
+ genre
+-------
+(0 rows)
+ </programlisting>
+
+ It may also happen that target <acronym>lsn</acronym> is not achieved
+ within the timeout. In that case the error is thrown.
+
+ <programlisting>
+postgres=# CALL pg_wal_replay_wait('0/306EE20', 100);
+ERROR: timed out while waiting for target LSN 0/306EE20 to be replayed; current replay LSN 0/306EA60
+ </programlisting>
+
+ </para>
+
+ <para>
+ <function>pg_wal_replay_wait</function> can't be used within
+ the transaction, another procedure or function. Any of them holds a
+ snapshot, which could prevent the replay of WAL records.
+
+ <programlisting>
+postgres=# BEGIN;
+BEGIN
+postgres=*# CALL pg_wal_replay_wait('0/306EE20');
+ERROR: pg_wal_replay_wait() must be only called in non-atomic context
+DETAIL: Make sure pg_wal_replay_wait() isn't called within a transaction, another procedure, or a function.
+ </programlisting>
+
+ </para>
</sect2>
<sect2 id="functions-snapshot-synchronization">
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 9bda1aa6bc..d9546f7761 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -38,6 +38,7 @@
#include "commands/async.h"
#include "commands/tablecmds.h"
#include "commands/trigger.h"
+#include "commands/waitlsn.h"
#include "common/pg_prng.h"
#include "executor/spi.h"
#include "libpq/be-fsstubs.h"
@@ -2771,6 +2772,11 @@ AbortTransaction(void)
*/
LWLockReleaseAll();
+ /*
+ * Cleanup waiting for LSN if any.
+ */
+ WaitLSNCleanup();
+
/* Clear wait information and command progress indicator */
pgstat_report_wait_end();
pgstat_progress_end_command();
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 330e058c5f..ffc0f3d239 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -66,6 +66,7 @@
#include "catalog/catversion.h"
#include "catalog/pg_control.h"
#include "catalog/pg_database.h"
+#include "commands/waitlsn.h"
#include "common/controldata_utils.h"
#include "common/file_utils.h"
#include "executor/instrument.h"
@@ -6129,6 +6130,12 @@ StartupXLOG(void)
UpdateControlFile();
LWLockRelease(ControlFileLock);
+ /*
+ * Wake up all waiters for replay LSN. They need to report an error that
+ * recovery was ended before achieving the target LSN.
+ */
+ WaitLSNSetLatches(InvalidXLogRecPtr);
+
/*
* Shutdown the recovery environment. This must occur after
* RecoverPreparedTransactions() (see notes in lock_twophase_recover())
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index b45b833172..a96603cdd7 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -43,6 +43,7 @@
#include "backup/basebackup.h"
#include "catalog/pg_control.h"
#include "commands/tablespace.h"
+#include "commands/waitlsn.h"
#include "common/file_utils.h"
#include "miscadmin.h"
#include "pgstat.h"
@@ -1828,6 +1829,16 @@ PerformWalRecovery(void)
break;
}
+ /*
+ * If we replayed an LSN that someone was waiting for then walk
+ * over the shared memory array and set latches to notify the
+ * waiters.
+ */
+ if (waitLSN &&
+ (XLogRecoveryCtl->lastReplayedEndRecPtr >=
+ pg_atomic_read_u64(&waitLSN->minWaitedLSN)))
+ WaitLSNSetLatches(XLogRecoveryCtl->lastReplayedEndRecPtr);
+
/* Else, try to fetch the next WAL record */
record = ReadRecord(xlogprefetcher, LOG, false, replayTLI);
} while (record != NULL);
diff --git a/src/backend/catalog/system_functions.sql b/src/backend/catalog/system_functions.sql
index ae099e328c..623b9539b1 100644
--- a/src/backend/catalog/system_functions.sql
+++ b/src/backend/catalog/system_functions.sql
@@ -414,6 +414,9 @@ CREATE OR REPLACE FUNCTION
json_populate_recordset(base anyelement, from_json json, use_json_as_text boolean DEFAULT false)
RETURNS SETOF anyelement LANGUAGE internal STABLE ROWS 100 AS 'json_populate_recordset' PARALLEL SAFE;
+CREATE OR REPLACE PROCEDURE pg_wal_replay_wait(target_lsn pg_lsn, timeout int8 DEFAULT 0)
+ LANGUAGE internal AS 'pg_wal_replay_wait';
+
CREATE OR REPLACE FUNCTION pg_logical_slot_get_changes(
IN slot_name name, IN upto_lsn pg_lsn, IN upto_nchanges int, VARIADIC options text[] DEFAULT '{}',
OUT lsn pg_lsn, OUT xid xid, OUT data text)
diff --git a/src/backend/commands/Makefile b/src/backend/commands/Makefile
index 48f7348f91..cede90c3b9 100644
--- a/src/backend/commands/Makefile
+++ b/src/backend/commands/Makefile
@@ -61,6 +61,7 @@ OBJS = \
vacuum.o \
vacuumparallel.o \
variable.o \
- view.o
+ view.o \
+ waitlsn.o
include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/commands/meson.build b/src/backend/commands/meson.build
index 6dd00a4abd..7549be5dc3 100644
--- a/src/backend/commands/meson.build
+++ b/src/backend/commands/meson.build
@@ -50,4 +50,5 @@ backend_sources += files(
'vacuumparallel.c',
'variable.c',
'view.c',
+ 'waitlsn.c',
)
diff --git a/src/backend/commands/waitlsn.c b/src/backend/commands/waitlsn.c
new file mode 100644
index 0000000000..bd28a2fe32
--- /dev/null
+++ b/src/backend/commands/waitlsn.c
@@ -0,0 +1,344 @@
+/*-------------------------------------------------------------------------
+ *
+ * waitlsn.c
+ * Implements waiting for the given replay LSN, which is used in
+ * CALL pg_wal_replay_wait(target_lsn pg_lsn, timeout float8).
