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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: Tomas Vondra <tomas@vondra.me>
Cc: Thomas Munro <thomas.munro@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, Andres Freund <andres@anarazel.de>
Date: 2024-12-10T10:00:12Z
Lists: pgsql-hackers 
On 09/12/2024 22:55, Heikki Linnakangas wrote:
> Not sure how to fix this. A small sleep in the test would work, but in 
> principle there's no delay that's guaranteed to be enough. A more robust 
> solution would be to run a "select count(*) from pg_stat_activity" and 
> wait until the number of connections are what's expected. I'll try that 
> and see how complicated that gets..

Checking pg_stat_activity doesn't help, because the backend doesn't 
register itself in pg_stat_activity until later. A connection that's 
rejected due to connection limits never shows up in pg_stat_activity.

Some options:

0. Do nothing

1. Add a small sleep to the test

2. Move the pgstat_bestart() call earlier in the startup sequence, so 
that a backend shows up in pg_stat_activity before it acquires a PGPROC 
entry, and stays visible until after it has released its PGPROC entry. 
This would give more visibility to backends that are starting up.

3. Rearrange the FATAL error handling so that the process removes itself 
from PGPROC before sending the error to the client. That would be kind 
of nice anyway. Currently, if sending the rejection error message to the 
client blocks, you are holding up a PGPROC slot until the message is 
sent. The error message packet is short, so it's highly unlikely to 
block, but still.

Option 3 seems kind of nice in principle, but looking at the code, it's 
a bit awkward to implement.  Easiest way to implement it would be to 
modify send_message_to_frontend() to not call pq_flush() on FATAL 
errors, and flush the data in socket_close() instead. Not a lot of code, 
but it's a pretty ugly special case.

Option 2 seems nice too, but seems like a lot of work.

-- 
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.