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

Hi, hackers!


I want to propose the patch that allows to define custom signals and 
their handlers on extension side. It is based on ProcSignal module, 
namely it defines the fixed set (number is specified by constant) of 
custom signals that could be reserved on postgres initialization stage 
(in _PG_init function of shared preloaded module) through specific 
interface functions. Functions that are custom signal handlers are 
defined in extension. The relationship between custom signals and 
assigned handlers (function addresses) is replicated from postmaster to 
child processes including working backends. Using this signals we are 
able to call specific handler (via SendProcSignal function) on remote 
backend that is actually come into action in CHECK_FOR_INTERRUPTS point.

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.

C&C welcome!


-- 
Regards,
Maksim Milyutin

+1
if this permits to use extension  pg_query_state
<https://github.com/postgrespro/pg_query_state>  , that would be great !





--
Sent from: http://www.postgresql-archive.org/PostgreSQL-hackers-f1928748.html

23.12.17 12:58, legrand legrand wrote:

> +1
> if this permits to use extension  pg_query_state
> <https://github.com/postgrespro/pg_query_state>  , that would be great !
>

Yes, attached patch is the single critical point. It allows to loose 
pg_query_state from the need to patch postgres core.

-- 
Regards,
Maksim Milyutin

Hello,

On Fri, Dec 22, 2017 at 03:05:25PM +0300, Maksim Milyutin wrote:
> Hi, hackers!
> 
> 
> I want to propose the patch that allows to define custom signals and their
> handlers on extension side.
> 

I've looked a little bit on the patch. The patch applyes and regression tests pass.
I have a couple comments.

> The relationship between custom signals and 
> assigned handlers (function addresses) is replicated from postmaster to 
> child processes including working backends.

I think this happens only if a custom signal registered during initializing shared_preload_libraries.
But from RegisterCustomProcSignalHandler()'s implementation I see that you can register the signal anytime. Am I
wrong?

If so then custom signal handlers may not work as expected.

What is purpose of AssignCustomProcSignalHandler() function? This function can erase a handler set by another
extension.
For example, extension 1 set handler passing reason PROCSIG_CUSTOM_1, and extension 2 set another handler passing
reasonPROCSIG_CUSTOM_1 too. Here the handler of the extension 2 will be purged.
 

> +
> +    Assert(reason >= PROCSIG_CUSTOM_1 && reason <= PROCSIG_CUSTOM_N);
> +

I think it is not good to use asserts within AssignCustomProcSignalHandler() and GetCustomProcSignalHandler(). Because
thisfunctions can be executed by an external extension, and it may pass a reason outside this boundary. It will be
betterif the reason will be checked in runtime.
 

But it is fine for this assert within CustomSignalInterrupt().

-- 
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company

> 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.

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.

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

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

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.

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.

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.

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



-- 
Teodor Sigaev                                   E-mail: teodor@sigaev.ru
                                                    WWW: http://www.sigaev.ru/

This comes from
https://github.com/postgrespro/pg_query_state

Regards
PAscal



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Sent from: http://www.postgresql-archive.org/PostgreSQL-hackers-f1928748.html

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

Hello!

The relationship between custom signals and 
assigned handlers (function addresses) is replicated from postmaster to 
child processes including working backends.

I think this happens only if a custom signal registered during initializing shared_preload_libraries.
But from RegisterCustomProcSignalHandler()'s implementation I see that you can register the signal anytime. Am I wrong?

If so then custom signal handlers may not work as expected.

Yeah, thanks. Added checks on process_shared_preload_libraries_in_progress flag.

What is purpose of AssignCustomProcSignalHandler() function? This function can erase a handler set by another extension.
For example, extension 1 set handler passing reason PROCSIG_CUSTOM_1, and extension 2 set another handler passing reason PROCSIG_CUSTOM_1 too. Here the handler of the extension 2 will be purged.

+
+	Assert(reason >= PROCSIG_CUSTOM_1 && reason <= PROCSIG_CUSTOM_N);
+

I think it is not good to use asserts within AssignCustomProcSignalHandler() and GetCustomProcSignalHandler(). Because this functions can be executed by an external extension, and it may pass a reason outside this boundary. It will be better if the reason will be checked in runtime.

I was guided by the consideration that assertions define preconditions for input parameter (in our case, procsignal) of functions. Meeting these preconditions have to be provided by function callers. Since these checks is not expensive it will be good to replace their to ereport calls.