+ *
+ * Copyright (c) 2024, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ * src/backend/commands/waitlsn.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include <float.h>
+#include <math.h>
+
+#include "pgstat.h"
+#include "access/xlog.h"
+#include "access/xlogrecovery.h"
+#include "commands/waitlsn.h"
+#include "funcapi.h"
+#include "miscadmin.h"
+#include "storage/latch.h"
+#include "storage/proc.h"
+#include "storage/shmem.h"
+#include "utils/fmgrprotos.h"
+#include "utils/injection_point.h"
+#include "utils/pg_lsn.h"
+#include "utils/snapmgr.h"
+#include "utils/wait_event_types.h"
+
+static int lsn_cmp(const pairingheap_node *a, const pairingheap_node *b,
+ void *arg);
+
+struct WaitLSNState *waitLSN = NULL;
+
+/* Report the amount of shared memory space needed for WaitLSNState. */
+Size
+WaitLSNShmemSize(void)
+{
+ Size size;
+
+ size = offsetof(WaitLSNState, procInfos);
+ size = add_size(size, mul_size(MaxBackends, sizeof(WaitLSNProcInfo)));
+ return size;
+}
+
+/* Initialize the WaitLSNState in the shared memory. */
+void
+WaitLSNShmemInit(void)
+{
+ bool found;
+
+ waitLSN = (WaitLSNState *) ShmemInitStruct("WaitLSNState",
+ WaitLSNShmemSize(),
+ &found);
+ if (!found)
+ {
+ pg_atomic_init_u64(&waitLSN->minWaitedLSN, PG_UINT64_MAX);
+ pairingheap_initialize(&waitLSN->waitersHeap, lsn_cmp, NULL);
+ memset(&waitLSN->procInfos, 0, MaxBackends * sizeof(WaitLSNProcInfo));
+ }
+}
+
+/*
+ * Comparison function for waitLSN->waitersHeap heap. Waiting processes are
+ * ordered by lsn, so that the waiter with smallest lsn is at the top.
+ */
+static int
+lsn_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
+{
+ const WaitLSNProcInfo *aproc = pairingheap_const_container(WaitLSNProcInfo, phNode, a);
+ const WaitLSNProcInfo *bproc = pairingheap_const_container(WaitLSNProcInfo, phNode, b);
+
+ if (aproc->waitLSN < bproc->waitLSN)
+ return 1;
+ else if (aproc->waitLSN > bproc->waitLSN)
+ return -1;
+ else
+ return 0;
+}
+
+/*
+ * Update waitLSN->minWaitedLSN according to the current state of
+ * waitLSN->waitersHeap.
+ */
+static void
+updateMinWaitedLSN(void)
+{
+ XLogRecPtr minWaitedLSN = PG_UINT64_MAX;
+
+ if (!pairingheap_is_empty(&waitLSN->waitersHeap))
+ {
+ pairingheap_node *node = pairingheap_first(&waitLSN->waitersHeap);
+
+ minWaitedLSN = pairingheap_container(WaitLSNProcInfo, phNode, node)->waitLSN;
+ }
+
+ pg_atomic_write_u64(&waitLSN->minWaitedLSN, minWaitedLSN);
+}
+
+/*
+ * Put the current process into the heap of LSN waiters.
