From: Antonin Houska <ah@cybertec.at>
To: Michael Paquier <michael@paquier.xyz>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Incorrect visibility test function assigned to snapshot
Date: Fri, 15 Feb 2019 13:01:29 +0100
Message-ID: <16034.1550232089@localhost> (raw)
In-Reply-To: <20190214071313.GF2366@paquier.xyz>
References: <23215.1527665193@localhost>
<20180530132854.tkv37qiamepu3slh@alvherre.pgsql>
<20180623151856.GG21575@momjian.us>
<23440.1549623545@localhost>
<20190214071313.GF2366@paquier.xyz>
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
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To: pgsql-hackers@postgresql.org
Cc: ah@cybertec.at, michael@paquier.xyz, bruce@momjian.us, alvherre@2ndquadrant.com
Subject: Re: Incorrect visibility test function assigned to snapshot
In-Reply-To: <16034.1550232089@localhost>
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