Repair two related errors in heap_lock_tuple: it was failing to recognize

cases where we already hold the desired lock "indirectly", either via
membership in a MultiXact or because the lock was originally taken by a
different subtransaction of the current transaction.  These cases must be
accounted for to avoid needless deadlocks and/or inappropriate replacement of
an exclusive lock with a shared lock.  Per report from Clarence Gardner and
subsequent investigation.

4 files changed, 111 insertions, 42 deletions

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 45e4afbaa7..0a98f0be44 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -2359,6 +2359,8 @@ heap_lock_tuple(Relation relation, HeapTuple tuple, Buffer *buffer,
 	ItemId		lp;
 	PageHeader	dp;
 	TransactionId xid;
+	TransactionId xmax;
+	uint16		old_infomask;
 	uint16		new_infomask;
 	LOCKMODE	tuple_lock_type;
 	bool		have_tuple_lock = false;
@@ -2396,6 +2398,25 @@ l3:
 		LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);
 
 		/*
+		 * If we wish to acquire share lock, and the tuple is already
+		 * share-locked by a multixact that includes any subtransaction of the
+		 * current top transaction, then we effectively hold the desired lock
+		 * already.  We *must* succeed without trying to take the tuple lock,
+		 * else we will deadlock against anyone waiting to acquire exclusive
+		 * lock.  We don't need to make any state changes in this case.
+		 */
+		if (mode == LockTupleShared &&
+			(infomask & HEAP_XMAX_IS_MULTI) &&
+			MultiXactIdIsCurrent((MultiXactId) xwait))
+		{
+			Assert(infomask & HEAP_XMAX_SHARED_LOCK);
+			/* Probably can't hold tuple lock here, but may as well check */
+			if (have_tuple_lock)
+				UnlockTuple(relation, tid, tuple_lock_type);
+			return HeapTupleMayBeUpdated;
+		}
+
+		/*
 		 * Acquire tuple lock to establish our priority for the tuple.
 		 * LockTuple will release us when we are next-in-line for the tuple.
 		 * We must do this even if we are share-locking.
@@ -2533,25 +2554,49 @@ l3:
 	}
 
 	/*
+	 * We might already hold the desired lock (or stronger), possibly under
+	 * a different subtransaction of the current top transaction.  If so,
+	 * there is no need to change state or issue a WAL record.  We already
+	 * handled the case where this is true for xmax being a MultiXactId,
+	 * so now check for cases where it is a plain TransactionId.
+	 *
+	 * Note in particular that this covers the case where we already hold
+	 * exclusive lock on the tuple and the caller only wants shared lock.
+	 * It would certainly not do to give up the exclusive lock.
+	 */
+	xmax = HeapTupleHeaderGetXmax(tuple->t_data);
+	old_infomask = tuple->t_data->t_infomask;
+
+	if (!(old_infomask & (HEAP_XMAX_INVALID |
+						  HEAP_XMAX_COMMITTED |
+						  HEAP_XMAX_IS_MULTI)) &&
+		(mode == LockTupleShared ?
+		 (old_infomask & HEAP_IS_LOCKED) :
+		 (old_infomask & HEAP_XMAX_EXCL_LOCK)) &&
+		TransactionIdIsCurrentTransactionId(xmax))
+	{
+		LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);
+		/* Probably can't hold tuple lock here, but may as well check */
+		if (have_tuple_lock)
+			UnlockTuple(relation, tid, tuple_lock_type);
+		return HeapTupleMayBeUpdated;
+	}
+
+	/*
 	 * Compute the new xmax and infomask to store into the tuple.  Note we do
 	 * not modify the tuple just yet, because that would leave it in the wrong
 	 * state if multixact.c elogs.
 	 */
 	xid = GetCurrentTransactionId();
 
-	new_infomask = tuple->t_data->t_infomask;
-
-	new_infomask &= ~(HEAP_XMAX_COMMITTED |
-					  HEAP_XMAX_INVALID |
-					  HEAP_XMAX_IS_MULTI |
-					  HEAP_IS_LOCKED |
-					  HEAP_MOVED);
+	new_infomask = old_infomask & ~(HEAP_XMAX_COMMITTED |
+									HEAP_XMAX_INVALID |
+									HEAP_XMAX_IS_MULTI |
+									HEAP_IS_LOCKED |
+									HEAP_MOVED);
 
