Don't mark pages all-visible spuriously

Dan Wood diagnosed a long-standing problem that pages containing tuples
that are locked by multixacts containing live lockers may spuriously end
up as candidates for getting their all-visible flag set.  This has the
long-term effect that multixacts remain unfrozen; this may previously
pass undetected, but since commit XYZ it would be reported as
  "ERROR: found multixact 134100944 from before relminmxid 192042633"
because when a later vacuum tries to freeze the page it detects that a
multixact that should have gotten frozen, wasn't.

Dan proposed a (correct) patch that simply sets a variable to its
correct value, after a bogus initialization.  But, per discussion, it
seems better coding to avoid the bogus initializations altogether, since
they could give rise to more bugs later.  Therefore this fix rewrites
the logic a little bit to avoid depending on the bogus initializations.

This bug was part of a family introduced in 9.6 by commit a892234f;
later, commit 38e9f90a fixed most of them, but this one was
unnoticed.

Authors: Dan Wood, Pavan Deolasee, Álvaro Herrera
Reviewed-by: Masahiko Sawada, Pavan Deolasee, Álvaro Herrera
Discussion: https://postgr.es/m/84EBAC55-F06D-4FBE-A3F3-8BDA093CE3E3@amazon.com

src/backend/access/heap/heapam.c

@@ -6803,9 +6803,10 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 						  xl_heap_freeze_tuple *frz, bool *totally_frozen_p)
 {
 	bool		changed = false;
-	bool		freeze_xmax = false;
+	bool		xmax_already_frozen = false;
+	bool		xmin_frozen;
+	bool		freeze_xmax;
 	TransactionId xid;
-	bool		totally_frozen = true;
 
 	frz->frzflags = 0;
 	frz->t_infomask2 = tuple->t_infomask2;
@@ -6814,6 +6815,8 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 
 	/* Process xmin */
 	xid = HeapTupleHeaderGetXmin(tuple);
+	xmin_frozen = ((xid == FrozenTransactionId) ||
+				   HeapTupleHeaderXminFrozen(tuple));
 	if (TransactionIdIsNormal(xid))
 	{
 		if (TransactionIdPrecedes(xid, relfrozenxid))
@@ -6832,9 +6835,8 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 
 			frz->t_infomask |= HEAP_XMIN_FROZEN;
 			changed = true;
+			xmin_frozen = true;
 		}
-		else
-			totally_frozen = false;
 	}
 
 	/*
@@ -6857,9 +6859,9 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 									relfrozenxid, relminmxid,
 									cutoff_xid, cutoff_multi, &flags);
 
-		if (flags & FRM_INVALIDATE_XMAX)
-			freeze_xmax = true;
-		else if (flags & FRM_RETURN_IS_XID)
+		freeze_xmax = (flags & FRM_INVALIDATE_XMAX);
+
+		if (flags & FRM_RETURN_IS_XID)
 		{
 			/*
 			 * NB -- some of these transformations are only valid because we
@@ -6873,7 +6875,6 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 			if (flags & FRM_MARK_COMMITTED)
 				frz->t_infomask |= HEAP_XMAX_COMMITTED;
 			changed = true;
-			totally_frozen = false;
 		}
 		else if (flags & FRM_RETURN_IS_MULTI)
 		{
@@ -6895,11 +6896,6 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 			frz->xmax = newxmax;
 
 			changed = true;
-			totally_frozen = false;
-		}
-		else
-		{
-			Assert(flags & FRM_NOOP);
 		}
 	}
 	else if (TransactionIdIsNormal(xid))
@@ -6927,11 +6923,24 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 			freeze_xmax = true;
 		}
 		else
-			totally_frozen = false;
+			freeze_xmax = false;
 	}
+	else if ((tuple->t_infomask & HEAP_XMAX_INVALID) ||
+			 !TransactionIdIsValid(HeapTupleHeaderGetRawXmax(tuple)))
+	{
+		freeze_xmax = false;
+		xmax_already_frozen = true;
+	}
+	else
+		ereport(ERROR,
+				(errcode(ERRCODE_DATA_CORRUPTED),
+				 errmsg_internal("found xmax %u (infomask 0x%04x) not frozen, not multi, not normal",
+								 xid, tuple->t_infomask)));
 
 	if (freeze_xmax)
 	{
+		Assert(!xmax_already_frozen);
+
 		frz->xmax = InvalidTransactionId;
 
 		/*
@@ -6984,7 +6993,8 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 		}
 	}
 
-	*totally_frozen_p = totally_frozen;
+	*totally_frozen_p = (xmin_frozen &&
+						 (freeze_xmax || xmax_already_frozen));
 	return changed;
 }