config: propagate launch_editor() failure in show_editor()

show_editor() calls launch_editor() to open the user's editor on
the configuration file, but discards the return value and
unconditionally returns 0 (success). When the editor fails to
launch (e.g., $EDITOR is not found, or the editor exits with a
nonzero status), the caller receives no indication that anything
went wrong.

This affects "git config edit" and "git config --edit": the
command silently succeeds even when the editor could not be
started. In contrast, other editor-launching paths in git (such
as "git commit" and "git rebase --edit-todo") properly propagate
editor failures and exit with an error.

Check the return value and propagate the failure by returning -1.
The two callers (cmd_config_edit at line 1315 and the legacy
cmd_config at line 1478) both propagate this return to
handle_builtin, which translates negative returns into an error
exit.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/config.c

@@ -1290,7 +1290,10 @@ static int show_editor(struct config_location_options *opts)
 		else if (errno != EEXIST)
 			die_errno(_("cannot create configuration file %s"), config_file);
 	}
-	launch_editor(config_file, NULL, NULL);
+	if (launch_editor(config_file, NULL, NULL)) {
+		free(config_file);
+		return -1;
+	}
 	free(config_file);
 
 	return 0;