 	if (mode == LockTupleShared)
 	{
-		TransactionId xmax = HeapTupleHeaderGetXmax(tuple->t_data);
-		uint16		old_infomask = tuple->t_data->t_infomask;
-
 		/*
 		 * If this is the first acquisition of a shared lock in the current
 		 * transaction, set my per-backend OldestMemberMXactId setting. We can
@@ -2592,32 +2637,13 @@ l3:
 			}
 			else if (TransactionIdIsInProgress(xmax))
 			{
-				if (TransactionIdEquals(xmax, xid))
-				{
-					/*
-					 * If the old locker is ourselves, we'll just mark the
-					 * tuple again with our own TransactionId.	However we
-					 * have to consider the possibility that we had exclusive
-					 * rather than shared lock before --- if so, be careful to
-					 * preserve the exclusivity of the lock.
-					 */
-					if (!(old_infomask & HEAP_XMAX_SHARED_LOCK))
-					{
-						new_infomask &= ~HEAP_XMAX_SHARED_LOCK;
-						new_infomask |= HEAP_XMAX_EXCL_LOCK;
-						mode = LockTupleExclusive;
-					}
-				}
-				else
-				{
-					/*
-					 * If the Xmax is a valid TransactionId, then we need to
-					 * create a new MultiXactId that includes both the old
-					 * locker and our own TransactionId.
-					 */
-					xid = MultiXactIdCreate(xmax, xid);
-					new_infomask |= HEAP_XMAX_IS_MULTI;
-				}
+				/*
+				 * If the XMAX is a valid TransactionId, then we need to
+				 * create a new MultiXactId that includes both the old
+				 * locker and our own TransactionId.
+				 */
+				xid = MultiXactIdCreate(xmax, xid);
+				new_infomask |= HEAP_XMAX_IS_MULTI;
 			}
 			else
 			{
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 13f6e11792..7db36b801c 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -366,7 +366,6 @@ bool
 MultiXactIdIsRunning(MultiXactId multi)
 {
 	TransactionId *members;
-	TransactionId myXid;
 	int			nmembers;
 	int			i;
 
@@ -380,12 +379,14 @@ MultiXactIdIsRunning(MultiXactId multi)
 		return false;
 	}
 
-	/* checking for myself is cheap */
-	myXid = GetTopTransactionId();
-
+	/*
+	 * Checking for myself is cheap compared to looking in shared memory,
+	 * so first do the equivalent of MultiXactIdIsCurrent().  This is not
+	 * needed for correctness, it's just a fast path.
+	 */
 	for (i = 0; i < nmembers; i++)
 	{
-		if (TransactionIdEquals(members[i], myXid))
+		if (TransactionIdIsCurrentTransactionId(members[i]))
 		{
 			debug_elog3(DEBUG2, "IsRunning: I (%d) am running!", i);
 			pfree(members);
@@ -417,6 +418,44 @@ MultiXactIdIsRunning(MultiXactId multi)
 }
 
 /*
+ * MultiXactIdIsCurrent
+ *		Returns true if the current transaction is a member of the MultiXactId.
+ *
+ * We return true if any live subtransaction of the current top-level
+ * transaction is a member.  This is appropriate for the same reason that a
+ * lock held by any such subtransaction is globally equivalent to a lock
+ * held by the current subtransaction: no such lock could be released without
+ * aborting this subtransaction, and hence releasing its locks.  So it's not
+ * necessary to add the current subxact to the MultiXact separately.
+ */
+bool
+MultiXactIdIsCurrent(MultiXactId multi)
+{
+	bool		result = false;
+	TransactionId *members;
+	int			nmembers;
+	int			i;
+
+	nmembers = GetMultiXactIdMembers(multi, &members);
+
+	if (nmembers < 0)
+		return false;
+
+	for (i = 0; i < nmembers; i++)
+	{
+		if (TransactionIdIsCurrentTransactionId(members[i]))
+		{
+			result = true;
+			break;
+		}
+	}
+
+	pfree(members);
+
+	return result;
+}
+
+/*
  * MultiXactIdSetOldestMember
  *		Save the oldest MultiXactId this transaction could be a member of.
  *
diff --git a/src/backend/utils/time/tqual.c b/src/backend/utils/time/tqual.c
index f973921edd..91edae9afb 100644
--- a/src/backend/utils/time/tqual.c
+++ b/src/backend/utils/time/tqual.c
@@ -511,7 +511,10 @@ HeapTupleSatisfiesToast(HeapTupleHeader tuple, Buffer buffer)
  *	HeapTupleUpdated: The tuple was updated by a committed transaction.
  *
  *	HeapTupleBeingUpdated: The tuple is being updated by an in-progress
- *	transaction other than the current transaction.
+ *	transaction other than the current transaction.  (Note: this includes
+ *	the case where the tuple is share-locked by a MultiXact, even if the
+ *	MultiXact includes the current transaction.  Callers that want to
+ *	distinguish that case must test for it themselves.)
  */
 HTSU_Result
 HeapTupleSatisfiesUpdate(HeapTupleHeader tuple, CommandId curcid,
diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h
index d465dff0a3..9f841fea86 100644
--- a/src/include/access/multixact.h
+++ b/src/include/access/multixact.h
@@ -45,6 +45,7 @@ typedef struct xl_multixact_create
 extern MultiXactId MultiXactIdCreate(TransactionId xid1, TransactionId xid2);
 extern MultiXactId MultiXactIdExpand(MultiXactId multi, TransactionId xid);
 extern bool MultiXactIdIsRunning(MultiXactId multi);
+extern bool MultiXactIdIsCurrent(MultiXactId multi);
 extern void MultiXactIdWait(MultiXactId multi);
 extern bool ConditionalMultiXactIdWait(MultiXactId multi);
 extern void MultiXactIdSetOldestMember(void);