Re: Injection points: some tools to wait and wake

On Tue, Feb 20, 2024 at 05:32:38PM +0300, Andrey M. Borodin wrote:
> I will try to simplify test to 2-step, but it would be much easier
> to implement if injection points could be awaken independently. 

I don't mind implementing something that wakes only a given point
name, that's just more state data to track in shmem for the module.

> No, "prepared injection point" is not about wait\wake logic. It's
> about having injection point in critical section. 
> Normal injection point will pstrdup(name) and fail. In [0] I need a
> test that waits after multixact generation before WAL-logging
> it. It's only possible in a critical section. 

It took me some time to understand what you mean here.  You are
referring to the allocations done in load_external_function() ->
expand_dynamic_library_name() -> substitute_libpath_macro(), which
is something that has to happen the first time a callback is loaded
into the local cache of a process.  So what you want to achieve is to
preload the callback in its cache in a first step without running it,
then be able to run it, so your Prepare[d] layer is just a way to
split InjectionPointRun() into two.  Fancy use case.  You could
disable the critical section around the INJECTION_POINT() as one
solution, though having something to pre-warm the local cache for a
point name and avoid the allocations done in the external load would
be a second way to achieve that.

"Prepare" is not the best term I would have used, perhaps just have
one INJECTION_POINT_PRELOAD() macro to warm up the cache outside the
critical section with a wrapper routine? You could then leave
InjectionPointRun() as it is now.  Having a check on CritSectionCount
in the injection point internals to disable temporarily a critical
section would not feel right as this is used as a safety check
anywhere else.
--
Michael