+ */
+static void
+addLSNWaiter(XLogRecPtr lsn)
+{
+ WaitLSNProcInfo *procInfo = &waitLSN->procInfos[MyProcNumber];
+
+ LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
+
+ Assert(!procInfo->inHeap);
+
+ procInfo->procnum = MyProcNumber;
+ procInfo->waitLSN = lsn;
+
+ pairingheap_add(&waitLSN->waitersHeap, &procInfo->phNode);
+ procInfo->inHeap = true;
+ updateMinWaitedLSN();
+
+ LWLockRelease(WaitLSNLock);
+}
+
+/*
+ * Remove the current process from the heap of LSN waiters if it's there.
+ */
+static void
+deleteLSNWaiter(void)
+{
+ WaitLSNProcInfo *procInfo = &waitLSN->procInfos[MyProcNumber];
+
+ LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
+
+ if (!procInfo->inHeap)
+ {
+ LWLockRelease(WaitLSNLock);
+ return;
+ }
+
+ pairingheap_remove(&waitLSN->waitersHeap, &procInfo->phNode);
+ procInfo->inHeap = false;
+ updateMinWaitedLSN();
+
+ LWLockRelease(WaitLSNLock);
+}
+
+/*
+ * Set latches of LSN waiters whose LSN has been replayed. Set latches of all
+ * LSN waiters when InvalidXLogRecPtr is given.
+ */
+void
+WaitLSNSetLatches(XLogRecPtr currentLSN)
+{
+ int i;
+ int *wakeUpProcNums;
+ int numWakeUpProcs = 0;
+
+ wakeUpProcNums = palloc(sizeof(int) * MaxBackends);
+
+ LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
+
+ /*
+ * Iterate the pairing heap of waiting processes till we find LSN not yet
+ * replayed. Record the process numbers to set their latches later.
+ */
+ while (!pairingheap_is_empty(&waitLSN->waitersHeap))
+ {
+ pairingheap_node *node = pairingheap_first(&waitLSN->waitersHeap);
+ WaitLSNProcInfo *procInfo = pairingheap_container(WaitLSNProcInfo, phNode, node);
+
+ if (!XLogRecPtrIsInvalid(currentLSN) &&
+ procInfo->waitLSN > currentLSN)
+ break;
+
+ wakeUpProcNums[numWakeUpProcs++] = procInfo->procnum;
+ (void) pairingheap_remove_first(&waitLSN->waitersHeap);
+ procInfo->inHeap = false;
+ }
+
+ updateMinWaitedLSN();
+
+ LWLockRelease(WaitLSNLock);
+
+ /*
+ * Set latches for processes, whose waited LSNs are already replayed. This
+ * involves spinlocks. So, we shouldn't do this under a spinlock.
+ */
+ for (i = 0; i < numWakeUpProcs; i++)
+ {
+ PGPROC *backend;
+
+ backend = GetPGProcByNumber(wakeUpProcNums[i]);
+ SetLatch(&backend->procLatch);
+ }
+ pfree(wakeUpProcNums);
+}
+
+/*
+ * Delete our item from shmem array if any.
+ */
+void
+WaitLSNCleanup(void)
+{
+ /*
+ * We do a fast-path check of the 'inHeap' flag without the lock. This
+ * flag is set to true only by the process itself. So, it's only possible
+ * to get a false positive. But that will be eliminated by a recheck
+ * inside deleteLSNWaiter().
+ */
+ if (waitLSN->procInfos[MyProcNumber].inHeap)
+ deleteLSNWaiter();
+}
+
+/*
+ * Wait using MyLatch till the given LSN is replayed, the postmaster dies or
+ * timeout happens.
+ */
+void
+WaitForLSN(XLogRecPtr targetLSN, int64 timeout)
+{
+ XLogRecPtr currentLSN;
+ TimestampTz endtime = 0;
+
+ /* Shouldn't be called when shmem isn't initialized */
+ Assert(waitLSN);
+
+ /* Should be only called by a backend */
+ Assert(MyBackendType == B_BACKEND && MyProcNumber <= MaxBackends);
+
+ if (!RecoveryInProgress())
+ ereport(ERROR,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("recovery is not in progress"),
+ errhint("Waiting for LSN can only be executed during recovery.")));
+
+ /* If target LSN is already replayed, exit immediately */
+ if (targetLSN <= GetXLogReplayRecPtr(NULL))
+ return;
+
+ if (timeout > 0)
+ endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(), timeout);
+
+ addLSNWaiter(targetLSN);
+
+ for (;;)
+ {
+ int rc;
+ int latch_events = WL_LATCH_SET | WL_EXIT_ON_PM_DEATH;
+ long delay_ms = 0;
+
+ /* Check if the waited LSN has been replayed */
+ currentLSN = GetXLogReplayRecPtr(NULL);
+ if (targetLSN <= currentLSN)
+ break;
+
+ /* Recheck that recovery is still in-progress */
+ if (!RecoveryInProgress())
+ ereport(ERROR,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("recovery is not in progress"),
+ errdetail("Recovery ended before replaying the target LSN %X/%X; last replay LSN %X/%X.",
+ LSN_FORMAT_ARGS(targetLSN),
+ LSN_FORMAT_ARGS(currentLSN))));
+
+ if (timeout > 0)
+ {
+ delay_ms = (endtime - GetCurrentTimestamp()) / 1000;
+ latch_events |= WL_TIMEOUT;
+ if (delay_ms <= 0)
+ break;
