Use a more granular approach to follow update chains

alvherre · alvherre · commit 4c71f48f4c6c · 2013-11-28T11:54:25.000-03:00
Instead of simply checking the KEYS_UPDATED bit, we need to check
whether each lock held on the future version of the tuple conflicts with
the lock we're trying to acquire.

Per bug report #8434 by Tomonari Katsumata
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
@@ -4777,6 +4777,83 @@ compute_new_xmax_infomask(TransactionId xmax, uint16 old_infomask,
 	*result_xmax = new_xmax;
 }
 
+/*
+ * Subroutine for heap_lock_updated_tuple_rec.
+ *
+ * Given an hypothetical multixact status held by the transaction identified
+ * with the given xid, does the current transaction need to wait, fail, or can
+ * it continue if it wanted to acquire a lock of the given mode?  "needwait"
+ * is set to true if waiting is necessary; if it can continue, then
+ * HeapTupleMayBeUpdated is returned.  In case of a conflict, a different
+ * HeapTupleSatisfiesUpdate return code is returned.
+ *
+ * The held status is said to be hypothetical because it might correspond to a
+ * lock held by a single Xid, i.e. not a real MultiXactId; we express it this
+ * way for simplicity of API.
+ */
+static HTSU_Result
+test_lockmode_for_conflict(MultiXactStatus status, TransactionId xid,
+						   LockTupleMode mode, bool *needwait)
+{
+	MultiXactStatus	wantedstatus;
+
+	*needwait = false;
+	wantedstatus = get_mxact_status_for_lock(mode, false);
+
+	/*
+	 * Note: we *must* check TransactionIdIsInProgress before
+	 * TransactionIdDidAbort/Commit; see comment at top of tqual.c for an
+	 * explanation.
+	 */
+	if (TransactionIdIsCurrentTransactionId(xid))
+	{
+		/*
+		 * Updated by our own transaction? Just return failure.  This shouldn't
+		 * normally happen.
+		 */
+		return HeapTupleSelfUpdated;
+	}
+	else if (TransactionIdIsInProgress(xid))
+	{
+		/*
+		 * If the locking transaction is running, what we do depends on whether
+		 * the lock modes conflict: if they do, then we must wait for it to
+		 * finish; otherwise we can fall through to lock this tuple version
+		 * without waiting.
+		 */
+		if (DoLockModesConflict(LOCKMODE_from_mxstatus(status),
+								LOCKMODE_from_mxstatus(wantedstatus)))
+		{
+			*needwait = true;
+		}
+
+		/*
+		 * If we set needwait above, then this value doesn't matter; otherwise,
+		 * this value signals to caller that it's okay to proceed.
+		 */
+		return HeapTupleMayBeUpdated;
+	}
+	else if (TransactionIdDidAbort(xid))
+		return HeapTupleMayBeUpdated;
+	else if (TransactionIdDidCommit(xid))
+	{
+		/*
+		 * If the updating transaction committed, what we do depends on whether
+		 * the lock modes conflict: if they do, then we must report error to
+		 * caller.  But if they don't, we can fall through to lock it.
+		 */
+		if (DoLockModesConflict(LOCKMODE_from_mxstatus(status),
+								LOCKMODE_from_mxstatus(wantedstatus)))
+			/* bummer */
+			return HeapTupleUpdated;
+
+		return HeapTupleMayBeUpdated;
+	}
+
+	/* Not in progress, not aborted, not committed -- must have crashed */
+	return HeapTupleMayBeUpdated;
+}
+
 
 /*
  * Recursive part of heap_lock_updated_tuple
@@ -4794,7 +4871,8 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
 	Buffer		buf;
 	uint16		new_infomask,
 				new_infomask2,
-				old_infomask;
+				old_infomask,
+				old_infomask2;
 	TransactionId xmax,
 				new_xmax;
 	TransactionId priorXmax = InvalidTransactionId;
@@ -4836,45 +4914,107 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
 		}
 
 		old_infomask = mytup.t_data->t_infomask;
+		old_infomask2 = mytup.t_data->t_infomask2;
 		xmax = HeapTupleHeaderGetRawXmax(mytup.t_data);
 
