Ticket #2988: When an unknown key is pressed, it is interpreted as garbage.

keyboard input: simplify code, no logic changes

This change slightly simplifies and rearranges the code
in get_key_code(), reduces indentation levels there,
adds a few comments. The logic remains the same.

This is a preparatory patch for subsequent changes.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Slava Zanko <slavazanko@gmail.com>
This change prevents misinterpreting an unknown ESC sequence's
tail as a garbage input. To reproduce, run "sleep 3" and
hold down Down_Arrow key until sleep runs.
With debugging log enabled, the following can be seen:

entered get_key_code(no_delay:0)
 c=tty_lowlevel_getch()=27
 push_char(27) !0
 c=xgetch_second()=91
 push_char(91) !0
 2 c=tty_lowlevel_getch()=66
 push_char(66)
 seq_buffer[0]:27   <---- the saved Down Arrow sequence "ESC [ B"
 seq_buffer[1]:91
 seq_buffer[2]:66
 seq_buffer[3]:0
pending_keys!=NULL. m=-1
d=*pending_keys++=27
d=ALT(*pending_keys++)=ALT(91)=8283
^^^^^^^^^^^^^^^^^^^^^^^ we misinterpret "ESC [ B" as "ESC ["
return correct_key_code(8283)
entered get_key_code(no_delay:0)
pending_keys!=NULL. m=-1
d=*pending_keys++=66
^^^^^^^^^^^^ we think user pressed "B"
return correct_key_code(66)

With this patch, no bogus "input" is generated.

Longer unknown sequences need an additional fix, coming next.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Slava Zanko <slavazanko@gmail.com>
When we see an unknown sequence, it is not enough
to drop already received part - there can be more of it
coming over e.g. a serial line.

To prevent interpreting it as a random garbage,
eat and discard all chars that follow.
Small, but non-zero timeout is needed to reconnect
escape sequence split up by a serial line.

Before this change, Ctrl-Alt-Shift-Right_Arrow generates "1;8C"
bogus "input" in MC on my machine; after the change,
nothing is generated.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Slava Zanko <slavazanko@gmail.com>
Merge branch '2988_ignore_unknown_esc_sequences'

* 2988_ignore_unknown_esc_sequences:
  When we see an unknown sequence, it is not enough to drop already received part - there can be more of it coming over e.g. a serial line.
  This change prevents misinterpreting an unknown ESC sequence's tail as a garbage input. To reproduce, run "sleep 3" and hold down Down_Arrow key until sleep runs. With debugging log enabled, the following can be seen:
  Ticket #2988: When an unknown key is pressed, it is interpreted as garbage.