bug#33749: 26.1; input-decode-map to empty vector should preserve echo area

[Yuri Khan]

See wider context here:
http://lists.gnu.org/archive/html/emacs-devel/2018-12/msg00254.html

The Kitty terminal emulator in full keyboard mode sends sequences for
every keyboard event. This includes key release events and modifier
key press events. I would like to ignore these, and it seems to be the
most appropriate method to map them to an empty vector via
‘input-decode-map’. Currently, this works, but every time such a
sequence is received from the terminal, the echo area is cleared,
rendering invisible any prompt displayed by the previous key press.


Here’s a playground recipe with no external dependencies:

$ emacs -Q

(define-key input-decode-map (kbd "<f5> <f5>") [])

C-s <f5> <f5>


The intuitive expectation is that pressing <f5> <f5> is a no-op. The
I-search prompt should continue to be displayed, and no other action
should be taken.

The actual behavior depends on the timing:

* Pressing <f5> the first time clears the echo area immediately.

* If <f5> is pressed again within ‘echo-keystrokes’ seconds, the echo
area stays blank and the prompt invisible until the next key (other
than <f5>) is pressed.

* If ‘echo-keystrokes’ seconds elapse before the next key is pressed,
then the echo area displays the current key sequence prefix and a
dash: ‘f5-’.

* In that case, pressing <f5> again changes the echo area to ‘f5 f5-’,
as if it’s waiting for more. And it is; since <f5> <f5> is replaced by
an empty vector, it is now a prefix of every possible binding.


The call sequence that causes the echo to be cleared looks like this:

command_loop_1 (in keyboard.c)
  read_key_sequence
    read_char
      redisplay

and much of the read_char and read_key_sequence mechanics maintain
echoing of the current prefix sequence.


My suggestion is to add two changes:

* If ‘read_key_sequence’ detects that the whole key sequence has been
rewritten to an empty sequence, return -1. This is the same code as a
canceled menu selection, upon receiving which clients call
‘read_key_sequence’ again.

* In ‘command_loop_1’, save the echo area contents before calling
‘read_key_sequence’, and restore it if the return value is -1.

A proposed patch will follow.


In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-05-29 built on lcy01-amd64-029
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description:    Ubuntu 18.04.1 LTS

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