postgres.git / summary / log / commit / refs

commit    07f32fcd23ac81898ed47f88beb569c631a2f223
Author:   Andres Freund <andres@anarazel.de>
Date:     Tue Aug 18 23:31:12 2020 +0000
First released in: 14.0

    Fix race condition in snapshot caching when 2PC is used.
    
    When preparing a transaction xactCompletionCount needs to be
    incremented, even though the transaction has not committed
    yet. Otherwise the snapshot used within the transaction otherwise can
    get reused outside of the prepared transaction. As GetSnapshotData()
    does not include the current xid when building a snapshot, reuse would
    not be correct.
    
    Somewhat surprisingly the regression tests only rarely show incorrect
    results without the fix. The reason for that is that often the
    snapshot's xmax will be >= the backend xid, yielding a snapshot that
    is correct, despite the bug.
    
    I'm working on a reliable test for the bug, but it seems worth seeing
    whether this fixes all the BF failures while I do.
    
    Author: Andres Freund <andres@anarazel.de>
    Discussion: https://postgr.es/m/E1k7tGP-0005V0-5k@gemulon.postgresql.org


 src/backend/storage/ipc/procarray.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index e687cde6f17..51f8099cad2 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -860,6 +860,15 @@ ProcArrayClearTransaction(PGPROC *proc)
 	Assert(!(proc->vacuumFlags & PROC_VACUUM_STATE_MASK));
 	Assert(!proc->delayChkpt);
 
+	/*
+	 * Need to increment completion count even though transaction hasn't
+	 * really committed yet. The reason for that is that GetSnapshotData()
+	 * omits the xid of the current transaction, thus without the increment we
+	 * otherwise could end up reusing the snapshot later. Which would be bad,
+	 * because it might not count the prepared transaction as running.
+	 */
+	ShmemVariableCache->xactCompletionCount++;
+
 	/* Clear the subtransaction-XID cache too */
 	Assert(ProcGlobal->subxidStates[pgxactoff].count == proc->subxidStatus.count &&
 		   ProcGlobal->subxidStates[pgxactoff].overflowed == proc->subxidStatus.overflowed);


[parent: 734478200aba]