Don't test HEAP_XMAX_INVALID when freezing xmax.

We shouldn't ever need to rely on whether HEAP_XMAX_INVALID is set in
t_infomask when considering whether or not an xmax should be deemed
already frozen, since that status flag is just a hint.  The only
acceptable representation for an "xmax_already_frozen" raw xmax field is
the transaction ID value zero (also known as InvalidTransactionId).

Adjust code that superficially appeared to rely on HEAP_XMAX_INVALID to
make the rule about xmax_already_frozen clear.  Also avoid needlessly
rereading the tuple's raw xmax.

Oversight in bugfix commit d2599ecf.  There is no evidence that this
ever led to incorrect behavior, so no backpatch.  The worst consequence
of this bug was that VACUUM could hypothetically fail to notice and
report on certain kinds of corruption, which seems fairly benign.

Author: Peter Geoghegan <[email protected]>
Discussion: https://postgr.es/m/CAH2-Wzkh3DMCDRPfhZxj9xCq9v3WmzvmbiCpf1dNKUBPadhCbQ@mail.gmail.com

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index d18c5ca6f586b0fc0431efb13e3669fd601a2fdb..747db50376154e737707347fe717660b64f1501d 100644 (file)
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -6542,6 +6542,7 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 
    if (tuple->t_infomask & HEAP_XMAX_IS_MULTI)
    {
+       /* Raw xmax is a MultiXactId */
        TransactionId newxmax;
        uint16      flags;
        TransactionId mxid_oldest_xid_out = *relfrozenxid_out;
@@ -6640,6 +6641,7 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
    }
    else if (TransactionIdIsNormal(xid))
    {
+       /* Raw xmax is normal XID */
        if (TransactionIdPrecedes(xid, relfrozenxid))
            ereport(ERROR,
                    (errcode(ERRCODE_DATA_CORRUPTED),
@@ -6670,9 +6672,10 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
                *relfrozenxid_out = xid;
        }
    }
-   else if ((tuple->t_infomask & HEAP_XMAX_INVALID) ||
-            !TransactionIdIsValid(HeapTupleHeaderGetRawXmax(tuple)))
+   else if (!TransactionIdIsValid(xid))
    {
+       /* Raw xmax is InvalidTransactionId XID */
+       Assert((tuple->t_infomask & HEAP_XMAX_IS_MULTI) == 0);
        freeze_xmax = false;
        xmax_already_frozen = true;
        /* No need for relfrozenxid_out handling for already-frozen xmax */