TOAST needs to do at least minimal time-qual checking in order not to

tglsfdc · tglsfdc · commit cf97080fa4af · 2002-01-16T20:29:02.000Z
mess up after an aborted VACUUM FULL, per today's pghackers discussion.
Add a suitable HeapTupleSatisfiesToast routine.  Remove useless special-
case test in HeapTupleSatisfiesVisibility macro for xmax =
BootstrapTransactionId; perhaps that was needed at one time, but it's
a waste of cycles now, not to mention actively wrong for SnapshotAny.
Along the way, add some much-needed comments to tqual.c, and simplify
toast_fetch_datum, which no longer needs to assume it may see chunks
out-of-order.
diff --git a/src/backend/access/heap/tuptoaster.c b/src/backend/access/heap/tuptoaster.c
@@ -4,11 +4,11 @@
  *	  Support routines for external and compressed storage of
  *	  variable size attributes.
  *
- * Copyright (c) 2000, PostgreSQL Global Development Group
+ * Copyright (c) 2000-2002, PostgreSQL Global Development Group
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/access/heap/tuptoaster.c,v 1.26 2001/11/05 17:46:23 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/access/heap/tuptoaster.c,v 1.27 2002/01/16 20:29:01 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -921,7 +921,7 @@ toast_delete_datum(Relation rel, Datum value)
 	while ((indexRes = index_getnext(toastscan, ForwardScanDirection)) != NULL)
 	{
 		toasttup.t_self = indexRes->heap_iptr;
-		heap_fetch(toastrel, SnapshotAny, &toasttup, &buffer, toastscan);
+		heap_fetch(toastrel, SnapshotToast, &toasttup, &buffer, toastscan);
 		pfree(indexRes);
 
 		if (!toasttup.t_data)
@@ -963,26 +963,18 @@ toast_fetch_datum(varattrib *attr)
 	TupleDesc	toasttupDesc;
 	RetrieveIndexResult indexRes;
 	Buffer		buffer;
-
 	varattrib  *result;
 	int32		ressize;
-	int32		residx;
-	int			numchunks;
+	int32		residx,
+				nextidx;
+	int32		numchunks;
 	Pointer		chunk;
 	bool		isnull;
 	int32		chunksize;
 
-	char	   *chunks_found;
-	char	   *chunks_expected;
-
 	ressize = attr->va_content.va_external.va_extsize;
 	numchunks = ((ressize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;
 
-	chunks_found = palloc(numchunks);
-	chunks_expected = palloc(numchunks);
-	memset(chunks_found, 0, numchunks);
-	memset(chunks_expected, 1, numchunks);
-
 	result = (varattrib *) palloc(ressize + VARHDRSZ);
 	VARATT_SIZEP(result) = ressize + VARHDRSZ;
 	if (VARATT_IS_COMPRESSED(attr))
@@ -1008,13 +1000,17 @@ toast_fetch_datum(varattrib *attr)
 	/*
 	 * Read the chunks by index
 	 *
-	 * Note we will not necessarily see the chunks in sequence-number order.
+	 * Note that because the index is actually on (valueid, chunkidx)
+	 * we will see the chunks in chunkidx order, even though we didn't
+	 * explicitly ask for it.
 	 */
+	nextidx = 0;
+
 	toastscan = index_beginscan(toastidx, false, 1, &toastkey);
 	while ((indexRes = index_getnext(toastscan, ForwardScanDirection)) != NULL)
 	{
 		toasttup.t_self = indexRes->heap_iptr;
-		heap_fetch(toastrel, SnapshotAny, &toasttup, &buffer, toastscan);
+		heap_fetch(toastrel, SnapshotToast, &toasttup, &buffer, toastscan);
 		pfree(indexRes);
 
 		if (toasttup.t_data == NULL)
@@ -1033,9 +1029,9 @@ toast_fetch_datum(varattrib *attr)
 		/*
 		 * Some checks on the data we've found
 		 */
-		if (residx < 0 || residx >= numchunks)
-			elog(ERROR, "unexpected chunk number %d for toast value %u",
-				 residx,
+		if (residx != nextidx)
+			elog(ERROR, "unexpected chunk number %d (expected %d) for toast value %u",
+				 residx, nextidx,
 				 attr->va_content.va_external.va_valueid);
 		if (residx < numchunks - 1)
 		{
@@ -1044,15 +1040,15 @@ toast_fetch_datum(varattrib *attr)
 					 chunksize, residx,
 					 attr->va_content.va_external.va_valueid);
 		}
-		else
+		else if (residx < numchunks)
 		{
 			if ((residx * TOAST_MAX_CHUNK_SIZE + chunksize) != ressize)
 				elog(ERROR, "unexpected chunk size %d in chunk %d for toast value %u",
 					 chunksize, residx,
 					 attr->va_content.va_external.va_valueid);
 		}
-		if (chunks_found[residx]++ > 0)
-			elog(ERROR, "chunk %d for toast value %u appears multiple times",
+		else
+			elog(ERROR, "unexpected chunk number %d for toast value %u",
 				 residx,
 				 attr->va_content.va_external.va_valueid);
 
