postgres.git / summary / log / commit / refs

commit    9f3e4c8131886ffb1f382d4ef8e76caa965983f4
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Tue May 24 19:20:12 2016 +0000

    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>


 src/backend/commands/vacuum.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index abba59e9d6d..057fd91bde1 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -1002,7 +1002,7 @@ vac_truncate_clog(TransactionId frozenXID,
 				  TransactionId lastSaneFrozenXid,
 				  MultiXactId lastSaneMinMulti)
 {
-	TransactionId myXID = GetCurrentTransactionId();
+	TransactionId nextXID = ReadNewTransactionId();
 	Relation	relation;
 	HeapScanDesc scan;
 	HeapTuple	tuple;
@@ -1051,7 +1051,7 @@ 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))
 		{


[parent: de521989a221]