Revert bogus fixes of HOT-freezing bug

alvherre · alvherre · commit ef0339ee5dcf · 2017-11-02T15:51:05.000+01:00
It turns out we misdiagnosed what the real problem was. Revert the previous changes, because they may have worse consequences going forward. A better fix is forthcoming. The simplistic test case is kept, though disabled. Discussion: https://postgr.es/m/20171102112019.33wb7g5wp4zpjelu@alap3.anarazel.de
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
@@ -1737,7 +1737,8 @@ heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
 		 * broken.
 		 */
 		if (TransactionIdIsValid(prev_xmax) &&
-			!HeapTupleUpdateXmaxMatchesXmin(prev_xmax, heapTuple->t_data))
+			!TransactionIdEquals(prev_xmax,
+								 HeapTupleHeaderGetXmin(heapTuple->t_data)))
 			break;
 
 		/*
@@ -1919,7 +1920,7 @@ heap_get_latest_tid(Relation relation,
 		 * tuple.  Check for XMIN match.
 		 */
 		if (TransactionIdIsValid(priorXmax) &&
-			!HeapTupleUpdateXmaxMatchesXmin(priorXmax, tp.t_data))
+		  !TransactionIdEquals(priorXmax, HeapTupleHeaderGetXmin(tp.t_data)))
 		{
 			UnlockReleaseBuffer(buffer);
 			break;
@@ -1951,50 +1952,6 @@ heap_get_latest_tid(Relation relation,
 	}							/* end of loop */
 }
 
-/*
- * HeapTupleUpdateXmaxMatchesXmin - verify update chain xmax/xmin lineage
- *
- * Given the new version of a tuple after some update, verify whether the
- * given Xmax (corresponding to the previous version) matches the tuple's
- * Xmin, taking into account that the Xmin might have been frozen after the
- * update.
- */
-bool
-HeapTupleUpdateXmaxMatchesXmin(TransactionId xmax, HeapTupleHeader htup)
-{
-	TransactionId	xmin = HeapTupleHeaderGetXmin(htup);
-
-	/*
-	 * If the xmax of the old tuple is identical to the xmin of the new one,
-	 * it's a match.
-	 */
-	if (TransactionIdEquals(xmax, xmin))
-		return true;
-
-	/*
-	 * If the Xmin that was in effect prior to a freeze matches the Xmax,
-	 * it's good too.
-	 */
-	if (HeapTupleHeaderXminFrozen(htup) &&
-		TransactionIdEquals(HeapTupleHeaderGetRawXmin(htup), xmax))
-		return true;
-
-	/*
-	 * When a tuple is frozen, the original Xmin is lost, but we know it's a
-	 * committed transaction.  So unless the Xmax is InvalidXid, we don't know
-	 * for certain that there is a match, but there may be one; and we must
-	 * return true so that a HOT chain that is half-frozen can be walked
-	 * correctly.
-	 *
-	 * We no longer freeze tuples this way, but we must keep this in order to
-	 * interpret pre-pg_upgrade pages correctly.
-	 */
-	if (TransactionIdEquals(xmin, FrozenTransactionId) &&
-		TransactionIdIsValid(xmax))
-		return true;
-
-	return false;
-}
 
 /*
  * UpdateXmaxHintBits - update tuple hint bits after xmax transaction ends
@@ -5283,7 +5240,8 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
 		 * end of the chain, we're done, so return success.
 		 */
 		if (TransactionIdIsValid(priorXmax) &&
-			!HeapTupleUpdateXmaxMatchesXmin(priorXmax, mytup.t_data))
+			!TransactionIdEquals(HeapTupleHeaderGetXmin(mytup.t_data),
+								 priorXmax))
 		{
 			UnlockReleaseBuffer(buf);
 			return HeapTupleMayBeUpdated;
@@ -5729,23 +5687,14 @@ FreezeMultiXactId(MultiXactId multi, uint16 t_infomask,
 			Assert(TransactionIdIsValid(xid));
 
 			/*
-			 * The updating transaction cannot possibly be still running, but
-			 * verify whether it has committed, and request to set the
-			 * COMMITTED flag if so.  (We normally don't see any tuples in
-			 * this state, because they are removed by page pruning before we
-			 * try to freeze the page; but this can happen if the updating
-			 * transaction commits after the page is pruned but before
-			 * HeapTupleSatisfiesVacuum).
+			 * If the xid is older than the cutoff, it has to have aborted,
+			 * otherwise the tuple would have gotten pruned away.
 			 */
 			if (TransactionIdPrecedes(xid, cutoff_xid))
 			{
-				if (TransactionIdDidCommit(xid))
-					*flags = FRM_MARK_COMMITTED | FRM_RETURN_IS_XID;
-				else
-				{
-					*flags |= FRM_INVALIDATE_XMAX;
-					xid = InvalidTransactionId;	/* not strictly necessary */
-				}
+				Assert(!TransactionIdDidCommit(xid));
+				*flags |= FRM_INVALIDATE_XMAX;
+				xid = InvalidTransactionId;		/* not strictly necessary */
 			}
 			else
 			{
@@ -5816,51 +5765,65 @@ FreezeMultiXactId(MultiXactId multi, uint16 t_infomask,
 
