bug#29349: read_key_sequence is only partially recursive. This is a bug.

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

In branch emacs-26.

I came across this bug whilst working on bug #29272 ("C-h k C-mouse-3"
followed by menu selection asks for more keys).

In a Linux tty using the GPM mouse package, doing read_key_sequence (the
C function in keyboard.c), when a menu action is activated,
read_key_sequence calls itself recursively to handle all the mouse
manipulation.

Unfortunately, the variable raw_keybuf_count is initialised to 0 in
r_k_s.  This includes in the recursive call.  This variable indexes the
global buffer raw_keybuf, which accumulates the raw events which make up
the key sequence.

The result of this is that the events in the recursive call overwrite
the stored events of the outer r_k_s call, leaving a mess.

r_k_s is static in keyboard.c and is called from three functions:
command_loop_1, read_menu_command (the one that gives the trouble), and
read_key_sequence_vs.

So I propose as a solution that raw_keybuf_count be initialised to zero
in two of these three functions, but not in read_menu_command (and no
longer in read_key_sequence).  I've tried this, and it seems to work.
It has the disadvantage of being ugly, and it makes read_menu_command
only callable as a subfunction of r_k_s.

Has anybody any thoughts on this?

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As a second problem, is there any way, preferably at the Lisp level, to
determine that a key sequence is a menu key sequence?

-- 
Alan Mackenzie (Nuremberg, Germany).