@@ -1064,16 +1060,16 @@ toast_fetch_datum(varattrib *attr)
 			   chunksize);
 
 		ReleaseBuffer(buffer);
+		nextidx++;
 	}
 
 	/*
 	 * Final checks that we successfully fetched the datum
 	 */
-	if (memcmp(chunks_found, chunks_expected, numchunks) != 0)
-		elog(ERROR, "not all toast chunks found for value %u",
+	if (nextidx != numchunks)
+		elog(ERROR, "missing chunk number %d for toast value %u",
+			 nextidx,
 			 attr->va_content.va_external.va_valueid);
-	pfree(chunks_expected);
-	pfree(chunks_found);
 
 	/*
 	 * End scan and close relations
diff --git a/src/backend/utils/time/tqual.c b/src/backend/utils/time/tqual.c
@@ -1,14 +1,22 @@
 /*-------------------------------------------------------------------------
  *
  * tqual.c
- *	  POSTGRES "time" qualification code.
+ *	  POSTGRES "time" qualification code, ie, tuple visibility rules.
+ *
+ * NOTE: all the HeapTupleSatisfies routines will update the tuple's
+ * "hint" status bits if we see that the inserting or deleting transaction
+ * has now committed or aborted.  The caller is responsible for noticing any
+ * change in t_infomask and scheduling a disk write if so.  Note that the
+ * caller must hold at least a shared buffer context lock on the buffer
+ * containing the tuple.  (VACUUM FULL assumes it's sufficient to have
+ * exclusive lock on the containing relation, instead.)
+ *
  *
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v 1.46 2002/01/11 20:07:03 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v 1.47 2002/01/16 20:29:02 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -36,8 +44,7 @@ bool		ReferentialIntegritySnapshotOverride = false;
  *
  * Note:
  *		Assumes heap tuple is valid.
- */
-/*
+ *
  * The satisfaction of "itself" requires the following:
  *
  * ((Xmin == my-transaction &&				the row was updated by the current transaction, and
@@ -153,8 +160,7 @@ HeapTupleSatisfiesItself(HeapTupleHeader tuple)
  *
  * Note:
  *		Assumes heap tuple is valid.
- */
-/*
+ *
  * The satisfaction of "now" requires the following:
  *
  * ((Xmin == my-transaction &&				changed by the current transaction
@@ -288,6 +294,71 @@ HeapTupleSatisfiesNow(HeapTupleHeader tuple)
 	return false;
 }
 
+/*
+ * HeapTupleSatisfiesToast
+ *		True iff heap tuple is valid for TOAST usage.
+ *
+ * This is a simplified version that only checks for VACUUM moving conditions.
+ * It's appropriate for TOAST usage because TOAST really doesn't want to do
+ * its own time qual checks; if you can see the main-table row that contains
+ * a TOAST reference, you should be able to see the TOASTed value.  However,
+ * vacuuming a TOAST table is independent of the main table, and in case such
+ * a vacuum fails partway through, we'd better do this much checking.
+ *
+ * Among other things, this means you can't do UPDATEs of rows in a TOAST
+ * table.
+ */
+bool
+HeapTupleSatisfiesToast(HeapTupleHeader tuple)
+{
+	if (!(tuple->t_infomask & HEAP_XMIN_COMMITTED))
+	{
+		if (tuple->t_infomask & HEAP_XMIN_INVALID)
+			return false;
+
+		if (tuple->t_infomask & HEAP_MOVED_OFF)
+		{
+			if (TransactionIdIsCurrentTransactionId((TransactionId) tuple->t_cmin))
+				return false;
+			if (!TransactionIdIsInProgress((TransactionId) tuple->t_cmin))
+			{
+				if (TransactionIdDidCommit((TransactionId) tuple->t_cmin))
+				{
+					tuple->t_infomask |= HEAP_XMIN_INVALID;
+					return false;
+				}
+				tuple->t_infomask |= HEAP_XMIN_COMMITTED;
+			}
+		}
+		else if (tuple->t_infomask & HEAP_MOVED_IN)
+		{
+			if (!TransactionIdIsCurrentTransactionId((TransactionId) tuple->t_cmin))
+			{
+				if (TransactionIdIsInProgress((TransactionId) tuple->t_cmin))
+					return false;
+				if (TransactionIdDidCommit((TransactionId) tuple->t_cmin))
+					tuple->t_infomask |= HEAP_XMIN_COMMITTED;
+				else
+				{
+					tuple->t_infomask |= HEAP_XMIN_INVALID;
+					return false;
+				}
+			}
+		}
+	}
+
+	/* otherwise assume the tuple is valid for TOAST. */
+	return true;
+}
+
+/*
+ * HeapTupleSatisfiesUpdate
+ *		Check whether a tuple can be updated.
+ *
+ * This applies exactly the same checks as HeapTupleSatisfiesNow,
+ * but returns a more-detailed result code, since UPDATE needs to know
+ * more than "is it visible?"
+ */
 int
 HeapTupleSatisfiesUpdate(HeapTuple htuple)
 {
@@ -404,6 +475,18 @@ HeapTupleSatisfiesUpdate(HeapTuple htuple)
 	return HeapTupleUpdated;	/* updated by other */
 }
 