 			/*
 			 * It's an update; should we keep it?  If the transaction is known
-			 * aborted or crashed then it's okay to ignore it, otherwise not.
+			 * aborted then it's okay to ignore it, otherwise not.  However,
+			 * if the Xid is older than the cutoff_xid, we must remove it.
+			 * Note that such an old updater cannot possibly be committed,
+			 * because HeapTupleSatisfiesVacuum would have returned
+			 * HEAPTUPLE_DEAD and we would not be trying to freeze the tuple.
+			 *
+			 * Note the TransactionIdDidAbort() test is just an optimization
+			 * and not strictly necessary for correctness.
 			 *
 			 * As with all tuple visibility routines, it's critical to test
-			 * TransactionIdIsInProgress before TransactionIdDidCommit,
+			 * TransactionIdIsInProgress before the transam.c routines,
 			 * because of race conditions explained in detail in tqual.c.
-			 *
-			 * We normally don't see committed updating transactions earlier
-			 * than the cutoff xid, because they are removed by page pruning
-			 * before we try to freeze the page; but it can happen if the
-			 * updating transaction commits after the page is pruned but
-			 * before HeapTupleSatisfiesVacuum.
 			 */
 			if (TransactionIdIsCurrentTransactionId(xid) ||
 				TransactionIdIsInProgress(xid))
 			{
 				Assert(!TransactionIdIsValid(update_xid));
 				update_xid = xid;
 			}
-			else if (TransactionIdDidCommit(xid))
+			else if (!TransactionIdDidAbort(xid))
 			{
 				/*
-				 * The transaction committed, so we can tell caller to set
-				 * HEAP_XMAX_COMMITTED.  (We can only do this because we know
-				 * the transaction is not running.)
+				 * Test whether to tell caller to set HEAP_XMAX_COMMITTED
+				 * while we have the Xid still in cache.  Note this can only
+				 * be done if the transaction is known not running.
 				 */
+				if (TransactionIdDidCommit(xid))
+					update_committed = true;
 				Assert(!TransactionIdIsValid(update_xid));
-				update_committed = true;
 				update_xid = xid;
 			}
 
-			/*
-			 * Not in progress, not committed -- must be aborted or crashed;
-			 * we can ignore it.
-			 */
-
 			/*
 			 * If we determined that it's an Xid corresponding to an update
 			 * that must be retained, additionally add it to the list of
-			 * members of the new Multi, in case we end up using that.  (We
+			 * members of the new Multis, in case we end up using that.  (We
 			 * might still decide to use only an update Xid and not a multi,
 			 * but it's easier to maintain the list as we walk the old members
 			 * list.)
+			 *
+			 * It is possible to end up with a very old updater Xid that
+			 * crashed and thus did not mark itself as aborted in pg_clog.
+			 * That would manifest as a pre-cutoff Xid.  Make sure to ignore
+			 * it.
 			 */
 			if (TransactionIdIsValid(update_xid))
-				newmembers[nnewmembers++] = members[i];
+			{
+				if (!TransactionIdPrecedes(update_xid, cutoff_xid))
+				{
+					newmembers[nnewmembers++] = members[i];
+				}
+				else
+				{
+					/* cannot have committed: would be HEAPTUPLE_DEAD */
+					Assert(!TransactionIdDidCommit(update_xid));
+					update_xid = InvalidTransactionId;
+					update_committed = false;
+				}
+			}
 		}
 		else
 		{
@@ -5927,10 +5890,7 @@ FreezeMultiXactId(MultiXactId multi, uint16 t_infomask,
  *
  * It is assumed that the caller has checked the tuple with
  * HeapTupleSatisfiesVacuum() and determined that it is not HEAPTUPLE_DEAD
- * (else we should be removing the tuple, not freezing it).  However, note
- * that we don't remove HOT tuples even if they are dead, and it'd be incorrect
- * to freeze them (because that would make them visible), so we mark them as
- * update-committed, and needing further freezing later on.
+ * (else we should be removing the tuple, not freezing it).
  *
  * NB: cutoff_xid *must* be <= the current global xmin, to ensure that any
  * XID older than it could neither be running nor seen as running by any
@@ -6035,18 +5995,7 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
 	else if (TransactionIdIsNormal(xid) &&
 			 TransactionIdPrecedes(xid, cutoff_xid))
 	{
-		/*
-		 * Must freeze regular XIDs older than the cutoff.  We must not freeze
-		 * a HOT-updated tuple, though; doing so would bring it back to life.
-		 */
-		if (HeapTupleHeaderIsHotUpdated(tuple))
-		{
-			frz->t_infomask |= HEAP_XMAX_COMMITTED;
-			changed = true;
-			/* must not freeze */
-		}
-		else
-			freeze_xmax = true;
+		freeze_xmax = true;
 	}
 
