[PATCH] Fix the processing of Meta-key commands in TUI

[Patrick Palka <patrick@parcs.ath.cx>]

This patch fixes the annoying bug where key sequences such as Alt_F or
Alt_B (go forward or backwards by a word) do not behave promptly in TUI.
You have to press a third key in order for the key sequence to register.

This is mostly ncurses' fault.  Calling wgetch() normally causes ncurses
to read only a single key from stdin.  However if the key read is the
start-sequence key (^[ a.k.a. ESC) then wgetch() reads TWO keys from
stdin, storing the 2nd key into an internal FIFO buffer and returning
the start-sequence key.  The extraneous read of the 2nd key makes us
miss its corresponding stdin event, so the event loop blocks until a
third key is pressed.  This explains why such key sequences do not
behave promptly in TUI.

To fix this issue, we must somehow compensate for the missed stdin event
corresponding to the 2nd byte of a key sequence.  This patch achieves
this by hacking  up the stdin event handler to conditionally execute the
readline callback multiple multiple times in a row.  This is done via a
new global variable, call_stdin_event_handler_again_p, which is set from
tui_getc() when we receive a start-sequence key and notice extra pending
input in the ncurses buffer.

Tested on x86_64-unknown-linux-gnu.
---
 gdb/event-top.c  | 13 ++++++++++++-
 gdb/event-top.h  |  1 +
 gdb/tui/tui-io.c | 22 +++++++++++++++++++++-
 3 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/gdb/event-top.c b/gdb/event-top.c
index 833f49d..df1351d 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -120,6 +120,11 @@ int exec_done_display_p = 0;
    read commands from.  */
 int input_fd;
 
+/* Used by the stdin event handler to compensate for missed stdin events.
+   Setting this value inside an stdin callback makes the callback run
+   again.  */
+int call_stdin_event_handler_again_p;
+
 /* Signal handling variables.  */
 /* Each of these is a pointer to a function that the event loop will
    invoke if the corresponding signal has received.  The real signal
@@ -370,7 +375,13 @@ stdin_event_handler (int error, gdb_client_data client_data)
       quit_command ((char *) 0, stdin == instream);
     }
   else
-    (*call_readline) (client_data);
+    {
+      do
+	{
+	  call_stdin_event_handler_again_p = 0;
+	  (*call_readline) (client_data);
+	} while (call_stdin_event_handler_again_p != 0);
+    }
 }
 
 /* Re-enable stdin after the end of an execution command in
diff --git a/gdb/event-top.h b/gdb/event-top.h
index 2d05d45..c7b1224 100644
--- a/gdb/event-top.h
+++ b/gdb/event-top.h
@@ -61,6 +61,7 @@ extern void (*call_readline) (void *);
 extern void (*input_handler) (char *);
 extern int input_fd;
 extern void (*after_char_processing_hook) (void);
+extern int call_stdin_event_handler_again_p;
 
 extern void cli_command_loop (void *);
 
diff --git a/gdb/tui/tui-io.c b/gdb/tui/tui-io.c
index a890678..32cc96e 100644
--- a/gdb/tui/tui-io.c
+++ b/gdb/tui/tui-io.c
@@ -695,7 +695,27 @@ tui_getc (FILE *fp)
     TUI_CMD_WIN->detail.command_info.curch = 0;
   if (ch == KEY_BACKSPACE)
     return '\b';
-  
+
+  if (async_command_editing_p && key_is_start_sequence (ch))
+    {
+      int ch_pending;
+
+      nodelay (w, TRUE);
+      ch_pending = wgetch (w);
+      nodelay (w, FALSE);
+
+      /* If we have pending input following a start sequence, call the event
+	 handler again.  ncurses may have already read and stored the input into
+	 its internal buffer, meaning that we won't get an stdin event for it.  If we
+	 don't compensate for this missed stdin event, key sequences as Alt_F (^[f)
+	 will not behave promptly.  */
+      if (ch_pending != ERR)
+	{
+	  ungetch (ch_pending);
+	  call_stdin_event_handler_again_p = 1;
+	}
+    }
+
   return ch;
 }
 
-- 
2.1.0