public inbox for [email protected]  

help / color / mirror / Atom feed

From: Zhijie Hou (Fujitsu) <[email protected]>
To: JoongHyuk Shin <[email protected]>
Cc: [email protected] <[email protected]>
Subject: RE: [PATCH] Fix TOCTOU race in ReplicationSlotsComputeRequiredLSN()
Date: Fri, 17 Apr 2026 06:59:14 +0000
Message-ID: <TYRPR01MB14195CA431CFC783F77FB8AEF94202@TYRPR01MB14195.jpnprd01.prod.outlook.com> (raw)
In-Reply-To: <CACSdjfMQYL3DV-3inrxdReqpFMOky4JxLbN0gTTY+qGJqrUJXw@mail.gmail.com>
References: <CACSdjfMQYL3DV-3inrxdReqpFMOky4JxLbN0gTTY+qGJqrUJXw@mail.gmail.com>

On Friday, April 17, 2026 2:50 PM JoongHyuk Shin <[email protected]>  wrote:
> Commit 2a5225b99d7 fixed a race in ReplicationSlotsComputeRequiredXmin()
> where ReplicationSlotControlLock was released before the global xmin
> update, allowing a concurrent backend to overwrite a correct value with
> a stale one.
> 
> ReplicationSlotsComputeRequiredLSN() has the same problem, 
> it releases the lock before calling XLogSetReplicationSlotMinimumLSN(), 
> so a stale minimum LSN can overwrite a correct (lower) one, 
> potentially leading to premature WAL removal.
> 
> The attached patch moves LWLockRelease() to after the LSN update,
> matching the xmin fix.
> Since 2a5225b99d7 was backpatched to all supported versions, 
> I believe this should be as well.

Thanks for noticing this. There is an existing thread [1] that I started
following 2a5225b99d7 to address the same issue. The patch you posted
only increases the lock scope in ReplicationSlotsComputeRequiredLSN() but does
not increase the lock level when reserving WALs, so I think it would not
fix the issue.

Please feel free to review the patch in that thread if you find it helpful.

[1] https://commitfest.postgresql.org/patch/6451/

Best Regards,
Hou zj

view thread (3+ messages)  latest in thread

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected]
  Subject: RE: [PATCH] Fix TOCTOU race in ReplicationSlotsComputeRequiredLSN()
  In-Reply-To: <TYRPR01MB14195CA431CFC783F77FB8AEF94202@TYRPR01MB14195.jpnprd01.prod.outlook.com>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox