Subject: [PATCH v3] pager: avoid setting COLUMNS when we're guessing its value

From: "Johannes Schindelin via GitGitGadget" <[email-masked]>
Date: Mon, 21 Jun 2021 16:57:58 +0000

From: Johannes Schindelin <[email-masked]>

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 <[email-masked]>
Signed-off-by: Johannes Schindelin <[email-masked]>
---
    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 v2:
    
     * We no longer avoid setting COLUMNS based on the Operating Systems and
       the presence of TIOCGWINSZ, but set a flag specifically when we
       successfully queried the terminal's dimensions, and otherwise skip
       the setenv(COLUMNS) call.
    
    Changes since v1:
    
     * The commit message was reworded to clarify the underlying issue
       better.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-982%2Fdscho%2Ffix-duplicated-lines-when-moving-in-pager-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-982/dscho/fix-duplicated-lines-when-moving-in-pager-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/982

Range-diff vs v2:

 1:  82099e53bbc9 ! 1:  fe2ee68de43e pager: do not unnecessarily set COLUMNS on Windows
     @@ Metadata
      Author: Johannes Schindelin <[email-masked]>
      
       ## Commit message ##
     -    pager: do not unnecessarily set COLUMNS on Windows
     +    pager: avoid setting COLUMNS when we're guessing its value
      
     -    Since https://github.com/gwsw/less/commit/bb0ee4e76c2, `less` prefers
     -    the `COLUMNS` variable over asking ncurses itself.
     +    We query `TIOCGWINSZ` in Git to determine the correct value for
     +    `COLUMNS`, and then set that environment variable.
      
     -    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.
     +    If `TIOCGWINSZ` is not available, we fall back to the hard-coded value
     +    80 _and still_ set the environment variable.
      
     -    However, on Windows it _is_ a problem. The reason is that Git for
     +    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
     @@ Commit message
          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.
     +    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 <[email-masked]>
          Signed-off-by: Johannes Schindelin <[email-masked]>
      
       ## pager.c ##
     +@@
     + 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 */
      @@ pager.c: void setup_pager(void)
     - 	 * to get the terminal size. Let's grab it now, and then set $COLUMNS
     - 	 * to communicate it to any sub-processes.
     - 	 */
     -+#if !defined(WIN32) || defined(TIOCGWINSZ)
       	{
       		char buf[64];
       		xsnprintf(buf, sizeof(buf), "%d", term_columns());
     - 		setenv("COLUMNS", buf, 0);
     +-		setenv("COLUMNS", buf, 0);
     ++		if (!term_columns_guessed)
     ++			setenv("COLUMNS", buf, 0);
       	}
     -+#endif
       
       	setenv("GIT_PAGER_IN_USE", "true", 1);
     +@@ pager.c: 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

 pager.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/pager.c b/pager.c
index 3d37dd7adaa2..52f27a6765c8 100644
--- 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

base-commit: ebf3c04b262aa27fbb97f8a0156c2347fecafafb
-- 
gitgitgadget