Re: Maybe the debugger should not ignore read errors

[Spencer Baugh]

Eli Zaretskii <eliz@gnu.org> writes:
>> From: Spencer Baugh <sbaugh@janestreet.com>
>> Date: Wed, 17 Jan 2024 13:51:03 -0500
>> 
>> 
>> If you read a file with a syntax error with debug-on-error=t, e.g.:
>> 
>> (let ((debug-on-error t)) (read ""))
>> 
>> Emacs will not drop to the debugger.  Instead, the error "End of file
>> during parsing" will just be printed.  This is because
>> debug-ignored-errors includes end-of-file, which is what read signals on
>> error.
>> 
>> This in turn means that --debug-init will not cause Emacs to drop to the
>> debugger if there's a read error in some file at load time, which can
>> happen in many ways.  For example, some state file such as created by
>> project.el or savehist.el might get corrupted one way or another.
>> 
>> This seems unnecessarily hostile to novice Emacs users who only know
>> that --debug-init seems to do nothing when faced with this kind of
>> error.  Certainly they have no idea that debug-ignored-errors even
>> exists.
>
> No, when Emacs is invoked with --debug-init, we reset
> debug-ignored-errors to nil while the init file is loaded.  This was
> done as part of fixing bug#64163, so if you see this now on master, it
> is some kind of regression, perhaps due to recent changes related to
> handler-bind.

Oh, that's great!  I didn't test the --debug-init part with master, so I
missed that this had changed.  That mostly resolves my concern, then.

>> Should we remove end-of-file from debug-ignored-errors?
>
> No, I don't think so.  The problem with debugging the init files is
> supposed to be solved (if that broke, we need to fix it), and other
> than that I don't see a reason to change this age-old behavior.
> Please keep in mind that end-of-file frequently happens when users
> type expressions at the M-: prompt and make syntax mistakes; we don't
> want to pop up the debugger in that case (and other similar ones).
>
>> end-of-file is not like the other errors in debug-ignored-errors,
>> which are all user-errors.
>
> Actually, end-of-file is frequently a cockpit error, see above.

That's a good point, it would be annoying to drop to the debugger in
that case.

(So then I'll just fix the related bug#68546 to improve the error
message, and I'll be happy)