+ }
+
+ CHECK_FOR_INTERRUPTS();
+
+ rc = WaitLatch(MyLatch, latch_events, delay_ms,
+ WAIT_EVENT_WAIT_FOR_WAL_REPLAY);
+
+ if (rc & WL_LATCH_SET)
+ ResetLatch(MyLatch);
+ }
+
+ /*
+ * Check if startup process is already replayed target lsn
+ */
+ INJECTION_POINT("pg-wal-replay-wait");
+
+ deleteLSNWaiter();
+
+ if (targetLSN > currentLSN)
+ {
+ ereport(ERROR,
+ (errcode(ERRCODE_QUERY_CANCELED),
+ errmsg("timed out while waiting for target LSN %X/%X to be replayed; current replay LSN %X/%X",
+ LSN_FORMAT_ARGS(targetLSN),
+ LSN_FORMAT_ARGS(currentLSN))));
+ }
+}
+
+Datum
+pg_wal_replay_wait(PG_FUNCTION_ARGS)
+{
+ XLogRecPtr target_lsn = PG_GETARG_LSN(0);
+ int64 timeout = PG_GETARG_INT64(1);
+ CallContext *context = (CallContext *) fcinfo->context;
+
+ if (timeout < 0)
+ ereport(ERROR,
+ (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
+ errmsg("\"timeout\" must not be negative")));
+
+ /*
+ * We are going to wait for the LSN replay. We should first care that we
+ * don't hold a snapshot and correspondingly our MyProc->xmin is invalid.
+ * Otherwise, our snapshot could prevent the replay of WAL records
+ * implying a kind of self-deadlock. This is the reason why
+ * pg_wal_replay_wait() is a procedure, not a function.
+ *
+ * At first, we check that pg_wal_replay_wait() is called in a non-atomic
+ * context. That is, a procedure call isn't wrapped into a transaction,
+ * another procedure call, or a function call.
+ *
+ * Secondly, according to PlannedStmtRequiresSnapshot(), even in an atomic
+ * context, CallStmt is processed with a snapshot. Thankfully, we can pop
+ * this snapshot, because PortalRunUtility() can tolerate this.
+ */
+ if (context->atomic)
+ ereport(ERROR,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("pg_wal_replay_wait() must be only called in non-atomic context"),
+ errdetail("Make sure pg_wal_replay_wait() isn't called within a transaction, another procedure, or a function.")));
+
+ if (ActiveSnapshotSet())
+ PopActiveSnapshot();
+ Assert(!ActiveSnapshotSet());
+ InvalidateCatalogSnapshot();
+ Assert(MyProc->xmin == InvalidTransactionId);
+
+ (void) WaitForLSN(target_lsn, timeout);
+
+ PG_RETURN_VOID();
+}
diff --git a/src/backend/lib/pairingheap.c b/src/backend/lib/pairingheap.c
index fe1deba13e..7858e5e076 100644
--- a/src/backend/lib/pairingheap.c
+++ b/src/backend/lib/pairingheap.c
@@ -44,12 +44,26 @@ pairingheap_allocate(pairingheap_comparator compare, void *arg)
pairingheap *heap;
heap = (pairingheap *) palloc(sizeof(pairingheap));
+ pairingheap_initialize(heap, compare, arg);
+
+ return heap;
+}
+
+/*
+ * pairingheap_initialize
+ *
+ * Same as pairingheap_allocate(), but initializes the pairing heap in-place
+ * rather than allocating a new chunk of memory. Useful to store the pairing
+ * heap in a shared memory.
+ */
+void
+pairingheap_initialize(pairingheap *heap, pairingheap_comparator compare,
+ void *arg)
+{
heap->ph_compare = compare;
heap->ph_arg = arg;
heap->ph_root = NULL;
-
- return heap;
}
/*
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 521ed5418c..90c84fec27 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -25,6 +25,7 @@
#include "access/xlogprefetcher.h"
#include "access/xlogrecovery.h"
#include "commands/async.h"
+#include "commands/waitlsn.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
@@ -152,6 +153,7 @@ CalculateShmemSize(int *num_semaphores)
size = add_size(size, WaitEventExtensionShmemSize());
size = add_size(size, InjectionPointShmemSize());
size = add_size(size, SlotSyncShmemSize());
+ size = add_size(size, WaitLSNShmemSize());
#ifdef EXEC_BACKEND
size = add_size(size, ShmemBackendArraySize());
#endif
@@ -357,6 +359,7 @@ CreateOrAttachShmemStructs(void)
StatsShmemInit();
WaitEventExtensionShmemInit();
InjectionPointShmemInit();
+ WaitLSNShmemInit();
}
/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index ce29da9012..18e07bf77d 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -36,6 +36,7 @@
#include "access/transam.h"
#include "access/twophase.h"
#include "access/xlogutils.h"
+#include "commands/waitlsn.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
@@ -862,6 +863,11 @@ ProcKill(int code, Datum arg)
*/
LWLockReleaseAll();
+ /*
+ * Cleanup waiting for LSN if any.
