From 793296739bd38e6166510311b9134e0da7db6e74 Mon Sep 17 00:00:00 2001 From: Alvaro Herrera Date: Wed, 27 Nov 2013 17:49:12 -0300 Subject: [PATCH 4/5] Compare Xmin to previous Xmax when locking an update chain Not doing so causes us to traverse an update chain that has been broken by concurrent page pruning. All other code that traverses update chains uses this check as one of the cases in which to stop iterating, so replicate it here too. Failure to do so leads to erroneous CLOG, subtrans or multixact lookups. Per bug report by J Smith in CADFUPgc5bmtv-yg9znxV-vcfkb+JPRqs7m2OesQXaM_4Z1JpdQ@mail.gmail.com diagnosed by Andres Freund. --- src/backend/access/heap/heapam.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c index 6d7cd0c..f70e1f7 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -4819,6 +4819,7 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid, old_infomask; TransactionId xmax, new_xmax; + TransactionId priorXmax = InvalidTransactionId; ItemPointerCopy(tid, &tupid); @@ -4844,6 +4845,18 @@ l4: CHECK_FOR_INTERRUPTS(); LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE); + /* + * Check the tuple XMIN against prior XMAX, if any. If we reached + * the end of the chain, we're done, so return success. + */ + if (TransactionIdIsValid(priorXmax) && + !TransactionIdEquals(HeapTupleHeaderGetXmin(mytup.t_data), + priorXmax)) + { + UnlockReleaseBuffer(buf); + return HeapTupleMayBeUpdated; + } + old_infomask = mytup.t_data->t_infomask; xmax = HeapTupleHeaderGetRawXmax(mytup.t_data); @@ -4944,6 +4957,7 @@ l4: } /* tail recursion */ + priorXmax = HeapTupleHeaderGetUpdateXid(mytup.t_data); ItemPointerCopy(&(mytup.t_data->t_ctid), &tupid); UnlockReleaseBuffer(buf); } -- 1.7.10.4