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Commit2a4b6ee

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Compare Xmin to previous Xmax when locking an update chain
Not doing so causes us to traverse an update chain that has been brokenby concurrent page pruning. All other code that traverses update chainsuses this check as one of the cases in which to stop iterating, soreplicate it here too. Failure to do so leads to erroneous CLOG,subtrans or multixact lookups.Per discussion following the bug report by J Smith inCADFUPgc5bmtv-yg9znxV-vcfkb+JPRqs7m2OesQXaM_4Z1JpdQ@mail.gmail.comas diagnosed by Andres Freund.
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‎src/backend/access/heap/heapam.c

Lines changed: 14 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -4797,6 +4797,7 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
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old_infomask;
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TransactionIdxmax,
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new_xmax;
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TransactionIdpriorXmax=InvalidTransactionId;
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ItemPointerCopy(tid,&tupid);
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@@ -4822,6 +4823,18 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
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CHECK_FOR_INTERRUPTS();
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LockBuffer(buf,BUFFER_LOCK_EXCLUSIVE);
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/*
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* Check the tuple XMIN against prior XMAX, if any. If we reached
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* the end of the chain, we're done, so return success.
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*/
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if (TransactionIdIsValid(priorXmax)&&
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!TransactionIdEquals(HeapTupleHeaderGetXmin(mytup.t_data),
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priorXmax))
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{
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UnlockReleaseBuffer(buf);
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returnHeapTupleMayBeUpdated;
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}
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old_infomask=mytup.t_data->t_infomask;
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xmax=HeapTupleHeaderGetRawXmax(mytup.t_data);
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@@ -4922,6 +4935,7 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
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}
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/* tail recursion */
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priorXmax=HeapTupleHeaderGetUpdateXid(mytup.t_data);
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ItemPointerCopy(&(mytup.t_data->t_ctid),&tupid);
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UnlockReleaseBuffer(buf);
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}

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