+ */
+ WaitLSNCleanup();
+
/* Cancel any pending condition variable sleep, too */
ConditionVariableCancelSleep();
diff --git a/src/backend/utils/activity/wait_event_names.txt b/src/backend/utils/activity/wait_event_names.txt
index 87cbca2811..f079d660a4 100644
--- a/src/backend/utils/activity/wait_event_names.txt
+++ b/src/backend/utils/activity/wait_event_names.txt
@@ -87,6 +87,7 @@ LIBPQWALRECEIVER_CONNECT "Waiting in WAL receiver to establish connection to rem
LIBPQWALRECEIVER_RECEIVE "Waiting in WAL receiver to receive data from remote server."
SSL_OPEN_SERVER "Waiting for SSL while attempting connection."
WAIT_FOR_STANDBY_CONFIRMATION "Waiting for WAL to be received and flushed by the physical standby."
+WAIT_FOR_WAL_REPLAY "Waiting for a replay of the particular WAL position on the physical standby."
WAL_SENDER_WAIT_FOR_WAL "Waiting for WAL to be flushed in WAL sender process."
WAL_SENDER_WRITE_DATA "Waiting for any activity when processing replies from WAL receiver in WAL sender process."
@@ -345,6 +346,7 @@ WALSummarizer "Waiting to read or update WAL summarization state."
DSMRegistry "Waiting to read or update the dynamic shared memory registry."
InjectionPoint "Waiting to read or update information related to injection points."
SerialControl "Waiting to read or update shared <filename>pg_serial</filename> state."
+WaitLSN "Waiting to read or update shared Wait-for-LSN state."
#
# END OF PREDEFINED LWLOCKS (DO NOT CHANGE THIS LINE)
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 6a5476d3c4..5d83c4e919 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -12156,6 +12156,11 @@
prorettype => 'bytea', proargtypes => 'pg_brin_minmax_multi_summary',
prosrc => 'brin_minmax_multi_summary_send' },
+{ oid => '16387', descr => 'wait for LSN with timeout',
+ proname => 'pg_wal_replay_wait', prokind => 'p', prorettype => 'void',
+ proargtypes => 'pg_lsn int8', proargnames => '{target_lsn,timeout}',
+ prosrc => 'pg_wal_replay_wait' },
+
{ oid => '6291', descr => 'arbitrary value from among input values',
proname => 'any_value', prokind => 'a', proisstrict => 'f',
prorettype => 'anyelement', proargtypes => 'anyelement',
diff --git a/src/include/commands/waitlsn.h b/src/include/commands/waitlsn.h
new file mode 100644
index 0000000000..da17b8be6f
--- /dev/null
+++ b/src/include/commands/waitlsn.h
@@ -0,0 +1,77 @@
+/*-------------------------------------------------------------------------
+ *
+ * waitlsn.h
+ * Declarations for LSN replay waiting routines.
+ *
+ * Copyright (c) 2024, PostgreSQL Global Development Group
+ *
+ * src/include/commands/waitlsn.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef WAIT_LSN_H
+#define WAIT_LSN_H
+
+#include "lib/pairingheap.h"
+#include "postgres.h"
+#include "port/atomics.h"
+#include "storage/spin.h"
+#include "tcop/dest.h"
+
+/*
+ * WaitLSNProcInfo â the shared memory structure representing information
+ * about the single process, which may wait for LSN replay. An item of
+ * waitLSN->procInfos array.
+ */
+typedef struct WaitLSNProcInfo
+{
+ /*
+ * A process number, same as the index of this item in waitLSN->procInfos.
+ * Stored for convenience.
+ */
+ int procnum;
+
+ /* LSN, which this process is waiting for */
+ XLogRecPtr waitLSN;
+
+ /* A pairing heap node for participation in waitLSN->waitersHeap */
+ pairingheap_node phNode;
+
+ /* A flag indicating that this item is added to waitLSN->waitersHeap */
+ bool inHeap;
+} WaitLSNProcInfo;
+
+/*
+ * WaitLSNState - the shared memory state for the replay LSN waiting facility.
