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  1. Fix corruption of toast indexes with REINDEX CONCURRENTLY

  1. table_open/table_close with different lock mode

    Junwang Zhao <zhjwpku@gmail.com> — 2023-07-21T06:05:56Z

    Hey hackers,

I noticed there are some places calling table_open with
RowExclusiveLock but table_close with NoLock, like in function
toast_save_datum.

Can anybody explain the underlying logic, thanks in advance.

-- 
Regards
Junwang Zhao

  2. Re: table_open/table_close with different lock mode

    Michael Paquier <michael@paquier.xyz> — 2023-07-21T06:26:22Z

    On Fri, Jul 21, 2023 at 02:05:56PM +0800, Junwang Zhao wrote:
> I noticed there are some places calling table_open with
> RowExclusiveLock but table_close with NoLock, like in function
> toast_save_datum.
> 
> Can anybody explain the underlying logic, thanks in advance.

This rings a bell.  This is a wanted behavior, see commit f99870d and
its related thread:
https://postgr.es/m/17268-d2fb426e0895abd4@postgresql.org

The tests added by this commit in src/test/isolation/ will show the
difference in terms of the way the toast values get handled with and
without the change.
--
Michael

  3. Re: table_open/table_close with different lock mode

    Junwang Zhao <zhjwpku@gmail.com> — 2023-07-21T06:38:34Z

    On Fri, Jul 21, 2023 at 2:26 PM Michael Paquier <michael@paquier.xyz> wrote:
>
> On Fri, Jul 21, 2023 at 02:05:56PM +0800, Junwang Zhao wrote:
> > I noticed there are some places calling table_open with
> > RowExclusiveLock but table_close with NoLock, like in function
> > toast_save_datum.
> >
> > Can anybody explain the underlying logic, thanks in advance.
>
> This rings a bell.  This is a wanted behavior, see commit f99870d and
> its related thread:
> https://postgr.es/m/17268-d2fb426e0895abd4@postgresql.org
>

I see this patch, so all the locks held by a transaction will be released
at the commit phase, right? Can you show me where the logic is located?

> The tests added by this commit in src/test/isolation/ will show the
> difference in terms of the way the toast values get handled with and
> without the change.
> --
> Michael



-- 
Regards
Junwang Zhao

  4. Re: table_open/table_close with different lock mode

    Gurjeet Singh <gurjeet@singh.im> — 2023-07-21T06:56:49Z

    On Thu, Jul 20, 2023 at 11:38 PM Junwang Zhao <zhjwpku@gmail.com> wrote:
>
> On Fri, Jul 21, 2023 at 2:26 PM Michael Paquier <michael@paquier.xyz> wrote:
> >
> > On Fri, Jul 21, 2023 at 02:05:56PM +0800, Junwang Zhao wrote:
> > > I noticed there are some places calling table_open with
> > > RowExclusiveLock but table_close with NoLock, like in function
> > > toast_save_datum.
> > >
> > > Can anybody explain the underlying logic, thanks in advance.
> >
> > This rings a bell.  This is a wanted behavior, see commit f99870d and
> > its related thread:
> > https://postgr.es/m/17268-d2fb426e0895abd4@postgresql.org
> >
>
> I see this patch, so all the locks held by a transaction will be released
> at the commit phase, right? Can you show me where the logic is located?

The NoLock is simple a marker that tells the underlying machinery to
not bother releasing any locks. As a matter of fact, you can pass
NoLock in *_open() calls, too, to indicate that you don't want any new
locks, perhaps because the transaction has already taken an
appropriate lock on the object.

As for lock-releasing codepath at transaction end, see
CommitTransaction() in xact.c, and specifically at the
ResourceOwnerRelease() calls in there.

Best regards,
Gurjeet
http://Gurje.et

  5. Re: table_open/table_close with different lock mode

    Junwang Zhao <zhjwpku@gmail.com> — 2023-07-21T07:01:59Z

    On Fri, Jul 21, 2023 at 2:57 PM Gurjeet Singh <gurjeet@singh.im> wrote:
>
> On Thu, Jul 20, 2023 at 11:38 PM Junwang Zhao <zhjwpku@gmail.com> wrote:
> >
> > On Fri, Jul 21, 2023 at 2:26 PM Michael Paquier <michael@paquier.xyz> wrote:
> > >
> > > On Fri, Jul 21, 2023 at 02:05:56PM +0800, Junwang Zhao wrote:
> > > > I noticed there are some places calling table_open with
> > > > RowExclusiveLock but table_close with NoLock, like in function
> > > > toast_save_datum.
> > > >
> > > > Can anybody explain the underlying logic, thanks in advance.
> > >
> > > This rings a bell.  This is a wanted behavior, see commit f99870d and
> > > its related thread:
> > > https://postgr.es/m/17268-d2fb426e0895abd4@postgresql.org
> > >
> >
> > I see this patch, so all the locks held by a transaction will be released
> > at the commit phase, right? Can you show me where the logic is located?
>
> The NoLock is simple a marker that tells the underlying machinery to
> not bother releasing any locks. As a matter of fact, you can pass
> NoLock in *_open() calls, too, to indicate that you don't want any new
> locks, perhaps because the transaction has already taken an
> appropriate lock on the object.
>
> As for lock-releasing codepath at transaction end, see
> CommitTransaction() in xact.c, and specifically at the
> ResourceOwnerRelease() calls in there.
>

Great, thanks for the thorough explanation, will look into the code :)

> Best regards,
> Gurjeet
> http://Gurje.et



-- 
Regards
Junwang Zhao