From: Simon Riggs <simon@2ndQuadrant.com>
Date: Sun, 24 Jan 2016 18:22:11 +0000 (-0800)
Subject: Correct comment in GetConflictingVirtualXIDs()
X-Git-Tag: REL9_6_BETA1~821
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=1129c2b0ad2732f301f696ae2cf98fb063a4c1f8;p=thirdparty%2Fpostgresql.git

Correct comment in GetConflictingVirtualXIDs()

We use Share lock because it is safe to do so.
---

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 813cea21a5d..bd0c96124e3 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -2540,8 +2540,11 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
 
 			/*
 			 * We ignore an invalid pxmin because this means that backend has
-			 * no snapshot and cannot get another one while we hold exclusive
-			 * lock.
+			 * no snapshot currently. We hold a Share lock to avoid contention
+			 * with users taking snapshots.  That is not a problem because
+			 * the current xmin is always at least one higher than the latest
+			 * removed xid, so any new snapshot would never conflict with the
+			 * test here.
 			 */
 			if (!TransactionIdIsValid(limitXmin) ||
 				(TransactionIdIsValid(pxmin) && !TransactionIdFollows(pxmin, limitXmin)))