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

Joel Brobecker
2014-11-23T10:08:24Z

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Patrick,

> 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?
[...]
> 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.

Your patch did not apply to today's sources. The conflict was trivially
resolved, but it would be good if you could check the "master" branch
out, and rebuild, to make sure your fix was correctly pushed.

Attached is what I did push.

Thanks for the patch.
--
Joel

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From d64e57faa89ba4de0ebacdc30fbee5d19310950c Mon Sep 17 00:00:00 2001
From: Patrick Palka <patrick@parcs.ath.cx>
Date: Sun, 23 Nov 2014 14:03:39 +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/ChangeLog | 9 +++++++++
gdb/event-top.c | 13 ++++++++++++-
gdb/event-top.h | 1 +
gdb/tui/tui-io.c | 28 +++++++++++++++++++++++++++-
4 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index e16f28f..326a0d6 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,14 @@
2014-11-23 Patrick Palka <patrick@parcs.ath.cx>

+ * 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.
+
+2014-11-23 Patrick Palka <patrick@parcs.ath.cx>
+
Pushed by Joel Brobecker <brobecker@adacore.com>
* linux-fork.c (checkpoint_command): Print index of new
checkpoint in response message.
diff --git a/gdb/event-top.c b/gdb/event-top.c
index 282c0fe..cb438ac 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
@@ -420,7 +425,13 @@ 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 ac0d47b..919287e 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;

/* Wrappers for rl_callback_handler_remove and
rl_callback_handler_install that keep track of whether the callback
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;
}

--
1.9.1