Updated patch is attached in [1].

1. https://www.postgresql.org/message-id/37a48ac6-aa14-4e36-5f08-cf8581fe1382%40gmail.com

-- 
Regards,
Maksim Milyutin

Hello,

On Mon, Jan 22, 2018 at 02:34:58PM +0300, Maksim Milyutin wrote:
> ...
> 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.

Thank you.

The patch is applied and build.

> +/*
> + * UnregisterCustomProcSignal
> + *      Release slot of specific custom signal.
> + *
> + * This function have to be called in _PG_init or _PG_fini functions of
> + * extensions at the stage of loading shared preloaded libraries. Otherwise it
> + * throws fatal error. Also it throws fatal error if argument is not valid
> + * custom signal.
> + */
> +void
> +UnregisterCustomProcSignal(ProcSignalReason reason)
> +{
> +    if (!process_shared_preload_libraries_in_progress)
> +        ereport(FATAL, (errcode(ERRCODE_INTERNAL_ERROR),
> +                        errmsg("cannot unregister custom signal after startup")));

Is there actual need in UnregisterCustomProcSignal() within _PG_init()?
An extension registers a handler and then unregister it doing
nothing. It seems useless.

Also process_shared_preload_libraries_in_progress within _PG_fini() will
be false I think. _PG_fini() won't be called though, because
implementation of internal_unload_library() is disabled.

Actually, is there need in UnregisterCustomProcSignal() at all? It
unregisters a handler only in current backend, for actual unergistering
we need to call it everywhere, if I'm not mistaken.

-- 
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company

Hello Maksim,

On 1/27/18 2:19 PM, Arthur Zakirov wrote:
> On Mon, Jan 22, 2018 at 02:34:58PM +0300, Maksim Milyutin wrote:
> 
> The patch is applied and build.
> 
>> +/*
>> + * UnregisterCustomProcSignal
>> + *      Release slot of specific custom signal.
>> + *
>> + * This function have to be called in _PG_init or _PG_fini functions of
>> + * extensions at the stage of loading shared preloaded libraries. Otherwise it
>> + * throws fatal error. Also it throws fatal error if argument is not valid
>> + * custom signal.
>> + */
>> +void
>> +UnregisterCustomProcSignal(ProcSignalReason reason)
>> +{
>> +    if (!process_shared_preload_libraries_in_progress)
>> +        ereport(FATAL, (errcode(ERRCODE_INTERNAL_ERROR),
>> +                        errmsg("cannot unregister custom signal after startup")));
> 
> Is there actual need in UnregisterCustomProcSignal() within _PG_init()?
> An extension registers a handler and then unregister it doing
> nothing. It seems useless.
> 
> Also process_shared_preload_libraries_in_progress within _PG_fini() will
> be false I think. _PG_fini() won't be called though, because
> implementation of internal_unload_library() is disabled.
> 
> Actually, is there need in UnregisterCustomProcSignal() at all? It
> unregisters a handler only in current backend, for actual unergistering
> we need to call it everywhere, if I'm not mistaken.

This patch has been in Waiting on Author state for almost three weeks.
Have you had a chance to consider Arthur's suggestions?

Do you know when you'll have an updated patch available?

Thanks,
-- 
-David
david@pgmasters.net

Hello David,


On 05.03.2018 18:50, David Steele wrote:
> Hello Maksim,
>
> On 1/27/18 2:19 PM, Arthur Zakirov wrote:
>
>> Is there actual need in UnregisterCustomProcSignal() within _PG_init()?
>> An extension registers a handler and then unregister it doing
>> nothing. It seems useless.
>>
>> Also process_shared_preload_libraries_in_progress within _PG_fini() will
>> be false I think. _PG_fini() won't be called though, because
>> implementation of internal_unload_library() is disabled.
>>
>> Actually, is there need in UnregisterCustomProcSignal() at all? It
>> unregisters a handler only in current backend, for actual unergistering
>> we need to call it everywhere, if I'm not mistaken.
> This patch has been in Waiting on Author state for almost three weeks.
> Have you had a chance to consider Arthur's suggestions?

Yes, I want to rework my patch to enable setup of custom signals on 
working backend without preload initialization.

> Do you know when you'll have an updated patch available?

I want to actuate the work on this patch for the next 12 release. Sorry, 
for now I can not keep up with the current release.

