bug#66912: With `require', the byte compiler reports the wrong file for errors.

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> I can definitely live with this syntax, but maybe we should use
>> something more like what GCC uses (e.g. for errors in #included files)
>> which puts the "While loading" info on separate lines.
> I thought about that, but seeing as how only one message at a time is
> visible in the message area, we'd probably want to output one message
> with embedded LFs, rather than several consecutive "While loading ..."s.

I don't have an opinion on that.  I only care about the case where that
info ends up in a file or buffer, along with other warnings and errors,
such as when I do `make` or `byte-compile-file`.  Ideally I'd like to be
able to click on each "While loading" to be brought to the place of the
corresponding `require`.  And ideally this would work with the existing
entries of `compilation-error-regexp-alist`.

>> `combine-error-info` is a bit problematic because we don't have clear
>> rules about the content of (cdr err), other than the fact that it should
>> be a list (tho we don't even enforce that very much).
>> Most likely we could append elements to that list, but we'd have to
>> worry about interactions with other libraries wanting to do similar
>> things.
>
> Do other libraries actually do such things?

Currently, this would be the first, but since I added `handler-bind`
I've already felt like doing such things on a few occasions, so it's
only a question of time.

>> So I was thinking that we should go instead with:
>
>>    (handler-bind ((error (lambda (err)
>>                            (push file (gethash err our-table-of-error-source)))))
>>      	  readevalloop (Qget_file_char, &input, hist_file_name,
>>      	      0, Qnil, Qnil, Qnil, Qnil);)
>
>> Where `our-table-of-error-source` would be a weak eq-hashtable.
>
> Do we need a hash table when it's only going to have a few elements at
> any time?  `require's rarely go more than 5 or 6 deep.  Why not just have
> a simple dynamically bound list?  Or have I misunderstood what you mean?

A hashtable is not the only solution, indeed.  But a weak hashtable
makes it possible to skip the need to use something that's "dynamically
bound", and hence to have to think about where we do the dynamic binding
and what to do if it's nested etc...
IOW, it seems simpler to me.

>> Emacs Lisp guarantees that the `err` we get here will be the exact same
>> object that any subsequent `condition-case` will get when it finally
>> handles the error so that it can use `gethash` to fetch our
>> side information.
>
>> Note that we don't `signal` the error again, instead we let the error
>> handling code propagate it further, which is what `handler-bind` does
>> when the handler returns normally (which should also eliminate the
>> possible problems of interaction with `debug-on-error`).
>
> The reason I suggested a signal call was so that the error information in
> the successive ERRs would accumulate, rather than just being the fixed
> ERR from the initial error.

In my suggestion I also accumulate them, but I put them in the side-info
hashtable instead of inside the error object.  I think it is important to
preserve the `eq`ness of the error object (since it embodies the fact
that we're still handling the same error), so if we don't use a side
table we would probably want to "combine" by mutating the error object.

> And I think any call to the debugger on account of debug-on-error should
> be in the innermost recursive `require', where the error is actually
> signalled, so as to be of maximum use to the person debugging it.

I think I agree tho "be in the innermost recursive `require'" seems
quite vague.  But in any case the handlers of `handler-bind` are run
before we unwind the stack (e.g. if your nesting looks like "require =>
require => error" the two handlers of your two `require`s will be run
before we get to the debugger but the debugger will still show the full
stack.  Tho with your use of "resignaling" within the handlers, the
stack will tend to be even "more full", with the two handlers nested
and still active), so no matter how we do it, I think we will indeed get
the behavior that I believe you describe.

More concretely, with your code, I think the debugger will be called
with a stack that looks like

     <calling the debugger>
     signal(...)
     ...
     <calling the handler of the outer handler-bind>
     signal(...)
     ...
     <calling the handler of the inner handler-bind>
     error("the actual error")
     ...
     handler-bind(...)    ; the inner one
     require(...)
     ...
     handler-bind(...)    ; the outer one
     require(...)

whereas with the code I suggest the stack should look like

     <calling the debugger>
     error("the actual error")
     ...
     handler-bind(...)    ; the inner one
     require(...)
     ...
     handler-bind(...)    ; the outer one
     require(...)

In any case, it should be easy to try out and change from one to the
other with very local changes (I'd expect that the code of the handlers
will be written in ELisp rather than C, right?).  So either way is fine.


        Stefan