Re: Incorrect visibility test function assigned to snapshot - Mailing list pgsql-hackers

Bruce Momjian <bruce@momjian.us> wrote:

> On Wed, May 30, 2018 at 09:28:54AM -0400, Alvaro Herrera wrote:
> > On 2018-May-30, Antonin Houska wrote:
> >
> > > In the header comment, SnapBuildInitialSnapshot() claims to set
> > > snapshot->satisfies to the HeapTupleSatisfiesMVCC test function, and indeed it
> > > converts the "xid" array to match its semantics (i.e. the xid items eventually
> > > represent running transactions as opposed to the committed ones). However the
> > > test function remains HeapTupleSatisfiesHistoricMVCC as set by
> > > SnapBuildBuildSnapshot().
> >
> > Interesting.  While this sounds like an oversight that should have
> > horrible consequences, it's seems not to because the current callers
> > don't seem to care about the ->satisfies function.  Are you able to come
> > up with some scenario in which it causes an actual problem?
>
> Uh, are we going to fix this anyway?  Seems we should.

Sorry, I forgot. Patch is below and I'm going to add an entry to the next CF.

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26, A-2700 Wiener Neustadt
Web: https://www.cybertec-postgresql.com

diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index 2f185f7823..3394abb602 100644
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -615,6 +615,8 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
 
         TransactionIdAdvance(xid);
     }
+    /* And of course, adjust snapshot type accordingly. */
+    snap->snapshot_type = SNAPSHOT_MVCC;
 
     snap->xcnt = newxcnt;
     snap->xip = newxip;
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 6e02585e10..bc0166cc6f 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -1502,6 +1502,13 @@ ImportSnapshot(const char *idstr)
      */
     memset(&snapshot, 0, sizeof(snapshot));
 
+    /*
+     * Do not rely on the fact that SNAPSHOT_MVCC is zero. (The core code
+     * currently does not use this field of imported snapshot, but let's keep
+     * things consistent.)
+     */
+    snapshot.snapshot_type = SNAPSHOT_MVCC;
+
     parseVxidFromText("vxid:", &filebuf, path, &src_vxid);
     src_pid = parseIntFromText("pid:", &filebuf, path);
     /* we abuse parseXidFromText a bit here ... */