-- 
Regards,
Maksim Milyutin

Hi Maksim,

On 3/5/18 11:24 AM, Maksim Milyutin wrote:
> Hello David,
> 
> 
> On 05.03.2018 18:50, David Steele wrote:
>> Hello Maksim,
>>
>> On 1/27/18 2:19 PM, Arthur Zakirov wrote:
>>
>>> Is there actual need in UnregisterCustomProcSignal() within _PG_init()?
>>> An extension registers a handler and then unregister it doing
>>> nothing. It seems useless.
>>>
>>> Also process_shared_preload_libraries_in_progress within _PG_fini() will
>>> be false I think. _PG_fini() won't be called though, because
>>> implementation of internal_unload_library() is disabled.
>>>
>>> Actually, is there need in UnregisterCustomProcSignal() at all? It
>>> unregisters a handler only in current backend, for actual unergistering
>>> we need to call it everywhere, if I'm not mistaken.
>> This patch has been in Waiting on Author state for almost three weeks.
>> Have you had a chance to consider Arthur's suggestions?
> 
> Yes, I want to rework my patch to enable setup of custom signals on
> working backend without preload initialization.
> 
>> Do you know when you'll have an updated patch available?
> 
> I want to actuate the work on this patch for the next 12 release. Sorry,
> for now I can not keep up with the current release.
Understood.  I'll mark it Returned with Feedback and you can enter it in
a CF when you have a new patch.

Regards,
-- 
-David
david@pgmasters.net

Hello Maksim,

reading about
https://www.postgresql-archive.org/Allow-auto-explain-to-log-plans-before-queries-are-executed-td6124646.html
makes me think (again) about your work and pg_query_state ...

Is there a chance to see you restarting working on this patch ?
Regards
PAscal



--
Sent from: https://www.postgresql-archive.org/PostgreSQL-hackers-f1928748.html

Is there a chance to see you restarting working on this patch ?

I noticed that despite interest to the matter, the discussion in this CF thread stopped quite a while ago. So I'd like to push here a patch revised according to the previous discussion. Actually the patch is being used as a part of the pg_query_state extension during several major PostgreSQL releases. So I'd like to raise the matter of reviewing this updated patch and include it into version 14 as I see much use of this change.

I welcome all your considerations,

I took a look at the patch. It looks fine and I see, that it contains fixes for the latest questions in the thread.

I think we should provide a test module for this feature, that will serve as both test and example of the use.

This is a feature for extension developers, so I don't know where we should document it. At the very least we can improve comments. For example, describe the fact that custom signals are handled after receiving SIGUSR1.

-- 
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

On Wed, Nov 25, 2020 at 06:34:48PM +0300, Anastasia Lubennikova wrote:
> I took a look at the patch. It looks fine and I see, that it contains fixes
> for the latest questions in the thread.
>
> I think we should provide a test module for this feature, that will serve as
> both test and example of the use.

Yep.  That's missing.  The trick here is usually to be able to come up
with something minimalist still useful enough for two reasons:
- This can be used as a template.
- This makes sure that the API work.
As the patch stands, nothing in this architecture is tested.

> This is a feature for extension developers, so I don't know where we should
> document it. At the very least we can improve comments. For example,
> describe the fact that custom signals are handled after receiving SIGUSR1.

As you say, this is for extension developers, so this should be
documented in the headers defining those APIs.  FWIW, I am -1 with the
patch as proposed.  First, it has zero documentation.  Second, it uses
a hardcoded custom number of signals of 64 instead of a flexible
design.  In most cases, 64 is a waste.  In some cases, 64 may not be
enough.  IMO, a design based on the registration of a custom
procsignal done at shmem init time would be much more flexible.  You
need one registration API and something to get an ID associated to the
custom entry, and that's roughly what Teodor tells upthread.

This needs more work, so I am marking it as returned with feedback.
--
Michael

On 1/12/18 20:51, 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.
> 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.
+1
> 
> 8) I'd like to see an example of usage this API somewhere in contrib in 
> exsting modules. Ideas?
I imagine, auto_explain could demonstrate usage of the API by 
implementing logging of current query state, triggered by a signal. Most 
of necessary code is already existed there.
Of course, this option will be enabled if auto_explain loads on startup.
But, maybe it breaks main concept of the auto_explain extension?

-- 
Regards
Andrey Lepikhov
Postgres Professional