bug#45474: Icomplete exhibiting in recursive minibuffer when it shouldn’t

[Stefan Monnier <monnier@iro.umontreal.ca>]

> Well, the fact that there were a few other pieces of code which do that was
> (at least as I understood it) part of the initial problem.  And the purpose
> of the discussion (at least as I understood it) was to solve the current
> problem without breaking these few other pieces of code.

Indeed, and I think my patch should by and large leave them unaffected
(it neither magically fixes them nor breaks them).

>>> If not, it means that your patch will fix the problem for
>>> completing-read-default, but not for other completion backends, who will
>>> have to resort on a similar trick to get the same effect.
>>
>> I think they'd need to make similar changes to fix the problem under
>> discussion in this longish thread, but they can keep using their old way
>> of working and the consequence will just be that they will keep suffering
>> from the old problem.
> With my patch all they have to do is to add (minibuffer-local-completion t)
> before calling read-from-minibuffer.

I think whichever approach we choose, the fix is pretty simple.
IMO the only real gain we can try and aim for is to minimize the need for
change at all.

>>> Not with the patch I'm proposing.  What it does is the following (in
>>> abbreviated form):
>>>
>>> (let ((minibuffer-local-* minibuffer-completion-*))
>>>    (let ((minibuffer-completion-* nil))
>>>       (internal-read-from-minibuffer ...)))
>>
>> Not quite.  The actual code is more like:
>>
>>    (let ((minibuffer-local-* minibuffer-completion-*))
>>       [SOMETHING1]
>>       (let ((minibuffer-completion-* nil))
>>          (internal-read-from-minibuffer ...))
>>       [SOMETHING2])
>>
>> and those two [SOMETHINGn] still leak.
>
> I admit that you lost me here.  What are these [SOMETHINGn]'s, and why are
> they happening?

Because inevitably code can run in there, e.g. because the debugger gets
triggered in there or because the caller of `read-from-minibuffer` was not
careful to place the let-bindings of `minibuffer-completion-*` as close
as possible to `read-from-minibuffer`.

[ Side note: you can't define `read-from-minibuffer` as a macro because
  it is not compatible with the byte-code generated from code which
  called `read-from-minibuffer` as a function.  So your patch would
  need to be adjusted to keep `read-from-minibuffer` as a function.
  That opens up yet more opportuninities for those [SOMETHINGn], such
  as advice placed on `read-from-minibuffer`, ...  ]

BTW, these corner case problems are the same kind of corner case
problems which plague `minibuffer-with-setup-hook` as well, of course.

>> You share the main downside of my proposal: `minibuffer-local-*` are now
>> only non-nil buffer-locally in the minibuffers.
>>
>> I mean it's good because it's what we want in the long term, but it's bad
>> because it's not backward compatible and there's probably some code out
>> there which will need to be adjusted to work with this.
>
> I have to admit again that you lost me here.

No wonder: I wrote `minibuffer-local-*` when I meant `minibuffer-completion-*`.
Sorry 'bout that.


        Stefan