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Signaling errors within process sentinels only works when DEBUG-ON-ERROR
From: |
Adam Porter |
Subject: |
Signaling errors within process sentinels only works when DEBUG-ON-ERROR is non-nil |
Date: |
Mon, 8 May 2023 20:07:37 -0500 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 |
Hi,
For a while now, in my work on plz.el[0], I've been trying to understand
what causes an intermittent problem in that some HTTP requests sometimes
return nil instead of the intended value. Some of my attempts are
mentioned in plz.el#3[1], and that resulted in my filing Emacs bug#50166[2].
Finally, today I may have had a breakthough: I noticed that synchronous
requests that are expected to signal an error which is intended to be
handled by a CONDITION-CASE form do not have their error handled by the
form; instead Emacs seems to intercept the error itself, and so the
value that the CONDITION-CASE would return is not returned, with NIL
being returned instead. For example, see this code:
(condition-case err
(plz 'get "https://httpbinnnnnn.org/get/status/404")
(error 'foo))
This makes an HTTP request to a non-existent domain name, for which plz
signals an error in the process sentinel. However, when evaluating this
form (with EVAL-EXPRESSION-DEBUG-ON-ERROR set to NIL), instead of
evaluating to 'FOO, I get this in the *Messages* buffer:
error in process sentinel: plz: Curl error: "plz--sentinel: Curl
error", #s(plz-error (6 . "Couldn't resolve host. The given remote host
was not resolved.") nil nil)
nil
The CONDITION-CASE is apparently not allowed to handle the error, and
NIL is returned instead of 'FOO (after the 2-second
process-sentinel-error delay, which I also learned about recently, which
Lars recently added a variable to control).
In re-reading the Info page `(elisp)Sentinels', I saw this:
If an error happens during execution of a sentinel, it is caught
automatically, so that it doesn’t stop the execution of whatever
programs was running when the sentinel was started. However, if
‘debug-on-error’ is non-‘nil’, errors are not caught.
So I tried binding DEBUG-ON-ERROR non-nil around the call to PLZ, and
sure enough, this solved the problem:
(let ((debug-on-error t))
(condition-case err
(plz 'get "https://httpbinnnnnn.org/get/status/404")
(error 'foo)))
That evaluates to 'FOO, immediately, as expected.
So my questions:
1. Is this an acceptable way to work around this problem (of not being
"allowed" to signal errors within a process sentinel)?
2. Are there any potential problems with this workaround?
3. Is there a better way?
4. Should Emacs be patched to change this behavior? It seems strange to
me that, unless a seemingly unrelated variable is bound, errors within
sentinels can't be caught by CONDITION-CASE forms enclosing the code
that signals the error.
FWIW, I have spent many hours of apparently wasted time trying to debug
this, what has seemed to be a mysterious problem. Admittedly, this has
been documented in the manual for quite some years, and I've read that
page many times, but it wasn't until today that I was able to recognize
what was happening in the code and connect the behavior with that
paragraph in the manual. So if Emacs could be made to behave more
"naturally" in this respect, it would probably be widely useful.
As a point of comparison, request.el, the popular HTTP library for
Emacs, seems to work around this by catching errors in a function called
from within the sentinel and returning a list including the error data,
rather than allowing the signal to propagate up to application code[3].
This certainly works, but it seems unnatural; wouldn't it be preferable
to allow callers to wrap the call in their own CONDITION-CASE?
Thanks,
Adam
0: https://github.com/alphapapa/plz.el
1: https://github.com/alphapapa/plz.el/issues/3
2: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=50166
3:
https://github.com/tkf/emacs-request/blob/01e338c335c07e4407239619e57361944a82cb8a/request.el#L1146
- Signaling errors within process sentinels only works when DEBUG-ON-ERROR is non-nil,
Adam Porter <=