bug#4093: Overlay keymap and timers

[Stefan Monnier]

>> > Yes, this is a really bad thing - sometimes, ie when you need to do
>> > just that.
>> The code in keyboard.c reads the set of active keymaps before reading
>> the next event.  That's most likely the explanation for this behavior.
> What you are saying does actually make sense.  If it set the keymap
> *before* serving an event, it should work fine.  But I think what
> happens is that it sets the keymap *after* executing an event...

> I looked at the code but couldn't figure out where is the keymap
> computed.  Could you point it out?

The relevant code is in .... read_key_sequence! (only those who've had
to deal with this function understand the "....").

You'll see that it first collects all the active keyamps (see where it
calls current_minor_maps), and later on calls read_char (which can do
redisplay, run timers, run process filters, etc...).

Basically the problem in the case of changing the keymap from a timer
comes down to:

  what happens if the use presses C-c, then your code runs then the user
  presses C-d:  should the C-c C-d be looked up in the original keymaps
  or in the new keymaps?

you worry about the case where the timer is run before the C-c, but from
Emacs's current point of view, it's no different whether the timer is
run after 1 key-press, or after 2 key-presses, or after 0 key-presses.

We can probably change the code to collect the list of active keymaps
later (e.g. right after the first key-press).
But maybe an alternative is to provide some way for your Elisp code to
cause a jump back to `replay_sequence' so that you can force the C-c C-d
to be interpreted in the new keymaps even if the C-c had already been
pressed when your code was run.

In any case, this function is a monster, so I'll only consider changes
to it if it makes it simpler (typically by moving code out of it into
some new function).  Maybe the idea of a new "need-to-replay-sequence"
variable could be a good way to simplify the code: we could maybe
arrange for most other "goto replay_sequence" to use this new var
(i.e. replace the current code that checks some relevant condition, by
some piece of code elsewhere (where that condition is created) which
sets the var).


        Stefan