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Wait for transactions of an initial decoding snapshot to commit

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SnapBuildInitialSnapshot() converts the snapshot builder's list of
committed transactions into a regular MVCC snapshot, which is then used
with HeapTupleSatisfiesMVCC().  However, a snapshot produced that way
and used as an MVCC snapshot can potentially cause data corruption, if
it captures a transaction after it writes its commit WAL record but
before it has updated its CLOG entry: it will be incorrectly used to set
hint bits as if that transaction had aborted.

Fix by having SnapBuildInitialSnapshot() wait until such transactions
have removed themselves from procarray, which guarantees correct
visibility.

Other uses of SnapBuildBuildSnapshot only use the snapshot as
historical, so they don't need the waits, but add commentary in that
function to alert possible future callers.

While at this, backpatch commit 504fe10d1d, "Fix misplaced comment in
snapbuild.c."  The comment it moved was nearby the place we modify.

This bug was diagnosed by stress-testing of REPACK (CONCURRENTLY) in
branch 19, so add a test case that tickles it using that feature in that
branch and master.  However, the bug exists in all branches, and in
branch 18 we can add a test with regular logical decoding using
injection points that tickles it.  Unfortunately that test cannot be
backpatched to earlier branches for lack of facilities.

Reported-by: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Author: Antonin Houska <ah@cybertec.at>
Author: Rui Zhao <zhaorui126@gmail.com>
Backpatch-through: 14
Discussion: https://postgr.es/m/85833.1768840165@localhost
Discussion: https://postgr.es/m/CADzfLwU78as45To9a%3D-Qkr5jEg3tMxc5rUtdKy2MTv4r_SDGng%40mail.gmail.com
This commit is contained in:
Álvaro Herrera
2026-09-24 11:52:47 +02:00
parent 6ceac2e125
commit a33ffaf7b8
1 changed files with 43 additions and 3 deletions
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46
src/backend/replication/logical/snapbuild.c
View File

@@ -480,6 +480,12 @@ SnapBuildSnapDecRefcount(Snapshot snap)
* In-progress transactions with catalog access are *not* allowed to modify
* these snapshots; they have to copy them and fill in appropriate ->curcid
* and ->subxip/subxcnt values.
*
* Note that some of the transactions in the returned snapshot->xip might not
* have finished committing yet (we saw their commit WAL records, but we don't
* know if they've removed themselves from procarray). Callers that want to
* use the returned snapshot as an MVCC one must wait for this to happen; see
* SnapBuildInitialSnapshot().
*/
static Snapshot
SnapBuildBuildSnapshot(SnapBuild *builder)
@@ -592,10 +598,39 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
snap = SnapBuildBuildSnapshot(builder);
/*
* We know that snap->xmin is alive, enforced by the logical xmin
* mechanism. Due to that we can do this without locks, we're only
* changing our own value.
* Transactions appear in snap->xip as soon as their commit WAL records
* are decoded, but that doesn't imply they can be considered committed in
* a regular MVCC snapshot: that happens only when they remove themselves
* from procarray. Wait for this to happen; otherwise somebody using the
* snapshot might set hint bits incorrectly.
*
* This is not needed during recovery, because the decoded commit record
* has been replayed already.
*/
if (!RecoveryInProgress())
{
RunningTransactions running;
running = GetRunningTransactionData();
LWLockRelease(ProcArrayLock);
LWLockRelease(XidGenLock);
/*
* Note we walk the array up to running->xcnt only, omitting this wait
* for subtransactions: the subxacts are covered by their top-level
* transaction already, there's no need for an additional wait.
*/
for (int i = 0; i < running->xcnt; i++)
{
TransactionId running_xid = running->xids[i];
if (bsearch(&running_xid, snap->xip, snap->xcnt,
sizeof(TransactionId), xidComparator) != NULL)
XactLockTableWait(running_xid, NULL, NULL, XLTW_None);
}
}
/*
* Building an initial snapshot is expensive and an unenforced xmin
* horizon would have bad consequences, therefore always double-check that
* the horizon is enforced.
@@ -608,6 +643,11 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
elog(ERROR, "cannot build an initial slot snapshot as oldest safe xid %u follows snapshot's xmin %u",
safeXid, snap->xmin);
/*
* We know that snap->xmin is alive, enforced by the logical xmin
* mechanism. Due to that we can do this without locks, we're only
* changing our own value.
*/
MyProc->xmin = snap->xmin;
/* allocate in transaction context */