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

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

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On Thu, Aug 28, 2014 at 12:13 PM, Pedro Alves <palves@redhat.com> wrote:
> On 08/28/2014 03:18 PM, Patrick Palka wrote:
>> On Thu, Aug 28, 2014 at 7:31 AM, Pedro Alves <palves@redhat.com> wrote:
>>> On 08/22/2014 09:44 PM, Patrick Palka wrote:
>>>> +
>>>> +  if (async_command_editing_p && key_is_start_sequence (ch))
>>>
>>> I think the key_is_start_sequence check means that we'll
>>> fail to compensate in case the sequence if longer than
>>> 2 bytes?  That is, we'll compensate for the second char,
>>> but fail to compensate for the third, because by then,
>>> ch will not be a start sequence key.
>>
>> Yes I think so.  I only took into account common 2-byte sequences such
>> as Alt_F.  I suppose that the check could be removed to account for
>> 3+-byte key sequences too, but I haven't thought out the consequences
>> of such change.  And I'm not sure that readline uses any of such
>> sequences.
>
> It's fine with me to leave this as is, if we add a comment
> mentioning this issue.
>
> Thanks,
> Pedro Alves
>

Attached is the patch that Pedro approved, augmented with a comment
describing the potential problem with 3+ byte sequences.  I wonder if
someone could commit this for me?

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From 0a88f9e0277442d586dc8dcb93164d02f922f071 Mon Sep 17 00:00:00 2001
From: Patrick Palka <patrick@parcs.ath.cx>
Date: Fri, 22 Aug 2014 16:26:51 -0400
Subject: [PATCH] Fix the processing of Meta-key commands in TUI

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 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/ChangeLog:

	* event-top.h (call_stdin_event_handler_again_p): Declare.
	* event-top.c (call_stdin_event_handler_again_p): Define.
	(stdin_event_handler): Use it.
	* tui/tui-io.c (tui_getc): Prepare to call the stdin event
	handler again if there is pending input following a
	start sequence.
---
 gdb/event-top.c  | 13 ++++++++++++-
 gdb/event-top.h  |  1 +
 gdb/tui/tui-io.c | 28 +++++++++++++++++++++++++++-
 3 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/gdb/event-top.c b/gdb/event-top.c
index 3f9deec..9654a0f 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -119,6 +119,11 @@ 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 to a non-zero 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
@@ -369,7 +374,13 @@ 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 601d278..aa1d1c7 100644
--- a/gdb/tui/tui-io.c
+++ b/gdb/tui/tui-io.c
@@ -691,7 +691,33 @@ 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 stdin
+	 event handler again because 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.
+
+	 (We only compensates for the missed 2nd byte of a key sequence because
+	 2-byte sequences are by far the most commonly used. ncurses may have
+	 buffered a larger, 3+-byte key sequence though it remains to be seen
+	 whether it is useful to compensate for all the bytes of such
+	 sequences.)  */
+      if (ch_pending != ERR)
+	{
+	  ungetch (ch_pending);
+	  call_stdin_event_handler_again_p = 1;
+	}
+    }
+
   return ch;
 }
 
-- 
2.2.0.rc1.23.gf570943