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

[Derek Upham <sand@blarg.net>]

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
sand@blarg.net