From 447aae13b0305780e87cac7b0dd669db6fab3d9d Mon Sep 17 00:00:00 2001 From: Alexander Korotkov Date: Wed, 5 Nov 2025 11:43:55 +0200 Subject: Implement WAIT FOR command MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit WAIT FOR is to be used on standby and specifies waiting for the specific WAL location to be replayed. This option is useful when the user makes some data changes on primary and needs a guarantee to see these changes are on standby. WAIT FOR 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 separate utility command seems appears to be the most robust way to implement this functionality. It's not possible to implement this as a function. Previous experience shows that stored procedures also have limitation in this aspect. Discussion: https://www.postgresql.org/message-id/flat/CAPpHfdsjtZLVzxjGT8rJHCYbM0D5dwkO+BBjcirozJ6nYbOW8Q@mail.gmail.com Discussion: https://www.postgresql.org/message-id/flat/CABPTF7UNft368x-RgOXkfj475OwEbp%2BVVO-wEXz7StgjD_%3D6sw%40mail.gmail.com Author: Kartyshov Ivan Author: Alexander Korotkov Author: Xuneng Zhou Reviewed-by: Michael Paquier Reviewed-by: Peter Eisentraut Reviewed-by: Dilip Kumar Reviewed-by: Amit Kapila Reviewed-by: Alexander Lakhin Reviewed-by: Bharath Rupireddy Reviewed-by: Euler Taveira Reviewed-by: Heikki Linnakangas Reviewed-by: Kyotaro Horiguchi Reviewed-by: jian he Reviewed-by: Álvaro Herrera Reviewed-by: Xuneng Zhou --- src/backend/access/transam/xact.c | 6 ++++++ src/backend/access/transam/xlog.c | 7 +++++++ src/backend/access/transam/xlogrecovery.c | 11 +++++++++++ 3 files changed, 24 insertions(+) (limited to 'src/backend/access/transam') diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index 2cf3d4e92b7..092e197eba3 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -31,6 +31,7 @@ #include "access/xloginsert.h" #include "access/xlogrecovery.h" #include "access/xlogutils.h" +#include "access/xlogwait.h" #include "catalog/index.h" #include "catalog/namespace.h" #include "catalog/pg_enum.h" @@ -2843,6 +2844,11 @@ 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 9900e3e0179..7c959051e11 100644 --- a/src/backend/access/transam/xlog.c +++ b/src/backend/access/transam/xlog.c @@ -62,6 +62,7 @@ #include "access/xlogreader.h" #include "access/xlogrecovery.h" #include "access/xlogutils.h" +#include "access/xlogwait.h" #include "backup/basebackup.h" #include "catalog/catversion.h" #include "catalog/pg_control.h" @@ -6227,6 +6228,12 @@ StartupXLOG(void) UpdateControlFile(); LWLockRelease(ControlFileLock); + /* + * Wake up all waiters for replay LSN. They need to report an error that + * recovery was ended before reaching the target LSN. + */ + WaitLSNWakeup(WAIT_LSN_TYPE_REPLAY, 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 93c50831b26..550de6e4a59 100644 --- a/src/backend/access/transam/xlogrecovery.c +++ b/src/backend/access/transam/xlogrecovery.c @@ -40,6 +40,7 @@ #include "access/xlogreader.h" #include "access/xlogrecovery.h" #include "access/xlogutils.h" +#include "access/xlogwait.h" #include "backup/basebackup.h" #include "catalog/pg_control.h" #include "commands/tablespace.h" @@ -1838,6 +1839,16 @@ 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 (waitLSNState && + (XLogRecoveryCtl->lastReplayedEndRecPtr >= + pg_atomic_read_u64(&waitLSNState->minWaitedLSN[WAIT_LSN_TYPE_REPLAY]))) + WaitLSNWakeup(WAIT_LSN_TYPE_REPLAY, XLogRecoveryCtl->lastReplayedEndRecPtr); + /* Else, try to fetch the next WAL record */ record = ReadRecord(xlogprefetcher, LOG, false, replayTLI); } while (record != NULL); -- cgit v1.2.3