diff options
| author | Peter Geoghegan | 2026-08-20 23:47:23 +0000 |
|---|---|---|
| committer | Peter Geoghegan | 2026-08-20 23:47:23 +0000 |
| commit | 4afe4b564d867c0a2752840a309dfe5fb2616c6e (patch) | |
| tree | a420ca5bedd22b8fd76f3026cd8b29f56a7ab7d2 | |
| parent | 539e6a916eef09dc6b7c3abb603722af772b1386 (diff) | |
Fix snapshot import xmin ProcArrayLock bug.
ProcArrayInstallImportedXmin verifies that the source transaction (the
transaction whose snapshot we're importing) is still running, and then
installs the caller's imported xmin. These steps have to be atomic.
But it was just about possible for VACUUM to fail to observe the
imported xmin in either the source proc or the importing one. This
could result in VACUUM pruning away deleted tuples that were still
visible to the imported snapshot.
To fix, take ProcArrayLock in exclusive mode while importing an exported
snapshot's xmin within ProcArrayInstallImportedXmin. That guarantees
that a concurrent VACUUM's OldestXmin cannot advance past the xmin (one
proc or the other always advertises an xmin that holds it back).
Author: Chee Wooson <chee.wooson@gmail.com>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/20260730042128.714201-1-chee.wooson@gmail.com
Backpatch-through: 14
| -rw-r--r-- | src/backend/storage/ipc/procarray.c | 9 |
1 files changed, 7 insertions, 2 deletions
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c index 13505b90347..e4170d4e136 100644 --- a/src/backend/storage/ipc/procarray.c +++ b/src/backend/storage/ipc/procarray.c @@ -2623,8 +2623,13 @@ ProcArrayInstallImportedXmin(TransactionId xmin, if (!sourcevxid) return false; - /* Get lock so source xact can't end while we're doing this */ - LWLockAcquire(ProcArrayLock, LW_SHARED); + /* + * Take the lock in exclusive mode to ensure that installing xmin is + * atomic with our check that the source transaction is still running. + * (Using shared mode risks a concurrent VACUUM whose ComputeXidHorizons() + * call fails to observe the xmin in either the source proc or our own.) + */ + LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE); for (index = 0; index < arrayP->numProcs; index++) { |