+ */
+typedef struct WaitLSNState
+{
+ /*
+ * The minimum LSN value some process is waiting for. Used for the
+ * fast-path checking if we need to wake up any waiters after replaying a
+ * WAL record. Could be read lock-less. Update protected by WaitLSNLock.
+ */
+ pg_atomic_uint64 minWaitedLSN;
+
+ /*
+ * A pairing heap of waiting processes order by LSN values (least LSN is
+ * on top). Protected by WaitLSNLock.
+ */
+ pairingheap waitersHeap;
+
+ /*
+ * An array with per-process information, indexed by the process number.
+ * Protected by WaitLSNLock.
+ */
+ WaitLSNProcInfo procInfos[FLEXIBLE_ARRAY_MEMBER];
+} WaitLSNState;
+
+extern PGDLLIMPORT struct WaitLSNState *waitLSN;
+
+extern void WaitForLSN(XLogRecPtr targetLSN, int64 timeout);
+extern Size WaitLSNShmemSize(void);
+extern void WaitLSNShmemInit(void);
+extern void WaitLSNSetLatches(XLogRecPtr currentLSN);
+extern void WaitLSNCleanup(void);
+
+#endif /* WAIT_LSN_H */
diff --git a/src/include/lib/pairingheap.h b/src/include/lib/pairingheap.h
index 7eade81535..9e1c26033a 100644
--- a/src/include/lib/pairingheap.h
+++ b/src/include/lib/pairingheap.h
@@ -77,6 +77,9 @@ typedef struct pairingheap
extern pairingheap *pairingheap_allocate(pairingheap_comparator compare,
void *arg);
+extern void pairingheap_initialize(pairingheap *heap,
+ pairingheap_comparator compare,
+ void *arg);
extern void pairingheap_free(pairingheap *heap);
extern void pairingheap_add(pairingheap *heap, pairingheap_node *node);
extern pairingheap_node *pairingheap_first(pairingheap *heap);
diff --git a/src/include/storage/lwlocklist.h b/src/include/storage/lwlocklist.h
index 85f6568b9e..c2bab5a794 100644
--- a/src/include/storage/lwlocklist.h
+++ b/src/include/storage/lwlocklist.h
@@ -83,3 +83,4 @@ PG_LWLOCK(49, WALSummarizer)
PG_LWLOCK(50, DSMRegistry)
PG_LWLOCK(51, InjectionPoint)
PG_LWLOCK(52, SerialControl)
+PG_LWLOCK(53, WaitLSN)
diff --git a/src/test/recovery/meson.build b/src/test/recovery/meson.build
index b1eb77b1ec..3efe4645ac 100644
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -51,6 +51,8 @@ tests += {
't/040_standby_failover_slots_sync.pl',
't/041_checkpoint_at_promote.pl',
't/042_low_level_backup.pl',
+ 't/043_wal_replay_wait.pl',
+ 't/044_wal_replay_wait_injection_test.pl',
],
},
}
diff --git a/src/test/recovery/t/043_wal_replay_wait.pl b/src/test/recovery/t/043_wal_replay_wait.pl
new file mode 100644
index 0000000000..867c3dd94a
--- /dev/null
+++ b/src/test/recovery/t/043_wal_replay_wait.pl
@@ -0,0 +1,176 @@
+# Checks waiting for the lsn replay on standby using
+# pg_wal_replay_wait() procedure.
+use strict;
+use warnings;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Initialize primary node
+my $node_primary = PostgreSQL::Test::Cluster->new('primary');
+$node_primary->init(allows_streaming => 1);
+$node_primary->start;
+
+# And some content and take a backup
+$node_primary->safe_psql('postgres',
+ "CREATE TABLE wait_test AS SELECT generate_series(1,10) AS a");
+my $backup_name = 'my_backup';
+$node_primary->backup($backup_name);
+
+# Create a streaming standby with a 1 second delay from the backup
+my $node_standby1 = PostgreSQL::Test::Cluster->new('standby');
+my $delay = 1;
+$node_standby1->init_from_backup($node_primary, $backup_name,
+ has_streaming => 1);
+$node_standby1->append_conf(
+ 'postgresql.conf', qq[
+ recovery_min_apply_delay = '${delay}s'
+]);
+$node_standby1->start;
+
+# I
+# Make sure that pg_wal_replay_wait() works: add new content to
+# primary and memorize primary's insert LSN, then wait for that LSN to be
+# replayed on standby.
+$node_primary->safe_psql('postgres',
+ "INSERT INTO wait_test VALUES (generate_series(11, 20))");
+my $lsn1 =
+ $node_primary->safe_psql('postgres', "SELECT pg_current_wal_insert_lsn()");
+my $output = $node_standby1->safe_psql(
+ 'postgres', qq[
+ CALL pg_wal_replay_wait('${lsn1}', 1000000);
+ SELECT pg_lsn_cmp(pg_last_wal_replay_lsn(), '${lsn1}'::pg_lsn);
+]);
+
+# Make sure the current LSN on standby is at least as big as the LSN we
+# observed on primary's before.
