Subject: Re: [PATCH 2/3] terminal: set VMIN and VTIME in non-canonical mode

From: Junio C Hamano <[email-masked]>
Date: Wed, 16 Feb 2022 13:40:49 -0800

"Phillip Wood via GitGitGadget" <[email-masked]> writes:

> From: Phillip Wood <[email-masked]>
>
> If VMIN and VTIME are both set to zero then the terminal performs
> non-blocking reads which means that read_key_without_echo() returns
> EOF if there is no key press pending. This results in the user being
> unable to select anything when running "git add -p".  Fix this by
> explicitly setting VMIN and VTIME when enabling non-canonical mode.

Makes sense.

>
> Signed-off-by: Phillip Wood <[email-masked]>
> ---
>  compat/terminal.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/compat/terminal.c b/compat/terminal.c
> index 20803badd03..d882dfa06e3 100644
> --- a/compat/terminal.c
> +++ b/compat/terminal.c
> @@ -57,6 +57,10 @@ static int disable_bits(tcflag_t bits)
>  	t = old_term;
>  
>  	t.c_lflag &= ~bits;
> +	if (bits & ICANON) {
> +		t.c_cc[VMIN] = 1;
> +		t.c_cc[VTIME] = 0;
> +	}

Quite sensible.  I wonder if we can simplify the "are we looking at
an ESC that is the first byte of a multi-byte control sequence?"
logic in the caller by using inter-character delay, but it is
probably better to go one step at a time like this patch does.

>  	if (!tcsetattr(term_fd, TCSAFLUSH, &t))
>  		return 0;
>  
> @@ -159,7 +163,11 @@ static int disable_bits(DWORD bits)
>  
>  		if (bits & ENABLE_LINE_INPUT) {
>  			string_list_append(&stty_restore, "icanon");
> -			strvec_push(&cp.args, "-icanon");
> +			/*
> +			 * POSIX allows VMIN and VTIME to overlap with VEOF and
> +			 * VEOL - let's hope that is not the case on windows.
> +			 */
> +			strvec_pushl(&cp.args, "-icanon", "min", "1", "time", "0", NULL);

Interesting.  So each call to read_key_without_echo() ends up being
a run_command("stty -icanon min 1 time 0") followed by a read
followed by another "stty".

At least while in -icanon mode, VEOF and VEOL do not take effect,
and the potential overlap would not matter.  It really depends on
what happens upon restore.

Do we have similar "let's hope" on the tcsetattr() side, too?

>  		}
>  
>  		if (bits & ENABLE_ECHO_INPUT) {

Thanks.