[PATCH 4/5] keyboard input: treat only ESC<char><end> or <ESC><char><ESC> as valid

[Denys Vlasenko <vda linux googlemail 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>
---
 lib/tty/key.c |   11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/lib/tty/key.c b/lib/tty/key.c
index 77185ce..9b97aa8 100644
--- a/lib/tty/key.c
+++ b/lib/tty/key.c
@@ -1874,7 +1874,6 @@ get_key_code (int no_delay)
         {
             int d = *pending_keys++;
             keylog (" d=*pending_keys++=%d\n", d);
-          check_pend:
             if (*pending_keys == 0)
             {
                 pending_keys = NULL;
@@ -1882,9 +1881,17 @@ get_key_code (int no_delay)
             }
             else if (d == ESC_CHAR)
             {
+                int bad_seq;
                 d = ALT (*pending_keys++);
                 keylog (" d=ALT(*pending_keys++)=ALT(%d)=%d\n", pending_keys[-1], d);
-                goto check_pend;
+                bad_seq = (*pending_keys != ESC_CHAR && *pending_keys != 0);
+                if (*pending_keys == 0 || bad_seq)
+                {
+                    pending_keys = NULL;
+                    seq_append = NULL;
+                }
+                if (bad_seq)
+                    goto nodelay_try_again;
             }
             if ((d > 127 && d < 256) && use_8th_bit_as_meta)
             {
-- 
1.7.7.6