 		/*
-		 * If this tuple is updated and the key has been modified (or
-		 * deleted), what we do depends on the status of the updating
-		 * transaction: if it's live, we sleep until it finishes; if it has
-		 * committed, we have to fail (i.e. return HeapTupleUpdated); if it
-		 * aborted, we ignore it. For updates that didn't touch the key, we
-		 * can just plough ahead.
+		 * If this tuple version has been updated or locked by some concurrent
+		 * transaction(s), what we do depends on whether our lock mode
+		 * conflicts with what those other transactions hold, and also on the
+		 * status of them.
 		 */
-		if (!(old_infomask & HEAP_XMAX_INVALID) &&
-			(mytup.t_data->t_infomask2 & HEAP_KEYS_UPDATED))
+		if (!(old_infomask & HEAP_XMAX_INVALID))
 		{
-			TransactionId update_xid;
+			TransactionId rawxmax;
+			bool		needwait;
 
-			/*
-			 * Note: we *must* check TransactionIdIsInProgress before
-			 * TransactionIdDidAbort/Commit; see comment at top of tqual.c for
-			 * an explanation.
-			 */
-			update_xid = HeapTupleHeaderGetUpdateXid(mytup.t_data);
-			if (TransactionIdIsCurrentTransactionId(update_xid))
+			rawxmax = HeapTupleHeaderGetRawXmax(mytup.t_data);
+			if (old_infomask & HEAP_XMAX_IS_MULTI)
 			{
-				UnlockReleaseBuffer(buf);
-				return HeapTupleSelfUpdated;
-			}
-			else if (TransactionIdIsInProgress(update_xid))
-			{
-				LockBuffer(buf, BUFFER_LOCK_UNLOCK);
-				/* No LockTupleTuplock here -- see heap_lock_updated_tuple */
-				XactLockTableWait(update_xid);
-				goto l4;
+				int		nmembers;
+				int		i;
+				MultiXactMember *members;
+
+				nmembers = GetMultiXactIdMembers(rawxmax, &members, false);
+				for (i = 0; i < nmembers; i++)
+				{
+					HTSU_Result	res;
+
+					res = test_lockmode_for_conflict(members[i].status,
+													 members[i].xid,
+													 mode, &needwait);
+
+					if (needwait)
+					{
+						LockBuffer(buf, BUFFER_LOCK_UNLOCK);
+						XactLockTableWait(members[i].xid);
+						pfree(members);
+						goto l4;
+					}
+					if (res != HeapTupleMayBeUpdated)
+					{
+						UnlockReleaseBuffer(buf);
+						pfree(members);
+						return res;
+					}
+				}
+				if (members)
+					pfree(members);
 			}
-			else if (TransactionIdDidAbort(update_xid))
-				;				/* okay to proceed */
-			else if (TransactionIdDidCommit(update_xid))
+			else
 			{
-				UnlockReleaseBuffer(buf);
-				return HeapTupleUpdated;
+				HTSU_Result	res;
+				MultiXactStatus status;
+
+				/*
+				 * For a non-multi Xmax, we first need to compute the
+				 * corresponding MultiXactStatus by using the infomask bits.
+				 */
+				if (HEAP_XMAX_IS_LOCKED_ONLY(old_infomask))
+				{
+					if (HEAP_XMAX_IS_KEYSHR_LOCKED(old_infomask))
+						status = MultiXactStatusForKeyShare;
+					else if (HEAP_XMAX_IS_SHR_LOCKED(old_infomask))
+						status = MultiXactStatusForShare;
+					else if (HEAP_XMAX_IS_EXCL_LOCKED(old_infomask))
+					{
+						if (old_infomask2 & HEAP_KEYS_UPDATED)
+							status = MultiXactStatusForUpdate;
+						else
+							status = MultiXactStatusForNoKeyUpdate;
+					}
+					else
+					{
+						/*
+						 * LOCK_ONLY present alone (a pg_upgraded tuple
+						 * marked as share-locked in the old cluster) shouldn't
+						 * be seen in the middle of an update chain.
+						 */
+						elog(ERROR, "invalid lock status in tuple");
+					}
+				}
+				else
+				{
+					/* it's an update, but which kind? */
+					if (old_infomask2 & HEAP_KEYS_UPDATED)
+						status = MultiXactStatusUpdate;
+					else
+						status = MultiXactStatusNoKeyUpdate;
+				}
+
+				res = test_lockmode_for_conflict(status, rawxmax, mode,
+												 &needwait);
+				if (needwait)
+				{
+					LockBuffer(buf, BUFFER_LOCK_UNLOCK);
+					XactLockTableWait(rawxmax);
+					goto l4;
+				}
+				if (res != HeapTupleMayBeUpdated)
+				{
+					UnlockReleaseBuffer(buf);
+					return res;
+				}
 			}
 		}
 
