Re: Fix race in ReplicationSlotRelease for ephemeral slots

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From: Amit Kapila <[email protected]>
To: Xuneng Zhou <[email protected]>
Cc: Fujii Masao <[email protected]>
Cc: Zhijie Hou (Fujitsu) <[email protected]>
Cc: Srinath Reddy Sadipiralla <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: Fix race in ReplicationSlotRelease for ephemeral slots
Date: Sat, 20 Jun 2026 15:12:42 +0530
Message-ID: <CAA4eK1LG3gwpqidgvAGaTdz4fQRZWbRMM_nz1+2UHh-muXCWKQ@mail.gmail.com> (raw)
In-Reply-To: <CABPTF7Um6sB-7WLg5OeBMD5DvWNkj3ZwACp5A69hgL3GUD2Ezg@mail.gmail.com>
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On Sat, Jun 20, 2026 at 12:11 PM Xuneng Zhou <[email protected]> wrote:
>
> On Fri, Jun 19, 2026 at 8:08 PM Amit Kapila <[email protected]> wrote:
> >
> > On Thu, Jun 18, 2026 at 2:06 PM Xuneng Zhou <[email protected]> wrote:
> > >
> > > OK, how about elaborate it a bit like this:
> > >
> > > /*
> > >  * In the small window between getting the slot to drop and
> > >  * locking the database, there is a possibility of a parallel
> > >  * database drop by the startup process and the creation of a new
> > >  * slot by the user. This new user-created slot may end up using
> > >  * the same shared memory as that of 'local_slot'.
> > >  *
> > >  * If that happens, local_slot now describes the replacement slot:
> > >  * local_sync_slot_required() may have made its drop decision using
> > >  * the replacement slot's name or invalidation state, and slot_database
> > >  * may refer to the replacement slot's database. Thus check if
> > >  * local_slot is still a synced slot before performing the actual drop.
> > >  * This does not prove it is the original slot, but it prevents dropping
> > >  * an ordinary user-created replacement slot, and the copied database OID
> > >  * keeps lock/unlock symmetric. The remaining risk is limited to this
> > >  * cleanup cycle, such as briefly holding an unrelated database lock, and
> > >  * is acceptable here because this race is rare.
> > >  */
> > >
> >
> > Okay inspired from your and Fujii-san's version, here is a third version:
> > /*
> >  * In the small window between getting the slot to drop and
> >  * locking the database, there is a possibility of a parallel
> >  * database drop by the startup process and the creation of a new
> >  * slot by the user. This new user-created slot may end up using
> >  * the same shared memory as that of 'local_slot'.
> >  *
> >  * Because local_slot still points to a reusable slot-array entry,
> >  * its fields (name, database OID, invalidation state) may already
> >  * describe such a replacement slot by the time we reach here. That
> >  * means the drop decision made by local_sync_slot_required() above
> >  * could have been based on the replacement slot's data, and
> >  * slot_database could refer to an unrelated database. The recheck
> >  * below keeps us from actually dropping a user-created replacement
> >  * slot; the residual risk is confined to this cycle (for example,
> >  * briefly locking an unrelated database) and is acceptable because
> >  * the race is rare and non-fatal.
> >  */
> >
> > Thoughts?
>
> LGTM. It looks well-articulated.
>

Thanks, I'll push this as soon as the PG20 branch opens.

-- 
With Regards,
Amit Kapila.

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  Subject: Re: Fix race in ReplicationSlotRelease for ephemeral slots
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