+ok($output >= 0,
+ "standby reached the same LSN as primary after pg_wal_replay_wait()");
+
+# II
+# Check that new data is visible after calling pg_wal_replay_wait()
+$node_primary->safe_psql('postgres',
+ "INSERT INTO wait_test VALUES (generate_series(21, 30))");
+my $lsn2 =
+ $node_primary->safe_psql('postgres', "SELECT pg_current_wal_insert_lsn()");
+$output = $node_standby1->safe_psql(
+ 'postgres', qq[
+ CALL pg_wal_replay_wait('${lsn2}');
+ SELECT count(*) FROM wait_test;
+]);
+
+# Make sure the current LSN on standby and is the same as primary's LSN
+ok($output eq 30, "standby reached the same LSN as primary");
+
+# III
+# Check two standby waiting LSN
+# Create a streaming second standby with a 1 second delay from the backup
+my $node_standby2 = PostgreSQL::Test::Cluster->new('standby2');
+$node_standby2->init_from_backup($node_primary, $backup_name,
+ has_streaming => 1);
+$node_standby2->append_conf(
+ 'postgresql.conf', qq[
+ recovery_min_apply_delay = '${delay}s'
+]);
+$node_standby2->start;
+
+# Check that new data is visible after calling pg_wal_replay_wait()
+$node_primary->safe_psql('postgres',
+ "INSERT INTO wait_test VALUES (generate_series(31, 40))");
+$lsn2 =
+ $node_primary->safe_psql('postgres', "SELECT pg_current_wal_insert_lsn()");
+
+$output = $node_standby1->safe_psql(
+ 'postgres', qq[
+ CALL pg_wal_replay_wait('${lsn2}');
+ SELECT count(*) FROM wait_test;
+]);
+my $output2 = $node_standby1->safe_psql(
+ 'postgres', qq[
+ CALL pg_wal_replay_wait('${lsn2}');
+ SELECT count(*) FROM wait_test;
+]);
+
+# Make sure the current LSN on standby and standby2 are the same as
+# primary's LSN
+ok($output eq 40, "standby1 reached the same LSN as primary");
+ok($output2 eq 40, "standby2 reached the same LSN as primary");
+
+# IV
+# Create a cascading standby waiting LSN
+$backup_name = 'cas_backup';
+$node_standby1->backup($backup_name);
+
+my $cascading_standby = PostgreSQL::Test::Cluster->new('cascading_standby');
+$cascading_standby->init_from_backup($node_standby1, $backup_name,
+ has_streaming => 1,
+ has_restoring => 1);
+
+my $cascading_connstr = $node_standby1->connstr;
+$cascading_standby->append_conf(
+ 'postgresql.conf', qq(
+ hot_standby_feedback = on
+ recovery_min_apply_delay = '${delay}s'
+));
+
+$cascading_standby->start;
+
+# Check that new data is visible after calling pg_wal_replay_wait()
+$node_primary->safe_psql('postgres',
+ "INSERT INTO wait_test VALUES (generate_series(41, 50))");
+$lsn2 =
+ $node_primary->safe_psql('postgres', "SELECT pg_current_wal_insert_lsn()");
+
+$output = $node_standby1->safe_psql(
+ 'postgres', qq[
+ CALL pg_wal_replay_wait('${lsn2}');
+ SELECT count(*) FROM wait_test;
+]);
+$output2 = $cascading_standby->safe_psql(
+ 'postgres', qq[
+ CALL pg_wal_replay_wait('${lsn2}');
+ SELECT count(*) FROM wait_test;
+]);
+
+# Make sure the current LSN on standby and standby2 are the same as
+# primary's LSN
+ok($output eq 50, "standby1 reached the same LSN as primary");
+ok($output2 eq 50, "cascading_standby reached the same LSN as primary");
+
+# V
+# Check that waiting for unreachable LSN triggers the timeout. The
+# unreachable LSN must be well in advance. So WAL records issued by
+# the concurrent autovacuum could not affect that.
+my $lsn3 =
+ $node_primary->safe_psql('postgres',
+ "SELECT pg_current_wal_insert_lsn() + 10000000000");
+my $stderr;
+$node_standby1->safe_psql('postgres',
+ "CALL pg_wal_replay_wait('${lsn2}', 10);");
+$node_standby1->psql(
+ 'postgres',
+ "CALL pg_wal_replay_wait('${lsn3}', 1000);",
+ stderr => \$stderr);
+ok( $stderr =~ /timed out while waiting for target LSN/,
+ "get timeout on waiting for unreachable LSN");
+
+# VI
+# Check that the standby promotion terminates the wait on LSN. Start
+# waiting for unreachable LSN then promote. Check the log for the relevant
+# error message.
