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: 

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 93361a0c99c964a506c87e36878e0f3ce94f787e..9cda92503227b9dbac27eb7f861c704091f0d1f1 100644 (file)
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -1051,7 +1051,7 @@ vac_truncate_clog(TransactionId frozenXID,
                  TransactionId lastSaneFrozenXid,
                  MultiXactId lastSaneMinMulti)
 {
-   TransactionId myXID = GetCurrentTransactionId();
+   TransactionId nextXID = ReadNewTransactionId();
    Relation    relation;
    HeapScanDesc scan;
    HeapTuple   tuple;
@@ -1100,7 +1100,7 @@ vac_truncate_clog(TransactionId frozenXID,
            MultiXactIdPrecedes(lastSaneMinMulti, dbform->datminmxid))
            bogus = true;
 
-       if (TransactionIdPrecedes(myXID, dbform->datfrozenxid))
+       if (TransactionIdPrecedes(nextXID, dbform->datfrozenxid))
            frozenAlreadyWrapped = true;
        else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID))
        {