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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: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2024-07-06T19:01:44Z
Lists: pgsql-hackers

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Reading through postmaster code, I spotted some refactoring 
opportunities to make it slightly more readable.

Currently, when a child process exits, the postmaster first scans 
through BackgroundWorkerList to see if it was a bgworker process. If not 
found, it scans through the BackendList to see if it was a backend 
process (which it really should be then). That feels a bit silly, 
because every running background worker process also has an entry in 
BackendList. There's a lot of duplication between 
CleanupBackgroundWorker and CleanupBackend.

Before commit 8a02b3d732, we used to created Backend entries only for 
background worker processes that connected to a database, not for other 
background worker processes. I think that's why we have the code 
structure we have. But now that we have a Backend entry for all bgworker 
processes, it's more natural to have single function to deal with both 
regular backends and bgworkers.

So I came up with the attached patches. This doesn't make any meaningful 
user-visible changes, except for some incidental changes in log messages 
(see commit message for details).

-- 
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.