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Re: Refactoring postmaster's code to cleanup after child exit

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2024-08-09T21:13:37Z
Lists: pgsql-hackers 
On 08/08/2024 13:47, Thomas Munro wrote:
>      On Windows, if a child process exits with ERROR_WAIT_NO_CHILDREN, it's
>      now logged with that exit code, instead of 0. Also, if a bgworker
>      exits with ERROR_WAIT_NO_CHILDREN, it's now treated as crashed and is
>      restarted. Previously it was treated as a normal exit.
> 
> Interesting.  So when that error was first specially handled in this thread:
> 
> https://www.postgresql.org/message-id/flat/AANLkTimCTkNKKrHCd3Ot6kAsrSS7SeDpOTcaLsEP7i%2BM%40mail.gmail.com#41f60947571b75377f04af67ba6baf40
> 
> ... it went from being considered a crash, to being considered like
> exit(0).  It's true that shared memory can't be corrupted by a process
> that never enters main(), but it's better not to hide the true reason
> for the failure (if it is still possible -- I don't find many
> references to that phenomenon in recent times).  Clobbering exitstatus
> with 0 doesn't seem right at all, now that we have background workers
> whose restart behaviour is affected by that.

I adjusted this ERROR_WAIT_NO_CHILDREN a little more, to avoid logging 
the death of the child twice in some cases.

>> * v3-0003-Fix-comment-on-processes-being-kept-over-a-restar.patch
> 
> +1

Committed the patches up to and including this one, with tiny comment 
changes.

>> * v3-0004-Consolidate-postmaster-code-to-launch-background-.patch
> 
>      Much of the code in process_pm_child_exit() to launch replacement
>      processes when one exits or when progressing to next postmaster state
>      was unnecessary, because the ServerLoop will launch any missing
>      background processes anyway. Remove the redundant code and let
>      ServerLoop handle it.

I'm going to work a little more on the comments on this one before 
committing; I had just moved all the "If we have lost the XXX, try to 
start a new one" comments as is, but they look pretty repetitive now.

Thanks for the review!

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix test name and username used in failed connection attempts

  2. tests: Don't fail due to high default timeout in postmaster/003_start_stop

  3. tests: Fix race condition in postmaster/002_connection_limits

  4. Pass MyPMChildSlot as an explicit argument to child process

  5. Assign a child slot to every postmaster child process

  6. Kill dead-end children when there's nothing else left

  7. Replace postmaster.c's own backend type codes with BackendType

  8. Use an shmem_exit callback to remove backend from PMChildFlags on exit

  9. Add test for dead-end backends

  10. Add test for connection limits

  11. Fix garbled process name on backend crash

  12. Consolidate postmaster code to launch background processes

  13. Fix comment on processes being kept over a restart

  14. Refactor code to handle death of a backend or bgworker in postmaster

  15. Make BackgroundWorkerList doubly-linked

  16. Minor refactoring of assign_backendlist_entry()

  17. Fix outdated comment; all running bgworkers are in BackendList

  18. Allow building with MSVC and Strawberry perl

  19. Fix rare failure in LDAP tests.

  20. Allow notifications to bgworkers without database connections.