Commit 9da0cc35284, which introduced parallel CREATE INDEX, failed to propagate relmapper.c backend local cache state to parallel worker processes. This could result in parallel index builds against mapped catalog relations where the leader process (participating as a worker) scans the new, pristine relfilenode, while worker processes scan the obsolescent relfilenode. When this happened, the final index structure was typically not consistent with the owning table's structure. The final index structure could contain entries formed from both heap relfilenodes. Only rebuilds on mapped catalog relations that occur as part of a VACUUM FULL or CLUSTER could become corrupt in practice, since their mapped relation relfilenode swap is what allows the inconsistency to arise.
On master, fix the problem by propagating the required relmapper.c backend state as part of standard parallel initialization (Cf. commit 29d58fd3). On v11, simply disallow builds against mapped catalog relations by deeming them parallel unsafe.
Author: Peter Geoghegan
4974d7f87e Handle parallel index builds on mapped relations.
src/backend/access/transam/README.parallel | 4 ++
src/backend/access/transam/parallel.c | 19 ++++++-
src/backend/access/transam/xact.c | 4 +-
src/backend/utils/cache/relmapper.c | 84 +++++++++++++++++++++++++++---
src/include/utils/relmapper.h | 6 ++-
5 files changed, 107 insertions(+), 10 deletions(-)