Thread

1. [BUG] Race in online checksums launcher_exit()

   Ayush Tiwari <ayushtiwari.slg01@gmail.com> — 2026-04-19T20:09:51Z

   Hi hackers,
   
   While using the pg_enable_data_checksums() feature, I found a likely bug, a
   race condition in  datachecksum_state.c's launcher_exit().
   
   When pg_enable_data_checksums() is called twice before the first launcher
   starts, two bg workers are registered (the code expects this).  The
   redundant launcher exits early, but it's launcher_exit() callback
   unconditionally clears the shared launcher_running flag and may call
   SetDataChecksumsOff() -- even though it never owned the flag.
   
   This allows a third pg_enable_data_checksums() call to launch another
   launcher concurrently with the first (duplicate work, doubled I/O, spurious
   warnings).  Worse, if the redundant launcher initialized after the winner
   transitioned to inprogress-on, its exit handler calls
   SetDataChecksumsOff(), silently aborting the enable operation.  (I have
   not triggered the SetDataChecksumsOff part though calling out ad it can be
   a likely scenario based on timing of workers)
   
   Reproduced by firing three calls in quick succession:
   
     psql -c "SELECT pg_enable_data_checksums();" &
     psql -c "SELECT pg_enable_data_checksums();" &
     sleep 0.5
     psql -c "SELECT pg_enable_data_checksums();" &
   
   Log shows two launchers processing databases concurrently:
   
     [2093292] LOG:  enabling data checksums requested
     [2093293] LOG:  already running, exiting
     [2093299] LOG:  enabling data checksums requested     -- third launcher
   admitted
     [2093292] LOG:  processing database "postgres"
     [2093299] LOG:  processing database "postgres"        -- same DB,
   concurrently
     [2093299] WARNING:  cannot set data checksums to "on", current state is
   not "inprogress-on"
   
   I think the process-local launcher_running flag exists for this purpose and
   is already used for the worker-kill block, but the flag-clear and
   state-revert blocks do not use it.
   
   The attached patch returns early from launcher_exit() when the local flag
   is false. Thoughts?
   
   Regards,
   Ayush
   

2. Re: [BUG] Race in online checksums launcher_exit()

   Daniel Gustafsson <daniel@yesql.se> — 2026-04-19T20:17:08Z

   > On 19 Apr 2026, at 22:09, Ayush Tiwari <ayushtiwari.slg01@gmail.com> wrote:
   > 
   > Hi hackers,
   > 
   > While using the pg_enable_data_checksums() feature, I found a likely bug, a race condition in  datachecksum_state.c's launcher_exit().
   
   Thanks for your report.  Tomas and I have worked over the past couple of days
   on a fixup series due to a rare race condition which was found after extensive
   longrunning testing.  While hacking on that we identified what I believe is the
   same bug you found and we have a fix for that, the patchset will be shared very
   shortly (we am literally putting the final touches on it as I write this).
   I'll compare notes and will if applicable incorporate your patch into it.
   
   --
   Daniel Gustafsson
   
   
   
   
   

3. Re: [BUG] Race in online checksums launcher_exit()

   Ayush Tiwari <ayushtiwari.slg01@gmail.com> — 2026-04-20T06:08:49Z

   Hi,
   
   On Mon, 20 Apr 2026 at 01:47, Daniel Gustafsson <daniel@yesql.se> wrote:
   
   > > On 19 Apr 2026, at 22:09, Ayush Tiwari <ayushtiwari.slg01@gmail.com>
   > wrote:
   > >
   > > Hi hackers,
   > >
   > > While using the pg_enable_data_checksums() feature, I found a likely
   > bug, a race condition in  datachecksum_state.c's launcher_exit().
   >
   > Thanks for your report.  Tomas and I have worked over the past couple of
   > days
   > on a fixup series due to a rare race condition which was found after
   > extensive
   > longrunning testing.  While hacking on that we identified what I believe
   > is the
   > same bug you found and we have a fix for that, the patchset will be shared
   > very
   > shortly (we am literally putting the final touches on it as I write this).
   > I'll compare notes and will if applicable incorporate your patch into it.
   >
   > --
   > Daniel Gustafsson
   >
   >
   Thanks Daniel! Glad to hear it's being addressed. I would be happy to
   test the patchset when it's posted.
   
   Regards,
   Ayush