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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: 2024-08-12T09:55:00Z
Lists: pgsql-hackers

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On 10/08/2024 00:13, Heikki Linnakangas wrote:
> On 08/08/2024 13:47, Thomas Munro wrote:
>>> * 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.

Pushed this now, after adjusting the comments a bit. Thanks again for 
the review!

Here are the remaining patches, rebased.

> commit a1c43d65907d20a999b203e465db1277ec842a0a
> Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
> Date:   Thu Aug 1 17:24:12 2024 +0300
> 
>     Introduce a separate BackendType for dead-end children
>     
>     And replace postmaster.c's own "backend type" codes with BackendType
>     
>     XXX: While working on this, many times I accidentally did something
>     like "foo |= B_SOMETHING" instead of "foo |= 1 << B_SOMETHING", when
>     constructing arguments to SignalSomeChildren or CountChildren, and
>     things broke in very subtle ways taking a long time to debug. The old
>     constants that were already bitmasks avoided that. Maybe we need some
>     macro magic or something to make this less error-prone.

While rebasing this today, I spotted another instance of that mistake 
mentioned in the XXX comment above. I called "CountChildren(B_BACKEND)" 
instead of "CountChildren(1 << B_BACKEND)". Some ideas on how to make 
that less error-prone:

1. Add a separate typedef for the bitmasks, and macros/functions to work 
with it. Something like:

typedef struct {
	uint32		mask;
} BackendTypeMask;

static const BackendTypeMask BTMASK_ALL = { 0xffffffff };
static const BackendTypeMask BTMASK_NONE = { 0 };

static inline BackendTypeMask
BTMASK_ADD(BackendTypeMask mask, BackendType t)
{
	mask.mask |= 1 << t;
	return mask;
}

static inline BackendTypeMask
BTMASK_DEL(BackendTypeMask mask, BackendType t)
{
	mask.mask &= ~(1 << t);
	return mask;
}

Now the compiler will complain if you try to pass a BackendType for the 
mask. We could do this just for BackendType, or we could put this in 
src/include/lib/ with a more generic name, like "bitmask_u32".

2. Another idea is to redefine the BackendType values to be separate 
bits, like the current BACKEND_TYPE_* values in postmaster.c:

typedef enum BackendType
{
	B_INVALID = 0,

	/* Backends and other backend-like processes */
	B_BACKEND = 1 << 1,
	B_DEAD_END_BACKEND = 1 << 2,
	B_AUTOVAC_LAUNCHER = 1 << 3,
	B_AUTOVAC_WORKER = 1 << 4,

	...
} BackendType;

Then you can use | and & on BackendTypes directly. It makes it less 
clear which function arguments are a BackendType and which are a 
bitmask, however.


Thoughts, other ideas?

-- 
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.