Wait for transactions of an initial decoding snapshot to commit 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 504fe10d1d26, "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 <[email protected]> Author: Antonin Houska <[email protected]> Author: Rui Zhao <[email protected]> Backpatch-through: 14 Discussion: https://postgr.es/m/85833.1768840165@localhost Discussion: https://postgr.es/m/CADzfLwU78as45To9a%3D-Qkr5jEg3tMxc5rUtdKy2MTv4r_SDGng%40mail.gmail.com Branch ------ REL_18_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/6210f00cac2fa62b24881b31130bdb2224f7b950 Modified Files -------------- src/backend/access/transam/xact.c | 10 ++ src/backend/replication/logical/snapbuild.c | 46 +++++++- src/test/recovery/meson.build | 1 + src/test/recovery/t/057_snapshot_commit_race.pl | 148 ++++++++++++++++++++++++ 4 files changed, 202 insertions(+), 3 deletions(-)
