Fix use of wrong index in ComputeXidHorizons().

anarazel · anarazel · commit f6661d3df228 · 2020-08-16T14:21:37.000-07:00
This bug, recently introduced in 941697c, at least lead to vacuum failing because it found tuples inserted by a running transaction, but below the freeze limit. The freeze limit in turn is directly affected by the aforementioned bug. Thanks to Tom Lane figuring how to make the bug reproducible. We should add a few more assertions to make sure this type of bug isn't as hard to notice, but it's not yet clear how to best do so. Co-Diagnosed-By: Tom Lane <tgl@sss.pgh.pa.us> Author: Andres Freund <andres@anarazel.de> Discussion: https://postgr.es/m/1013484.1597609043@sss.pgh.pa.us
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
@@ -1663,7 +1663,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 		TransactionId xmin;
 
 		/* Fetch xid just once - see GetNewTransactionId */
-		xid = UINT32_ACCESS_ONCE(other_xids[pgprocno]);
+		xid = UINT32_ACCESS_ONCE(other_xids[index]);
 		xmin = UINT32_ACCESS_ONCE(proc->xmin);
 
 		/*
