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Ensure that the sync slots reach a consistent state after promotion without losing data.
We were directly copying the LSN locations while syncing the slots on the standby. Now, it is possible that at some particular restart_lsn there are some running xacts, which means if we start reading the WAL from that location after promotion, we won't reach a consistent snapshot state at that point. However, on the primary, we would have already been in a consistent snapshot state at that restart_lsn so we would have just serialized the existing snapshot. To avoid this problem we will use the advance_slot functionality unless the snapshot already exists at the synced restart_lsn location. This will help us to ensure that snapbuilder/slot statuses are updated properly without generating any changes. Note that the synced slot will remain as RS_TEMPORARY till the decoding from corresponding restart_lsn can reach a consistent snapshot state after which they will be marked as RS_PERSISTENT. Per buildfarm Author: Hou Zhijie Reviewed-by: Bertrand Drouvot, Shveta Malik, Bharath Rupireddy, Amit Kapila Discussion: https://postgr.es/m/OS0PR01MB5716B3942AE49F3F725ACA92943B2@OS0PR01MB5716.jpnprd01.prod.outlook.com
This commit is contained in:
Amit Kapila
@@ -36,6 +36,7 @@
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#include "replication/decode.h"
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#include "replication/logical.h"
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#include "replication/reorderbuffer.h"
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#include "replication/slotsync.h"
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#include "replication/snapbuild.h"
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#include "storage/proc.h"
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#include "storage/procarray.h"
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@@ -516,17 +517,23 @@ CreateDecodingContext(XLogRecPtr start_lsn,
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(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
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errmsg("cannot use physical replication slot for logical decoding")));
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if (slot->data.database != MyDatabaseId)
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/*
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* We need to access the system tables during decoding to build the
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* logical changes unless we are in fast_forward mode where no changes are
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* generated.
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*/
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if (slot->data.database != MyDatabaseId && !fast_forward)
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ereport(ERROR,
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(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
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errmsg("replication slot \"%s\" was not created in this database",
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NameStr(slot->data.name))));
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/*
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* Do not allow consumption of a "synchronized" slot until the standby
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* gets promoted.
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* The slots being synced from the primary can't be used for decoding as
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* they are used after failover. However, we do allow advancing the LSNs
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* during the synchronization of slots. See update_local_synced_slot.
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*/
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if (RecoveryInProgress() && slot->data.synced)
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if (RecoveryInProgress() && slot->data.synced && !IsSyncingReplicationSlots())
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ereport(ERROR,
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errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
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errmsg("cannot use replication slot \"%s\" for logical decoding",
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@@ -2034,3 +2041,135 @@ LogicalReplicationSlotHasPendingWal(XLogRecPtr end_of_wal)
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return has_pending_wal;
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}
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/*
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* Helper function for advancing our logical replication slot forward.
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*
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* The slot's restart_lsn is used as start point for reading records, while
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* confirmed_flush is used as base point for the decoding context.
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*
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* We cannot just do LogicalConfirmReceivedLocation to update confirmed_flush,
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* because we need to digest WAL to advance restart_lsn allowing to recycle
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* WAL and removal of old catalog tuples. As decoding is done in fast_forward
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* mode, no changes are generated anyway.
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*
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* *found_consistent_snapshot will be true if the initial decoding snapshot has
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* been built; Otherwise, it will be false.
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*/
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XLogRecPtr
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LogicalSlotAdvanceAndCheckSnapState(XLogRecPtr moveto,
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bool *found_consistent_snapshot)
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{
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LogicalDecodingContext *ctx;
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ResourceOwner old_resowner = CurrentResourceOwner;
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XLogRecPtr retlsn;
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Assert(moveto != InvalidXLogRecPtr);
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if (found_consistent_snapshot)
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*found_consistent_snapshot = false;
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PG_TRY();
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{
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/*
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* Create our decoding context in fast_forward mode, passing start_lsn
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* as InvalidXLogRecPtr, so that we start processing from my slot's
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* confirmed_flush.
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*/
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ctx = CreateDecodingContext(InvalidXLogRecPtr,
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NIL,
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true, /* fast_forward */
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XL_ROUTINE(.page_read = read_local_xlog_page,
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.segment_open = wal_segment_open,
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.segment_close = wal_segment_close),
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NULL, NULL, NULL);
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/*
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* Wait for specified streaming replication standby servers (if any)
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* to confirm receipt of WAL up to moveto lsn.
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*/
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WaitForStandbyConfirmation(moveto);
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/*
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* Start reading at the slot's restart_lsn, which we know to point to
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* a valid record.
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*/
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XLogBeginRead(ctx->reader, MyReplicationSlot->data.restart_lsn);
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/* invalidate non-timetravel entries */
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InvalidateSystemCaches();
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/* Decode records until we reach the requested target */
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while (ctx->reader->EndRecPtr < moveto)
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{
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char *errm = NULL;
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XLogRecord *record;
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/*
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* Read records. No changes are generated in fast_forward mode,
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* but snapbuilder/slot statuses are updated properly.
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*/
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record = XLogReadRecord(ctx->reader, &errm);
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if (errm)
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elog(ERROR, "could not find record while advancing replication slot: %s",
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errm);
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/*
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* Process the record. Storage-level changes are ignored in
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* fast_forward mode, but other modules (such as snapbuilder)
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* might still have critical updates to do.
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*/
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if (record)
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LogicalDecodingProcessRecord(ctx, ctx->reader);
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CHECK_FOR_INTERRUPTS();
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}
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if (found_consistent_snapshot && DecodingContextReady(ctx))
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*found_consistent_snapshot = true;
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/*
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* Logical decoding could have clobbered CurrentResourceOwner during
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* transaction management, so restore the executor's value. (This is
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* a kluge, but it's not worth cleaning up right now.)
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*/
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CurrentResourceOwner = old_resowner;
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if (ctx->reader->EndRecPtr != InvalidXLogRecPtr)
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{
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LogicalConfirmReceivedLocation(moveto);
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/*
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* If only the confirmed_flush LSN has changed the slot won't get
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* marked as dirty by the above. Callers on the walsender
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* interface are expected to keep track of their own progress and
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* don't need it written out. But SQL-interface users cannot
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* specify their own start positions and it's harder for them to
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* keep track of their progress, so we should make more of an
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* effort to save it for them.
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*
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* Dirty the slot so it is written out at the next checkpoint. The
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* LSN position advanced to may still be lost on a crash but this
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* makes the data consistent after a clean shutdown.
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*/
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ReplicationSlotMarkDirty();
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}
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retlsn = MyReplicationSlot->data.confirmed_flush;
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/* free context, call shutdown callback */
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FreeDecodingContext(ctx);
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InvalidateSystemCaches();
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}
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PG_CATCH();
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{
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/* clear all timetravel entries */
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InvalidateSystemCaches();
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PG_RE_THROW();
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}
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PG_END_TRY();
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return retlsn;
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}
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