From 23b25586dccfe81b6ac46ba8b4e505d48192732b Mon Sep 17 00:00:00 2001 From: Heikki Linnakangas Date: Thu, 15 Jan 2026 20:57:12 +0200 Subject: [PATCH] Fix 'unexpected data beyond EOF' on replica restart On restart, a replica can fail with an error like 'unexpected data beyond EOF in block 200 of relation T/D/R'. These are the steps to reproduce it: - A relation has a size of 400 blocks. - Blocks 201 to 400 are empty. - Block 200 has two rows. - Blocks 100 to 199 are empty. - A restartpoint is done - Vacuum truncates the relation to 200 blocks - A FPW deletes a row in block 200 - A checkpoint is done - A FPW deletes the last row in block 200 - Vacuum truncates the relation to 100 blocks - The replica restarts When the replica restarts: - The relation on disk starts at 100 blocks, because all the truncations were applied before restart. - The first truncate to 200 blocks is replayed. It silently fails, but it will still (incorrectly!) update the cache size to 200 blocks - The first FPW on block 200 is applied. XLogReadBufferForRead relies on the cached size and incorrectly assumes that the page already exists in the file, and thus won't extend the relation. - The online checkpoint record is replayed, calling smgrdestroyall which causes the cached size to be discarded - The second FPW on block 200 is applied. This time, the detected size is 100 blocks, an extend is attempted. However, the block 200 is already present in the buffer cache due to the first FPW. This triggers the 'unexpected data beyond EOF'. To fix, update the cached size in SmgrRelation with the current size rather than the requested new size, when the requested new size is greater. Author: Anthonin Bonnefoy Discussion: https://www.postgresql.org/message-id/CAO6_Xqrv-snNJNhbj1KjQmWiWHX3nYGDgAc=vxaZP3qc4g1Siw@mail.gmail.com Backpatch-through: 14 --- src/backend/storage/smgr/md.c | 3 +++ src/backend/storage/smgr/smgr.c | 11 ++++++++++- 2 files changed, 13 insertions(+), 1 deletion(-) diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c index 3a74dbf0ed9..a2625871185 100644 --- a/src/backend/storage/smgr/md.c +++ b/src/backend/storage/smgr/md.c @@ -1282,6 +1282,9 @@ mdnblocks(SMgrRelation reln, ForkNumber forknum) * functions for this relation or handled interrupts in between. This makes * sure we have opened all active segments, so that truncate loop will get * them all! + * + * If nblocks > curnblk, the request is ignored when we are InRecovery, + * otherwise, an error is raised. */ void mdtruncate(SMgrRelation reln, ForkNumber forknum, diff --git a/src/backend/storage/smgr/smgr.c b/src/backend/storage/smgr/smgr.c index 8943528fe1c..378c2a03f39 100644 --- a/src/backend/storage/smgr/smgr.c +++ b/src/backend/storage/smgr/smgr.c @@ -910,8 +910,17 @@ smgrtruncate(SMgrRelation reln, ForkNumber *forknum, int nforks, * backends to invalidate their copies of smgr_cached_nblocks, and * these ones too at the next command boundary. But ensure they aren't * outright wrong until then. + * + * We can have nblocks > old_nblocks when a relation was truncated + * multiple times, a replica applied all the truncations, and later + * restarts from a restartpoint located before the truncations. The + * relation on disk will be the size of the last truncate. When + * replaying the first truncate, we will have nblocks > current size. + * In such cases, smgr_truncate does nothing, so set the cached size + * to the old size rather than the requested size. */ - reln->smgr_cached_nblocks[forknum[i]] = nblocks[i]; + reln->smgr_cached_nblocks[forknum[i]] = + nblocks[i] > old_nblocks[i] ? old_nblocks[i] : nblocks[i]; } }