Re: Improve LWLock tranche name visibility across backends

Hi,

On Wed, Jul 09, 2025 at 04:39:48PM -0500, Sami Imseih wrote:
> Hi,
> 
> When querying pg_stat_activity, the function pgstat_get_wait_event is
> called, which internally uses GetLWLockIdentifier and GetLWTrancheName
> to map the LWLock to its tranche name. If the backend does not recognize
> the tranche ID, a fallback name "extension" is used. Therefore, backends
> that have registered the tranche will report the correct extension-defined
> tranche name, while others will report the generic fallback of "extension".
> 
> i.e.
> ````
> postgres=# select wait_event, wait_event_type from pg_stat_activity;
> -[ RECORD 1 ]---+--------------------
> wait_event      | extension
> wait_event_type | LWLock
> ```
> instead of
> ```
> postgres=# select wait_event, wait_event_type from pg_stat_activity;
> -[ RECORD 1 ]---+--------------------
> wait_event      | MyUsefulExtension
> wait_event_type | LWLock
> ```
> 
> This is the current design, but I think we can do better to avoid inconsitencies
> this my lead for monitoring tools and diagnostics.

+1 on finding a way to improve this, thanks for looking at it.

> Attached is a proof of concept that does not alter the
> LWLockRegisterTranche API. Instead, it detects when a registration is
> performed by a normal backend and stores the tranche name in shared memory,
> using a dshash keyed by tranche ID. Tranche name lookup now proceeds in
> the order of built-in names, the local list, and finally the shared memory.
> The fallback name "extension" can still be returned if an extension does
> not register a tranche.

I did not look in details, but do you think we could make use of
WaitEventCustomNew()?

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com