postgres.git / summary / log / commit / refs

commit    d22ac5378fbd9605dceabe1546f737ee005fb7b1
Author:   Thomas Munro <tmunro@postgresql.org>
Date:     Thu Mar 12 05:06:54 2020 +0000
First released in: 9.5.22, 9.6.18, 10.13, 11.8

    Fix nextXid tracking bug on standbys (9.5-11 only).
    
    RecordKnownAssignedTransactionIds() should never move
    nextXid backwards.  Before this commit, that could happen
    if some other code path had advanced it without advancing
    latestObservedXid.
    
    One consequence is that a well timed XLOG_CHECKPOINT_ONLINE
    could cause hot standby feedback messages to get confused
    and report an xmin from a future epoch, potentially allowing
    vacuum to run too soon on the primary.
    
    Repair, by making sure RecordKnownAssignedTransactionIds()
    can only move nextXid forwards.
    
    In release 12 and master, this was already done by commit
    2fc7af5e, which consolidated similar code and straightened
    out this bug.  Back-patch to supported releases before that.
    
    Author: Eka Palamadai <ekanatha@amazon.com>
    Discussion: https://postgr.es/m/98BB4805-D0A2-48E1-96F4-15014313EADC@amazon.com


 src/backend/storage/ipc/procarray.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index f1bbc4567c2..0389af5b98b 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -3016,7 +3016,8 @@ RecordKnownAssignedTransactionIds(TransactionId xid)
 		next_expected_xid = latestObservedXid;
 		TransactionIdAdvance(next_expected_xid);
 		LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
-		ShmemVariableCache->nextXid = next_expected_xid;
+		if (TransactionIdFollows(next_expected_xid, ShmemVariableCache->nextXid))
+			ShmemVariableCache->nextXid = next_expected_xid;
 		LWLockRelease(XidGenLock);
 	}
 }


[parent: 0b02476442d5]