pager: avoid setting COLUMNS when we're guessing its value

We query `TIOCGWINSZ` in Git to determine the correct value for
`COLUMNS`, and then set that environment variable.

If `TIOCGWINSZ` is not available, we fall back to the hard-coded value
80 _and still_ set the environment variable.

On Windows this 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).
Since https://github.com/gwsw/less/commit/bb0ee4e76c2, `less` prefers
the `COLUMNS` variable over asking ncurses itself.

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.

Let's just not set `COLUMNS` unless we managed to query the actual value
from the terminal.

This fixes https://github.com/git-for-windows/git/issues/3235

Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/pager.c b/pager.c
index 3d37dd7adaa27ed1b6bc3952711b11ff6dfc0f56..52f27a6765c8def12d66066d5ef409bd7b1f9de5 100644 (file)
--- a/pager.c
+++ b/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