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From: Amit Kapila <amit.kapila16@gmail.com>
Date: Fri, 19 Jun 2026 17:38:37 +0530
Message-ID: 
 <CAA4eK1JBBcS9R3m8nR93E5P-WxRwRx=AM+STZrp9g1Ma13kfag@mail.gmail.com>
Subject: Re: Fix race in ReplicationSlotRelease for ephemeral slots
To: Xuneng Zhou <xunengzhou@gmail.com>
Cc: Fujii Masao <masao.fujii@gmail.com>,
 "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>,
	Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Thu, Jun 18, 2026 at 2:06=E2=80=AFPM Xuneng Zhou <xunengzhou@gmail.com> =
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?

--=20
With Regards,
Amit Kapila.