Re: read_char() does not detect, handle special-event-map buffer changes

[Derek Upham]

Here's an even smaller repro case.  Eval the following in an Emacs
instance:

    (defun sigusr1-handler ()
      (interactive)
      (let* ((buf (get-buffer-create "*foo*")))
        (with-current-buffer buf
          (let ((inhibit-read-only t))
            (erase-buffer)
            (insert "foo foo foo\n")
            (switch-to-buffer buf)
            (goto-char (point-min)))
          (view-mode))))

    (define-key special-event-map [sigusr1] 'sigusr1-handler)

Then run

    sleep 5; kill -USR1 $EMACS_PID

in a separate terminal.  Move focus back to Emacs before the signal goes
out.  Hit `q' in the new buffer and Emacs will complain about "*foo*"
being read-only.  The second time you hit `q', Emacs will exit view mode
and bury the buffer.

Derek

Derek Upham writes:

> Here's a small repro case for the problem.  Start up two instances of
> Emacs, and load this file into both.  In the first, run
> `bad-keymap-startup-server'.  In the second, run `bad-keymap-send-signal',
> and immediately switch back to the first Emacs.
>
> The first Emacs will switch to a "*foo*" buffer with some dummy text.
> That buffer will be in view mode.  If you hit `q' after the buffer pops
> up, Emacs will complain about "*foo*" being read-only.  Hit `q' again and
> the buffer disappears.
>
> Kill the "*foo*" buffer and run the test again.  This time, instead of
> hitting `q', use `C-n' to move down a line.  After that, when you hit
> `q' you exit view mode immediately.
>
> In the original code, I had theorized that the event was going to the
> read-only temporary buffer, which had no binding for `q'.  But if that
> were the case, we would see different behavior when the "old" buffer is
> the writable "bad-keymap.el".  The "*foo*" buffer is getting the `q'
> event, but it looks like read_char() has not started using the buffer's
> new `view-mode' keymap, due to the way we are jumping to "retry:".  The
> proposed fix to exit read_char() after every special event gives the I/O
> system a chance to see the new keymap.
>
> Derek
>
> ------------------------- bad-keymap.el -------------------------
>
> (require 'dbus)
>
> (defun bad-keymap-switch-handler ()
>   (let* ((buf (get-buffer-create "*foo*")))
>     (with-current-buffer buf
>       (let ((inhibit-read-only t))
>       (erase-buffer)
>       (insert "foo foo foo\n")
>       (switch-to-buffer buf)
>       (goto-char (point-min)))
>       (view-mode))))
>
> (defun bad-keymap-startup-server ()
>   (interactive)
>   (dbus-register-signal :session "bad-keymap" "/bad/keymap" "bad.keymap"
>                         "Switch" 'bad-keymap-switch-handler))
>
> (defun bad-keymap-send-signal ()
>   (interactive)
>   (sit-for 5)
>   (dbus-send-signal :session "bad-keymap" "/bad/keymap" "bad.keymap"
>                     "Switch"))
>
> -----------------------------------------------------------------
>
>
> Derek Upham writes:
>> There is an Emacs package that uses process buffers to communicate with a
>> spawned worker process.  Due to locking on underlying files, this limits
>> me to one worker, and hence one Emacs.  I'm trying to add D-Bus support
>> to both sides, and have come across a bug in Emacs' support for special
>> events.
>>
>> The package includes a chunk of code that:
>>
>> 1. Brings up a temporary buffer in a visible window.
>> 2. Queries the worker through the process buffer channel.
>> 3. Handles the worker's response in a process filter handler, putting
>>    the text into a new buffer and replacing the temporary buffer in the
>>    same window.
>>
>> The D-Bus code attempts to do the exact same steps, and appears to
>> succeed.  But the first event after displaying the response (usually a
>> keystroke) goes to the /temporary buffer/, generating a "buffer
>> read-only" error.  After the error, the command loop resyncs the current
>> buffer to the visible buffer and later events work normally.
>>
>> I ran Emacs 24.2.2 in GDB and found that the read_char() function has
>> code that is supposed to detect this case:
>>
>>       if (current_buffer != prev_buffer)
>>         {
>>           /* The command may have changed the keymaps.  Pretend there
>>              is input in another keyboard and return.  This will
>>              recalculate keymaps.  */
>>           c = make_number (-2);
>>           goto exit;
>>         }
>>       else
>>         goto retry;
>>
>> However, `current_buffer' and `prev_buffer' are showing up as the same
>> in the debugger.  I think this is because we haven't gone through a
>> display refresh at this point in the code; the Emacs window still shows
>> the temporary buffer, for example.  This specific error case affects
>> D-Bus, but any similar activity by a special event handler should show
>> the same bug.
>>
>> I have a fix that seems to work: remove the test and assume that any
>> special event handler could have changed the keymaps.
>>
>>       /* The command may have changed the keymaps.  Pretend there
>>          is input in another keyboard and return.  This will
>>          recalculate keymaps.  */
>>       c = make_number (-2);
>>       goto exit;
>>
>> This removes a potential optimization, as the code goes up to a higher
>> level before restarting `read_char'.  But looking at the list of
>> special events (in `special-event-map'), those special events should be
>> infrequent enough that this change won't cause a performance impact.
>> Does anyone know of a reason not to make this change?
>>
>> Thanks,
>>
>> Derek


-- 
Derek Upham
address@hidden