freespace: Don't return blocks past the end of the main fork. GetPageWithFreeSpace() callers assume the returned block exists in the main fork, failing with "could not read block" errors if that doesn't hold. Make that assumption reliable now. It hadn't been guaranteed, due to the weak WAL and data ordering of participating components. Most operations on the fsm fork are not WAL-logged. Relation extension is not WAL-logged. Hence, an fsm-fork block on disk can reference a main-fork block that no WAL record has initialized. That could happen after an OS crash, a replica promote, or a PITR restore. wal_log_hints makes the trouble easier to hit; a replica promote or PITR ending just after a relevant fsm-fork FPI_FOR_HINT may yield this broken state. The v16 RelationAddBlocks() mechanism also makes the trouble easier to hit, since it bulk-extends even without extension lock waiters. Commit 917dc7d2393ce680dea7a59418be9ff341df3c14 stopped trouble around truncation, but vectors involving PageIsNew() pages remained.
This implementation adds a RelationGetNumberOfBlocks() call when the cached relation size doesn't confirm a block exists. We've been unable to identify a benchmark that slows materially, but this may show up as additional time in lseek(). An alternative without that overhead would be a new ReadBufferMode such that ReadBufferExtended() returns NULL after a 0-byte read, with all other errors handled normally. However, each GetFreeIndexPage() caller would then need code for the return-NULL case. Back-patch to v14, due to earlier versions not caching relation size and the absence of a pre-v16 problem report. Ronan Dunklau. Reported by Ronan Dunklau. Discussion: https://postgr.es/m/1878547.tdWV9SEqCh%40aivenlaptop Branch ------ REL_15_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/7c490a18b75b895c757a5340c33e05ca44a178f9 Modified Files -------------- src/backend/storage/freespace/README | 16 ++++- src/backend/storage/freespace/freespace.c | 106 ++++++++++++++++++++++++++---- src/backend/storage/smgr/smgr.c | 5 +- src/test/recovery/t/008_fsm_truncation.pl | 5 +- 4 files changed, 112 insertions(+), 20 deletions(-)
