Avoid consuming an XID during vac_truncate_clog().

vac_truncate_clog() uses its own transaction ID as the comparison point in
a sanity check that no database's datfrozenxid has already wrapped around
"into the future".  That was probably fine when written, but in a lazy
vacuum we won't have assigned an XID, so calling GetCurrentTransactionId()
causes an XID to be assigned when otherwise one would not be.  Most of the
time that's not a big problem ... but if we are hard up against the
wraparound limit, consuming XIDs during antiwraparound vacuums is a very
bad thing.

Instead, use ReadNewTransactionId(), which not only avoids this problem
but is in itself a better comparison point to test whether wraparound
has already occurred.

Report and patch by Alexander Korotkov.  Back-patch to all versions.

Report: <CAPpHfdspOkmiQsxh-UZw2chM6dRMwXAJGEmmbmqYR=yvM7-s6A@mail.gmail.com>

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index c5170efea507fd8fbfaa71a09712deac4793067e..15c36eca94ffe09448f8f9d5733d354915795884 100644 (file)
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -784,7 +784,7 @@ vac_update_datfrozenxid(void)
 static void
 vac_truncate_clog(TransactionId frozenXID)
 {
-       TransactionId myXID = GetCurrentTransactionId();
+       TransactionId nextXID = ReadNewTransactionId();
        Relation        relation;
        HeapScanDesc scan;
        HeapTuple       tuple;
@@ -816,7 +816,7 @@ vac_truncate_clog(TransactionId frozenXID)
 
                Assert(TransactionIdIsNormal(dbform->datfrozenxid));
 
-               if (TransactionIdPrecedes(myXID, dbform->datfrozenxid))
+               if (TransactionIdPrecedes(nextXID, dbform->datfrozenxid))
                        frozenAlreadyWrapped = true;
                else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID))
                {