Re: Incorrect visibility test function assigned to snapshot

Michael Paquier <michael(at)paquier(dot)xyz> wrote:

> On Fri, Feb 08, 2019 at 11:59:05AM +0100, Antonin Houska wrote:
> > Sorry, I forgot. Patch is below and I'm going to add an entry to the
> > next CF.
>
> > @@ -615,6 +615,8 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
> >
> > TransactionIdAdvance(xid);
> > }
> > + /* And of course, adjust snapshot type accordingly. */
> > + snap->snapshot_type = SNAPSHOT_MVCC;
>
> Wouldn't it be cleaner to have an additional argument to
> SnapBuildBuildSnapshot() for the snapshot type? It looks confusing to
> me to overwrite the snapshot type after initializing it once.

I'm not sure. SnapBuildBuildSnapshot() only creates snapshot for the logical
replication purposes and the "snapshot_type" argument would make the function
look like it can handle all snapshot types. If adding an argument, I'd prefer
a boolean variable (e.g. "historic") telling whether user wants
SNAPSHOT_HISTORIC_MVCC or SNAPSHOT_MVCC. But that may deserve a separate
patch.

As for the bug fix, I think the additional assignment does not make things
worse because SnapBuildInitialSnapshot() already does overwrite some fields:
"xip" and "xnt".

> > @@ -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;
>
> Okay for this one, however the comment does not add much value.

o.k. I don't insist on using this comment.

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