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 gwsw/less@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 #3235

Co-authored-by: Junio C Hamano <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: 2 people

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