bug#65267: 30.0.50; modifying debug-ignored-errors during startup with --debug-init is broken

[Štěpán Němec]

On Sun, 13 Aug 2023 13:12:56 +0300
Eli Zaretskii wrote:

>> From: Štěpán Němec <stepnem@smrk.net>
>> Cc: 65267@debbugs.gnu.org, monnier@iro.umontreal.ca
>> Date: Sun, 13 Aug 2023 12:03:41 +0200
>> 
>> On Sun, 13 Aug 2023 12:49:42 +0300
>> Eli Zaretskii wrote:
>> 
>> > when the user says --debug-init, they need to be shown all the errors
>> > raised during startup; hiding some of the errors was a misfeature in
>> > previous versions of Emacs, now fixed.
>> 
>> That would seem OK to me.  The problem is that, as it is now, this
>> breaks normal Emacs usage of anyone who starts with --debug-init and
>> happens to modify `debug-ignored-errors' as part of their
>> initialization.
>
> You are not supposed to suppress errors during startup when you use
> the --debug-init switch.  You are supposed to resolve the errors one
> by one, until no errors are left.  (Resolving errors could include
> catching them and ignoring them, e.g., with the likes of
> ignore-errors.)  That's what --debug-init is for, so leaving some
> errors suppressed in that case makes no sense.  Moreover, most users
> don't even know that debug-ignored-errors exists and has a non-nil
> value by default, so the effect is to hide some errors during startup
> from those users -- and that is the anti-thesis of --debug-init.

I fail to see how any of that pertains to the issue at hand.  Just to be
a bit more certain we understand each other: no errors occur during
startup; I'm not talking about any kind of debugging scenario, or
suppressing errors during startup.  The bug I describe causes a
perfectly cleanly started Emacs to end up with bogus
debug-ignored-errors, possibly (even likely) without the user
intentionally modifying any defaults, other than passing --debug-init to
Emacs and loading (as part of initialization) a package that might add
to debug-ignored-errors, both of which seem perfectly fine things to do
to me (and had been working for years until the recent changes).

>> Note that it might not even be user-intentional/explicit modification,
>> e.g., I noticed the problem because I load yasnippet from my init file,
>> and yasnippet adds a custom regexp to debug-ignored-errors; so now I end
>> up with bogus debug-ignored-errors value, which is further aggravated by
>> the fact that I normally run with debug-on-errors set, so, for example,
>> I now end up in the debugger whenever I press C-n at the end of buffer
>> (until I manually restore debug-ignored-errors to a sane value).
>
> The above works perfectly okay as long as --debug-init is not used,
> right?  (If it doesn't, then this is not a problem with a recent
> change that is supposed to affect only --debug-init.)

Yes, I stated so in my report.

-- 
Štěpán