Re: [PATCH v2] pager: do not unnecessarily set COLUMNS on Windows

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Junio,

On Thu, 17 Jun 2021, Junio C Hamano wrote:

> "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
> writes:
>
> > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> >
> > Since https://github.com/gwsw/less/commit/bb0ee4e76c2, `less` prefers
> > the `COLUMNS` variable over asking ncurses itself.
> >
> > This is typically not a problem because we ask `TIOCGWINSZ` in Git, to
> > determine the correct value for `COLUMNS`, and then set that environment
> > variable.
> >
> > However, on Windows it _is_ a problem. The reason is that Git for
> > Windows uses a version of `less` that relies on the MSYS2 runtime to
> > interact with the pseudo terminal (typically inside a MinTTY window,
> > which is also aware of the MSYS2 runtime). Both MinTTY and `less.exe`
> > interact with that pseudo terminal via `ioctl()` calls (which the MSYS2
> > runtime emulates even if there is no such thing on Windows).
> >
> > But `git.exe` itself is _not_ aware of the MSYS2 runtime, or for that
> > matter of that pseudo terminal, and has no way to call `ioctl()` or
> > `TIOCGWINSZ`.
> >
> > Therefore, `git.exe` will fall back to hard-coding 80 columns, no matter
> > what the actual terminal size is.
> >
> > But `less.exe` is totally able to interact with the MSYS2 runtime and
> > would not actually require Git's help (which actually makes things
> > worse here). So let's not override `COLUMNS` on Windows.
> >
> > Note: we do this _only_ on Windows, and _only_ if `TIOCGWINSZ` is not
> > defined, to reduce any potential undesired fall-out from this patch.
> >
> > This fixes https://github.com/git-for-windows/git/issues/3235
> >
> > Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> > ---
> >     pager: do not unnecessarily set COLUMNS on Windows
> >
> >     A recent upgrade of the "less" package in Git for Windows causes
> >     problems. Here is a work-around.
> >
> >     Changes since v1:
> >
> >      * The commit message was reworded to clarify the underlying issue
> >        better.
>
> Thanks for an updated log message to clarify the problem
> description.
>
> I think treating this as "less" specific band-aid is OK, but I do
> not think tying this to Windows is a good design choice.
>
> The guiding principle for this change is more like "if we do not
> know and cannot learn the true value, internally assuming 80-columns
> as a last resort fallback may be OK, but do not export it for
> consumption for other people---they cannot tell if COLUMNS=80 they
> see us export is because we actually measured the terminal width and
> know it to be 80, or we just punted and used a fallback default", I
> think, and there is nothing Windows-specific in there, no?
>
> In other words, if we use something like the attached as a "less
> specific band-aid" for now (i.e. direct replacement of your patch to
> fix the specific 'less' problem), and then later clean it up by
> actually returning -1 (or -80) from term_columns() as "we do not
> know" (or "we do not know---use the negation of this value as
> default"), we can help not just this paticular caller you touched,
> but all other callers of term_columns(), to make a more intelligent
> decision in the future if they wanted to.  The root of the issue I
> think is because term_columns() does not give callers to tell if its
> returned value is merely a guess.

That approach should also work. Do you want me to take custody of your
patch and issue a v3? If yes, I will mark you as co-author because the
patch is not really only mine any longer.

Ciao,
Dscho

>
>  pager.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git c/pager.c w/pager.c
> index 3d37dd7ada..52f27a6765 100644
> --- c/pager.c
> +++ w/pager.c
> @@ -11,6 +11,10 @@
>  static struct child_process pager_process = CHILD_PROCESS_INIT;
>  static const char *pager_program;
>
> +/* Is the value coming back from term_columns() just a guess? */
> +static int term_columns_guessed;
> +
> +
>  static void close_pager_fds(void)
>  {
>  	/* signal EOF to pager */
> @@ -114,7 +118,8 @@ void setup_pager(void)
>  	{
>  		char buf[64];
>  		xsnprintf(buf, sizeof(buf), "%d", term_columns());
> -		setenv("COLUMNS", buf, 0);
> +		if (!term_columns_guessed)
> +			setenv("COLUMNS", buf, 0);
>  	}
>
>  	setenv("GIT_PAGER_IN_USE", "true", 1);
> @@ -158,15 +163,20 @@ int term_columns(void)
>  		return term_columns_at_startup;
>
>  	term_columns_at_startup = 80;
> +	term_columns_guessed = 1;
>
>  	col_string = getenv("COLUMNS");
> -	if (col_string && (n_cols = atoi(col_string)) > 0)
> +	if (col_string && (n_cols = atoi(col_string)) > 0) {
>  		term_columns_at_startup = n_cols;
> +		term_columns_guessed = 0;
> +	}
>  #ifdef TIOCGWINSZ
>  	else {
>  		struct winsize ws;
> -		if (!ioctl(1, TIOCGWINSZ, &ws) && ws.ws_col)
> +		if (!ioctl(1, TIOCGWINSZ, &ws) && ws.ws_col) {
>  			term_columns_at_startup = ws.ws_col;
> +			term_columns_guessed = 0;
> +		}
>  	}
>  #endif
>
>