Re: [HACKERS] PoC: custom signal handler for extensions

On 12.01.2018 18:51, Teodor Sigaev wrote:

In perspective, this mechanism provides the low-level instrument to define remote procedure call on extension side. The simple RPC that defines effective userid on remote backend (remote_effective_user function) is attached for example.

Suppose, it's useful patch. Some notes:

1) AssignCustomProcSignalHandler is unneeded API function, easy replaced by GetCustomProcSignalHandler/ReleaseCustomProcSignal/RegisterCustomProcSignalHandler functions, but in other hand, it isn't looked as widely used feature to reassign custom signal handler.

Agreed. AssignCustomProcSignalHandler is unnecessary. Also I have removed GetCustomProcSignalHandler as not see any application of this function.

2) Naming RegisterCustomProcSignalHandler and ReleaseCustomProcSignal  is not self-consistent. Other parts suggest pair Register/Unregister or Aquire/Release. Seems, Register/Unregister pair is preferred here.

Thanks for notice. Fixed.

3) Agree that Assert(reason >= PROCSIG_CUSTOM_1 && reason <= PROCSIG_CUSTOM_N) should be replaced with ereport. By the way, ReleaseCustomProcSignal() is a single place where there isn't a range check of reason arg

Oops, I missed this assert check.
I considered assert check in user available routines accepting procsignal as a precondition for routine's clients, i.e. violation of this check have to involve uncertain behavior. This checks is not expensive itself therefore it makes sense to replace their to runtime checks via ereport calls.

4) CustomSignalInterrupt() - play with errno and SetLatch() seems unneeded here - there isn't call of handler of custom signal, SetLatch will be called several lines below

I have noticed that even simple HandleNotifyInterrupt() and HandleParallelMessageInterrupt() routines add at least SetLatch(MyLatch). I think it makes sense to leave this call in our case. Perhaps, I'm wrong. I don't understand entirely the intention of SetLatch() in those cases.

5) Using a special memory context for handler call looks questionably, I think that complicated handlers could manage memory itself (and will do, if they need to store something between calls). So, I prefer to remove context.

Fixed. But in this case we have to explicitly reflect in documentation the ambiguity of running memory context under signal handler and the intention to leave memory management on extension's developer.

6) As I can see, all possible (not only custom) signal handlers could perfectly use this API, or, at least, be store in CustomHandlers array, which could be renamed to InterruptHandlers, for example. Next, custom handler type is possible to make closier to built-in handlers, let its signature extends to
void (*ProcSignalHandler_type) (ProcSignalReason reason) as others. It will allow to use single handler to support several reasons.

OK, fixed.

7) Suppose, API allows to use different handlers in different processes for the same reason, it's could be reason of confusion. I suggest to restrict Register/Unregister call only for shared_preload_library, ie only during startup.

Yeah, added checks on process_shared_preload_libraries_in_progress flag. Thanks for notice.

8) I'd like to see an example of usage this API somewhere in contrib in exsting modules. Ideas?

Besides of usage in the extension pg_query_state [1] that provides the way to extract query state from running backend, I see the possibility with this patch to implement statistical sampling-based profiler for plpgsql functions on extension side. If we could get a call stack of plpgsql functions on interruptible backend then there are no obstacles to organize capturing of stack frames at some intervals over fixed period of time defined by arguments in extension's function.


I have attached a new version of patch and updated version of remote_effective_user function implementation that demonstrates the usage of custom signals API.


1. https://github.com/postgrespro/pg_query_state

-- 
Regards,
Maksim Milyutin