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

Patrick Palka
2014-08-27T18:25:29Z

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On Wed, 27 Aug 2014, Pedro Alves wrote:

> Hey there,
>
> Thanks for looking at this!
>
> On 08/22/2014 09:44 PM, Patrick Palka wrote:
>> 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.
>
> Hmm, I've been staring at this for a while trying to make sense of the
> whole thing. I think there's a larger issue here.
>
> This happens because we enable the "keypad" behavior. From the ncurses manual:
>
> The keypad option enables the keypad of the user's terminal. If enabled (bf is TRUE), the user can press a function key (such as an arrow key) and wgetch reâ€
> turns a single value representing the function key, as in KEY_LEFT. If disabled (bf is FALSE), curses does not treat function keys specially and the program
> has to interpret the escape sequences itself. If the keypad in the terminal can be turned on (made to transmit) and off (made to work locally), turning on this
> option causes the terminal keypad to be turned on when wgetch is called. The default value for keypad is false.
>
> readline must already be processing escape sequences itself, so
> then why we do enable this in the first place? The answer is that
> we want to intercept things like up/down -- when the TUI is active,
> those should scroll the source window instead of navigating readline's
> history.
>
> This is done in tui_getc:
>
> if (key_is_command_char (ch))
> { /* Handle prev/next/up/down here. */
> ch = tui_dispatch_ctrl_char (ch);
> }
>
> Hacking away keypad, like:
>
> gdb/tui/tui.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/gdb/tui/tui.c b/gdb/tui/tui.c
> index c30b76c..6174c0f 100644
> --- a/gdb/tui/tui.c
> +++ b/gdb/tui/tui.c
> @@ -398,7 +398,7 @@ tui_enable (void)
> tui_show_frame_info (0);
> tui_set_layout (SRC_COMMAND, TUI_UNDEFINED_REGS);
> tui_set_win_focus_to (TUI_SRC_WIN);
> - keypad (TUI_CMD_WIN->generic.handle, TRUE);
> + // keypad (TUI_CMD_WIN->generic.handle, TRUE);
> wrefresh (TUI_CMD_WIN->generic.handle);
> tui_finish_init = 0;
> }
>
> indeed makes Alt_F, etc. work.
>
> The main reason I think there's a larger problem here, is that
> if curses is reading more than one char from stdin, then that means
> that is must be blocking for a bit waiting for the next character,
> which is a no-no in an async world, where we want to be processing
> target events at the same time. The man page says:
>
> While interpreting an input escape sequence, wgetch sets a timer while waiting for the next character. If notimeout(win, TRUE) is called, then wgetch does not
> set a timer. The purpose of the timeout is to differentiate between sequences received from a function key and those typed by a user.
>
> Looks like there's a default timeout of 1 second. Indeed if I set a
> breakpoint in wgetch and another right after wgetch is called, and
> then I press escape, I see that gdb is stuck inside wgetch for around
> one second. During that time, gdb's own event loop isn't being processed.
>
> Not sure exactly how this is usually handled. Seems like there
> are several knobs that might be able to turn this delay off.
> Sounds like we should enable that (whatever the option is),
> and handle the timeout ourselves?

I don't think the timeout is the issue here. Even if the timeout is
disabled via notimeout(), wgetch() will still attempt to interpret keypad
sequences by reading multiple characters from stdin -- except that the
read will now be a non-blocking one instead of a blocking one.

One way or another, someone must read multiple keys from stdin in order
to semantically distinguish between keypad keys and regular key
sequences. And when it turns out that the input is not or cannot be a
keypad key then that someone must place the extraneous keys into a
buffer and notify GDB's event handler that we missed their stdin events.

If we handle the timeout ourselves (for instance by disabling keypad()
and enabling notimeout()) then we'll be responsible for doing the
lookahead, interpreting the sequences and buffering the keypresses. I
say let ncurses continue to handle the timeout so that we'll only be
responsible for notifying the event handler.

Though I may just be misunderstanding your proposal.

Patrick

>
> Thanks,
> Pedro Alves
>