+my $psql_session = $node_standby1->background_psql('postgres');
+$psql_session->query_until(
+ qr/start/, qq[
+ \\echo start
+ CALL pg_wal_replay_wait('${lsn3}');
+]);
+
+my $log_offset = -s $node_standby1->logfile;
+$node_standby1->promote;
+$node_standby1->wait_for_log('recovery is not in progress', $log_offset);
+
+$node_standby1->stop;
+$node_standby2->stop;
+$node_primary->stop;
+done_testing();
diff --git a/src/test/recovery/t/044_wal_replay_wait_injection_test.pl b/src/test/recovery/t/044_wal_replay_wait_injection_test.pl
new file mode 100644
index 0000000000..2429ef0da2
--- /dev/null
+++ b/src/test/recovery/t/044_wal_replay_wait_injection_test.pl
@@ -0,0 +1,98 @@
+# Copyright (c) 2024, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Time::HiRes qw(usleep);
+use Test::More;
+
+##################################################
+# Test race condition when timeout reached and new wal replayed on standby.
+#
+# This test relies on an injection point that cause to wait before Delete
+# from heap and wait when new lsn added to heap.
+##################################################
+
+if ($ENV{enable_injection_points} ne 'yes')
+{
+ plan skip_all => 'Injection points not supported by this build';
+}
+
+# Initialize primary node.
+my $node_primary = PostgreSQL::Test::Cluster->new('master');
+$node_primary->init(allows_streaming => 1);
+$node_primary->append_conf(
+ 'postgresql.conf', q[
+restart_after_crash = on
+]);
+$node_primary->start;
+
+my $backup_name = 'my_backup';
+$node_primary->backup($backup_name);
+
+# Setup first standby.
+my $node_standby = PostgreSQL::Test::Cluster->new('standby1');
+$node_standby->init_from_backup($node_primary, $backup_name,
+ has_streaming => 1);
+$node_standby->start;
+
+# Setup second standby.
+my $node_standby2 = PostgreSQL::Test::Cluster->new('standby2');
+$node_standby2->init_from_backup($node_primary, $backup_name,
+ has_streaming => 1);
+$node_standby2->start;
+
+# Create table
+$node_primary->safe_psql('postgres', 'CREATE TABLE prim_tab (a int);');
+
+# Create extention injection point
+$node_primary->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+# Wait until the extension has been created on the standby
+$node_primary->wait_for_replay_catchup($node_standby);
+
+# Attach the point in before deleteLSNWaiter.
+$node_standby->safe_psql('postgres',
+ "SELECT injection_points_attach('pg-wal-replay-wait', 'wait');");
+
+# Get log offset
+my $logstart = -s $node_standby->logfile;
+
+# Get current lsn from primary
+my $lsn_pr = $node_primary->safe_psql('postgres',
+ "SELECT pg_current_wal_insert_lsn() + 10000");
+
+# CALL pg_wal_replay_wait with timeout on standby1
+my $psql_session1 = $node_standby->background_psql('postgres');
+$psql_session1->query_until(
+ qr/start/, qq[
+ \\echo start
+ CALL pg_wal_replay_wait('${lsn_pr}',1000);
+]);
+
+# Wait till pg-wal-replay-wait apperied in pg_stat_activity
+$node_standby->wait_for_event('client backend', 'pg-wal-replay-wait');
+
+# Generate some WAL
+$node_primary->safe_psql('postgres', 'INSERT INTO prim_tab VALUES (1);');
+$node_primary->wait_for_replay_catchup($node_standby);
+$node_primary->wait_for_replay_catchup($node_standby2);
+
+# CALL pg_wal_replay_wait with timeout on standby2
+my $psql_session2 = $node_standby2->background_psql('postgres');
+$psql_session2->query_until(
+ qr/start/, qq[
+ \\echo start
+ CALL pg_wal_replay_wait('${lsn_pr}',1000);
+]);
+
+# Wake up to check the message
+$node_standby->safe_psql('postgres',
+ "SELECT injection_points_wakeup('pg-wal-replay-wait');");
+
+# Check the log
+ok( $node_standby->log_contains(
+ "timed out while waiting for target LSN", $logstart),
+ "pg_wal_replay_wait timeout");
+
+done_testing();
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 4f57078d13..85859158cb 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -3106,6 +3106,8 @@ WaitEventIO
WaitEventIPC
WaitEventSet
WaitEventTimeout
+WaitLSNProcInfo
+WaitLSNState
WaitPMResult
WalCloseMethod
WalCompression
