Re: trapping process filter error in a thread

[Thien-Thi Nguyen <ttn@gnuvola.org>]

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() Felix Dietrich <felix.dietrich@sperrhaken.name>
() Thu, 24 Mar 2022 03:33:50 +0100

   > would like to use Emacs' threads and ‘url-http’ funcs.

   You are much braver than I ;).  I am not sure if the url
   library is robust enough for that task, but you can certainly
   try and play around with it.

'Tis a fine line between brave and foolish, i'm told.  :-D

   > (a) Can anyone else reproduce this on their Emacs?

   Yes (also Debianʼs 27.1), but it does not appear to have
   anything to do with threads: it happens just as well when you
   call ‘func’ directly.

I think this tells us that error handing wrt threads are as if
they were invoked as a (top-level) command.

   [analysis: chain of failures] I have not found a good way to
   handle these errors with the url library.

Thanks for digging into the details.  After some poking around,
i realize that the url*.el facility has quite a lot of baggage
to deal w/, as well.  It's amazing how much cruft RFCs output!

   > (c) Is there a recommended way to trap process filter
   > errors for a noninteractive (perhaps batch) session?

   Fix the process filter ;).

Haha!

   I found the variable ‘command-error-function’ in the Emacs
   Lisp manual [...].

Good eye!  I have managed to adapt your code fragment to produce
a bad.el (attached here):

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that is able to trap the error (see bad.out, here):

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-*- mode: compilation; default-directory: "/tmp/" -*-
Compilation started at Sat Apr  2 19:42:08

for url in https://www.gnu.org/ https://www.gnu.org/WRONG http://www.oecd.org/pisa/test/ ; do printf '\nurl: %s\n' $url ; emacs -batch -Q -l bad.el $url ; done

url: https://www.gnu.org/
"ok"

url: https://www.gnu.org/WRONG
"dead"

url: http://www.oecd.org/pisa/test/
var: (no-catch error (("Process www.oecd.org<1> not running") "error in process filter: "))
"error"

Compilation finished at Sat Apr  2 19:42:11

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Curiously, there is ‘no-catch’ in the CAR of ‘var’ contents, but
i was not able to find any mention of it in the Elisp reference
manual.

   > (d) Is there a more idiomatic way to work w/ threads and
   >     ‘url-http’ (or async network connections, in general)?

   [url-*.el overview, plus emacs-aio]

I hope to be able to study these Someday.

   > Thank-you for any insight into this corner of Emacs Lisp!

   [‘read-string’ in batch mode]

Thanks for the tip.  It's good to see Emacs can be useful in
many different contexts.

-- 
Thien-Thi Nguyen -----------------------------------------------
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     (`(technical ,ml) (correctp ml))
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