 	if (freeze_xmax)
diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
@@ -465,7 +465,7 @@ heap_prune_chain(Relation relation, Buffer buffer, OffsetNumber rootoffnum,
 		 * Check the tuple XMIN against prior XMAX, if any
 		 */
 		if (TransactionIdIsValid(priorXmax) &&
-			!HeapTupleUpdateXmaxMatchesXmin(priorXmax, htup))
+			!TransactionIdEquals(HeapTupleHeaderGetXmin(htup), priorXmax))
 			break;
 
 		/*
@@ -805,7 +805,7 @@ heap_get_root_tuples(Page page, OffsetNumber *root_offsets)
 			htup = (HeapTupleHeader) PageGetItem(page, lp);
 
 			if (TransactionIdIsValid(priorXmax) &&
-				!HeapTupleUpdateXmaxMatchesXmin(priorXmax, htup))
+				!TransactionIdEquals(priorXmax, HeapTupleHeaderGetXmin(htup)))
 				break;
 
 			/* Remember the root line pointer for this item */
diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
@@ -1688,15 +1688,15 @@ lazy_record_dead_tuple(LVRelStats *vacrelstats,
 					   ItemPointer itemptr)
 {
 	/*
-	 * The array must never overflow, since we rely on all deletable tuples
-	 * being removed; inability to remove a tuple might cause an old XID to
-	 * persist beyond the freeze limit, which could be disastrous later on.
+	 * The array shouldn't overflow under normal behavior, but perhaps it
+	 * could if we are given a really small maintenance_work_mem. In that
+	 * case, just forget the last few tuples (we'll get 'em next time).
 	 */
-	if (vacrelstats->num_dead_tuples >= vacrelstats->max_dead_tuples)
-		elog(ERROR, "dead tuple array overflow");
-
-	vacrelstats->dead_tuples[vacrelstats->num_dead_tuples] = *itemptr;
-	vacrelstats->num_dead_tuples++;
+	if (vacrelstats->num_dead_tuples < vacrelstats->max_dead_tuples)
+	{
+		vacrelstats->dead_tuples[vacrelstats->num_dead_tuples] = *itemptr;
+		vacrelstats->num_dead_tuples++;
+	}
 }
 
 /*
diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
@@ -2080,7 +2080,8 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode, bool noWait,
 			 * buffer's content lock, since xmin never changes in an existing
 			 * tuple.)
 			 */
-			if (!HeapTupleUpdateXmaxMatchesXmin(priorXmax, tuple.t_data))
+			if (!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data),
+									 priorXmax))
 			{
 				ReleaseBuffer(buffer);
 				return NULL;
@@ -2209,7 +2210,8 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode, bool noWait,
 		/*
 		 * As above, if xmin isn't what we're expecting, do nothing.
 		 */
-		if (!HeapTupleUpdateXmaxMatchesXmin(priorXmax, tuple.t_data))
+		if (!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data),
+								 priorXmax))
 		{
 			ReleaseBuffer(buffer);
 			return NULL;
diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
@@ -129,9 +129,6 @@ extern void heap_get_latest_tid(Relation relation, Snapshot snapshot,
 					ItemPointer tid);
 extern void setLastTid(const ItemPointer tid);
 
-extern bool HeapTupleUpdateXmaxMatchesXmin(TransactionId xmax,
-							   HeapTupleHeader htup);
-
 extern BulkInsertState GetBulkInsertState(void);
 extern void FreeBulkInsertState(BulkInsertState);
 
diff --git a/src/test/isolation/isolation_schedule b/src/test/isolation/isolation_schedule
@@ -30,7 +30,6 @@ test: lock-committed-keyupdate
 test: update-locked-tuple
 test: propagate-lock-delete
 test: tuplelock-conflict
-test: freeze-the-dead
 test: nowait
 test: nowait-2
 test: nowait-3
