Code cleanup for heap_freeze_tuple.

It used to be case that lazy vacuum could call this function with only
a shared lock on the buffer, but neither lazy vacuum nor any other
code path does that any more.  Simplify the code accordingly and clean
up some related, obsolete comments.

src/backend/access/heap/heapam.c

@@ -3947,10 +3947,8 @@ heap_inplace_update(Relation relation, HeapTuple tuple)
  * because this function is applied during WAL recovery, when we don't have
  * access to any such state, and can't depend on the hint bits to be set.)
  *
- * In lazy VACUUM, we call this while initially holding only a shared lock
+ * If the tuple is in a shared buffer, caller must hold an exclusive lock on
- * on the tuple's buffer.  If any change is needed, we trade that in for an
+ * that buffer.
- * exclusive lock before making the change.  Caller should pass the buffer ID
- * if shared lock is held, InvalidBuffer if exclusive lock is already held.
  *
  * Note: it might seem we could make the changes without exclusive lock, since
  * TransactionId read/write is assumed atomic anyway.  However there is a race
@@ -3962,8 +3960,7 @@ heap_inplace_update(Relation relation, HeapTuple tuple)
  * infomask bits.
  */
 bool
-heap_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
+heap_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid)
-				  Buffer buf)
 {
 	bool		changed = false;
 	TransactionId xid;
@@ -3972,13 +3969,6 @@ heap_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
 	if (TransactionIdIsNormal(xid) &&
 		TransactionIdPrecedes(xid, cutoff_xid))
 	{
-		if (buf != InvalidBuffer)
-		{
-			/* trade in share lock for exclusive lock */
-			LockBuffer(buf, BUFFER_LOCK_UNLOCK);
-			LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
-			buf = InvalidBuffer;
-		}
 		HeapTupleHeaderSetXmin(tuple, FrozenTransactionId);
 
 		/*
@@ -3990,28 +3980,12 @@ heap_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
 		changed = true;
 	}
 
-	/*
-	 * When we release shared lock, it's possible for someone else to change
-	 * xmax before we get the lock back, so repeat the check after acquiring
-	 * exclusive lock.	(We don't need this pushup for xmin, because only
-	 * VACUUM could be interested in changing an existing tuple's xmin, and
-	 * there's only one VACUUM allowed on a table at a time.)
-	 */
-recheck_xmax:
 	if (!(tuple->t_infomask & HEAP_XMAX_IS_MULTI))
 	{
 		xid = HeapTupleHeaderGetXmax(tuple);
 		if (TransactionIdIsNormal(xid) &&
 			TransactionIdPrecedes(xid, cutoff_xid))
 		{
-			if (buf != InvalidBuffer)
-			{
-				/* trade in share lock for exclusive lock */
-				LockBuffer(buf, BUFFER_LOCK_UNLOCK);
-				LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
-				buf = InvalidBuffer;
-				goto recheck_xmax;		/* see comment above */
-			}
 			HeapTupleHeaderSetXmax(tuple, InvalidTransactionId);
 
 			/*
@@ -4046,30 +4020,15 @@ recheck_xmax:
 	}
 
 	/*
-	 * Although xvac per se could only be set by old-style VACUUM FULL, it
-	 * shares physical storage space with cmax, and so could be wiped out by
-	 * someone setting xmax.  Hence recheck after changing lock, same as for
-	 * xmax itself.
-	 *
 	 * Old-style VACUUM FULL is gone, but we have to keep this code as long as
 	 * we support having MOVED_OFF/MOVED_IN tuples in the database.
 	 */
-recheck_xvac:
 	if (tuple->t_infomask & HEAP_MOVED)
 	{
 		xid = HeapTupleHeaderGetXvac(tuple);
 		if (TransactionIdIsNormal(xid) &&
 			TransactionIdPrecedes(xid, cutoff_xid))
 		{
-			if (buf != InvalidBuffer)
-			{
-				/* trade in share lock for exclusive lock */
-				LockBuffer(buf, BUFFER_LOCK_UNLOCK);
-				LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
-				buf = InvalidBuffer;
-				goto recheck_xvac;		/* see comment above */
-			}
-
 			/*
 			 * If a MOVED_OFF tuple is not dead, the xvac transaction must
 			 * have failed; whereas a non-dead MOVED_IN tuple must mean the
@@ -4711,7 +4670,7 @@ heap_xlog_freeze(XLogRecPtr lsn, XLogRecord *record)
 			ItemId		lp = PageGetItemId(page, *offsets);
 			HeapTupleHeader tuple = (HeapTupleHeader) PageGetItem(page, lp);
 
-			(void) heap_freeze_tuple(tuple, cutoff_xid, InvalidBuffer);
+			(void) heap_freeze_tuple(tuple, cutoff_xid);
 			offsets++;
 		}
 	}

src/backend/access/heap/rewriteheap.c

@@ -335,13 +335,8 @@ rewrite_heap_tuple(RewriteState state,
 	/*
 	 * While we have our hands on the tuple, we may as well freeze any
 	 * very-old xmin or xmax, so that future VACUUM effort can be saved.
-	 *
-	 * Note we abuse heap_freeze_tuple() a bit here, since it's expecting to
-	 * be given a pointer to a tuple in a disk buffer.	It happens though that
-	 * we can get the right things to happen by passing InvalidBuffer for the
-	 * buffer.
 	 */
-	heap_freeze_tuple(new_tuple->t_data, state->rs_freeze_xid, InvalidBuffer);
+	heap_freeze_tuple(new_tuple->t_data, state->rs_freeze_xid);
 
 	/*
 	 * Invalid ctid means that ctid should point to the tuple itself. We'll

src/backend/commands/vacuumlazy.c

@@ -784,8 +784,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 				 * Each non-removable tuple must be checked to see if it needs
 				 * freezing.  Note we already have exclusive buffer lock.
 				 */
-				if (heap_freeze_tuple(tuple.t_data, FreezeLimit,
+				if (heap_freeze_tuple(tuple.t_data, FreezeLimit))
-									  InvalidBuffer))
 					frozen[nfrozen++] = offnum;
 			}
 		}						/* scan along page */

src/include/access/heapam.h

@@ -111,8 +111,7 @@ extern HTSU_Result heap_lock_tuple(Relation relation, HeapTuple tuple,
 				TransactionId *update_xmax, CommandId cid,
 				LockTupleMode mode, bool nowait);
 extern void heap_inplace_update(Relation relation, HeapTuple tuple);
-extern bool heap_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
+extern bool heap_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid);
-				  Buffer buf);
 extern bool heap_tuple_needs_freeze(HeapTupleHeader tuple, TransactionId cutoff_xid,
 				  Buffer buf);