+/*
+ * HeapTupleSatisfiesDirty
+ *		True iff heap tuple is valid, including effects of concurrent xacts.
+ *
+ * This is essentially like HeapTupleSatisfiesItself as far as effects of
+ * the current transaction and committed/aborted xacts are concerned.
+ * However, we also include the effects of other xacts still in progress.
+ *
+ * Returns extra information in the global variable SnapshotDirty, namely
+ * xids of concurrent xacts that affected the tuple.  Also, the tuple's
+ * t_ctid (forward link) is returned if it's being updated.
+ */
 bool
 HeapTupleSatisfiesDirty(HeapTupleHeader tuple)
 {
@@ -516,6 +599,18 @@ HeapTupleSatisfiesDirty(HeapTupleHeader tuple)
 	return false;				/* updated by other */
 }
 
+/*
+ * HeapTupleSatisfiesSnapshot
+ *		True iff heap tuple is valid for the given snapshot.
+ *
+ * This is the same as HeapTupleSatisfiesNow, except that transactions that
+ * were in progress or as yet unstarted when the snapshot was taken will
+ * be treated as uncommitted, even if they really have committed by now.
+ *
+ * (Notice, however, that the tuple status hint bits will be updated on the
+ * basis of the true state of the transaction, even if we then pretend we
+ * can't see it.)
+ */
 bool
 HeapTupleSatisfiesSnapshot(HeapTupleHeader tuple, Snapshot snapshot)
 {
@@ -658,11 +753,6 @@ HeapTupleSatisfiesSnapshot(HeapTupleHeader tuple, Snapshot snapshot)
  * deleted by XIDs >= OldestXmin are deemed "recently dead"; they might
  * still be visible to some open transaction, so we can't remove them,
  * even if we see that the deleting transaction has committed.
- *
- * As with the other HeapTupleSatisfies routines, we may update the tuple's
- * "hint" status bits if we see that the inserting or deleting transaction
- * has now committed or aborted.  The caller is responsible for noticing any
- * change in t_infomask and scheduling a disk write if so.
  */
 HTSV_Result
 HeapTupleSatisfiesVacuum(HeapTupleHeader tuple, TransactionId OldestXmin)
@@ -808,13 +898,21 @@ HeapTupleSatisfiesVacuum(HeapTupleHeader tuple, TransactionId OldestXmin)
 }
 
 
+/*
+ * SetQuerySnapshot
+ *		Initialize query snapshot for a new query
+ *
+ * The SerializableSnapshot is the first one taken in a transaction.
+ * In serializable mode we just use that one throughout the transaction.
+ * In read-committed mode, we take a new snapshot at the start of each query.
+ */
 void
 SetQuerySnapshot(void)
 {
 	/* Initialize snapshot overriding to false */
 	ReferentialIntegritySnapshotOverride = false;
 
-	/* 1st call in xaction */
+	/* 1st call in xaction? */
 	if (SerializableSnapshot == NULL)
 	{
 		SerializableSnapshot = GetSnapshotData(true);
@@ -837,6 +935,10 @@ SetQuerySnapshot(void)
 	Assert(QuerySnapshot != NULL);
 }
 
+/*
+ * FreeXactSnapshot
+ *		Free snapshot(s) at end of transaction.
+ */
 void
 FreeXactSnapshot(void)
 {
diff --git a/src/include/utils/tqual.h b/src/include/utils/tqual.h
