Calling internal-default-process-sentinel from another sentinel?

Calling internal-default-process-sentinel from another sentinel?

[Štěpán Němec]

For an asynchronous process I'd like to preserve the default sentinel
behaviour (insert a status message into the process's buffer, if there
is one), and only in one particular case (successful completion) add
some other functionality (follow up with a custom action).

Such a use case doesn't seem uncommon and simply calling
'internal-default-process-sentinel' from a custom sentinel would seem a
good solution, but the "internal-" prefix or the fact that such usage is
completely absent from Emacs core and very rare even in 3rd party
code[1] don't inspire confidence.

I'd appreciate advice or opinions on whether reusing
internal-default-process-sentinel is acceptable or if there are good
alternatives other than emulating its behaviour manually in the custom
sentinel.

Thank you,

  Štěpán

[1] As indicated e.g. by https://github.com/search?q=internal-default-process-sentinel+language%3A%22Emacs+Lisp%22&type=Code

Re: Calling internal-default-process-sentinel from another sentinel?

[Stefan Monnier]

> Such a use case doesn't seem uncommon and simply calling
> 'internal-default-process-sentinel' from a custom sentinel would seem a
> good solution, but the "internal-" prefix or the fact that such usage is
> completely absent from Emacs core and very rare even in 3rd party
> code[1] don't inspire confidence.

`add-function` is your friend.

    (add-function :around (process-sentinel proc)
                  (lambda (orig-fun proc state)
                    (if (one particular case)
                        (do the thing)
                      (funcall orig-fun proc state))))


-- Stefan

Re: Calling internal-default-process-sentinel from another sentinel?

[Štěpán Němec]

On Wed, 18 Nov 2020 09:19:36 -0500
Stefan Monnier wrote:

>> Such a use case doesn't seem uncommon and simply calling
>> 'internal-default-process-sentinel' from a custom sentinel would seem a
>> good solution, but the "internal-" prefix or the fact that such usage is
>> completely absent from Emacs core and very rare even in 3rd party
>> code[1] don't inspire confidence.
>
> `add-function` is your friend.
>
>     (add-function :around (process-sentinel proc)
>                   (lambda (orig-fun proc state)
>                     (if (one particular case)
>                         (do the thing)
>                       (funcall orig-fun proc state))))

Thanks, I've seen similar examples in the code base, but always advising
an Elisp sentinel, not 'internal-default-process-sentinel', which is a C
function, and IIUC also would/might be called from C (e.g. from
status_notify ?). Or is the actual default sentinel some kind of a
wrapper?

And even for Elisp sentinels, I figured I'd rather avoid advice, as even
when using a self-removing piece of advice, it still applies to all
(even unrelated) calls of the same sentinel function occurring until the
removal. Or am I missing something?

-- 
Štěpán

Re: Calling internal-default-process-sentinel from another sentinel?

[Stefan Monnier]

>>> Such a use case doesn't seem uncommon and simply calling
>>> 'internal-default-process-sentinel' from a custom sentinel would seem a
>>> good solution, but the "internal-" prefix or the fact that such usage is
>>> completely absent from Emacs core and very rare even in 3rd party
>>> code[1] don't inspire confidence.
>>
>> `add-function` is your friend.
>>
>>     (add-function :around (process-sentinel proc)
>>                   (lambda (orig-fun proc state)
>>                     (if (one particular case)
>>                         (do the thing)
>>                       (funcall orig-fun proc state))))
>
> Thanks, I've seen similar examples in the code base, but always advising
> an Elisp sentinel, not 'internal-default-process-sentinel', which is a C
> function, and IIUC also would/might be called from C (e.g. from
> status_notify ?). Or is the actual default sentinel some kind of a
> wrapper?

This is not advising 'internal-default-process-sentinel'.  It just
replaces the sentinel by a new function which often delegates to the
previous function, whichever that previous function was and without
affecting that previous function's definition.

> And even for Elisp sentinels, I figured I'd rather avoid advice, as even
> when using a self-removing piece of advice, it still applies to all
> (even unrelated) calls of the same sentinel function occurring until the
> removal. Or am I missing something?

You're confused: this is not an "advice".


        Stefan

Re: Calling internal-default-process-sentinel from another sentinel?

[Štěpán Němec]

On Wed, 18 Nov 2020 09:44:18 -0500
Stefan Monnier wrote:

>>> `add-function` is your friend.
>>>
>>>     (add-function :around (process-sentinel proc)
>>>                   (lambda (orig-fun proc state)
>>>                     (if (one particular case)
>>>                         (do the thing)
>>>                       (funcall orig-fun proc state))))
>>
>> Thanks, I've seen similar examples in the code base, but always advising
>> an Elisp sentinel, not 'internal-default-process-sentinel', which is a C
>> function, and IIUC also would/might be called from C (e.g. from
>> status_notify ?). Or is the actual default sentinel some kind of a
>> wrapper?
>
> This is not advising 'internal-default-process-sentinel'.  It just
> replaces the sentinel by a new function which often delegates to the
> previous function, whichever that previous function was and without
> affecting that previous function's definition.
>
>> And even for Elisp sentinels, I figured I'd rather avoid advice, as even
>> when using a self-removing piece of advice, it still applies to all
>> (even unrelated) calls of the same sentinel function occurring until the
>> removal. Or am I missing something?
>
> You're confused: this is not an "advice".

Indeed, I hadn't realized that, even though "(process-sentinel proc)"
above would (before the add-function modification) evaluate to
'internal-default-process-sentinel', using `add-function' on that form
really works through `set-process-sentinel' (via a gv place reference)
and is thus different from advising 'internal-default-process-sentinel'
directly.

Thank you!

-- 
Štěpán