Re: Incorrect visibility test function assigned to snapshot

Michael Paquier <michael@paquier.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