Re: make MaxBackends available in _PG_init

On 9/21/20, 4:52 AM, "Bharath Rupireddy" <bharath.rupireddyforpostgres@gmail.com> wrote:
> On Mon, Sep 21, 2020 at 9:14 AM Wang, Shenhao
> <wangsh.fnst@cn.fujitsu.com> wrote:
>>
>> In source, I find that the postmaster will first load library, and then calculate the value of MaxBackends.
>>
>> In the old version, the MaxBackends was calculated by:
>>          MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
>>                 GetNumShmemAttachedBgworkers();
>> Because any extension can register workers which will affect the return value of GetNumShmemAttachedBgworkers.
>> InitializeMaxBackends must be called after shared_preload_libraries. This is also mentioned in comments.
>>
>> Now, function GetNumShmemAttachedBgworkers was deleted and replaced by guc max_worker_processes,
>> so if we changed the calling order like:
>>         Step1: calling InitializeMaxBackends.
>>         Step2: calling process_shared_preload_libraries
>>
>
> Yes, the GetNumShmemAttachedBgworkers() was removed by commit #
> dfbba2c86cc8f09cf3ffca3d305b4ce54a7fb49a. ASAICS, changing the order
> of InitializeMaxBackends() and process_shared_preload_libraries() has
> no problem, as InitializeMaxBackends() doesn't calculate the
> MaxBackends based on bgworker infra code, it does calculate based on
> GUCs.
>
> Having said that, I'm not quite sure whether any of the bgworker
> registration code, for that matter process_shared_preload_libraries()
> code path will somewhere use MaxBackends?
>
>>
>> In this order extension can get the correct value of MaxBackends in _PG_init.
>>
>
> Is there any specific use case that any of the _PG_init will use MaxBackends?

I just encountered the same thing, so I am bumping this thread.  I was
trying to use MaxBackends in a call to RequestAddinShmemSpace() in a
_PG_init() function for a module, but since MaxBackends is not yet
initialized, you essentially need to open-code InitializeMaxBackends()
instead.

I think the comments about needing to register background workers
before initializing MaxBackends have been incorrect since the addition
of max_worker_processes in v9.4 (6bc8ef0b).  Furthermore, I think the
suggested reordering is a good idea because it is not obvious that
MaxBackends will be uninitialized in _PG_init(), and use-cases like
the RequestAddinShmemSpace() one are not guaranteed to fail when
MaxBackends is used incorrectly (presumably due to the 100 KB buffer
added in CreateSharedMemoryAndSemaphores()).

I've attached a new version of the proposed patch with some slight
adjustments and an attempt at a commit message.

Nathan