Check whether diff supports --label before using it

* lisp/vc/diff.el (diff-use-labels): New variable.
(diff-no-select): Use --label rather than -L, and first
check that it is supported.

Fixes: debbugs:11067

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 979c0808b7ba36e653e0f71c339656bf59076bb2..e0cc3f3b595779466e858b26c3a735d9c8fe7c29 100644 (file)
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,5 +1,9 @@
 2013-02-11  Glenn Morris  <rgm@gnu.org>
 
+       * vc/diff.el (diff-use-labels): New variable.
+       (diff-no-select): Use --label rather than -L, and first
+       check that it is supported.  (Bug#11067)
+
        * files.el (enable-dir-local-variables): New variable.
        (hack-dir-local-variables): Respect enable-dir-local-variables.
        * tutorial.el (help-with-tutorial):

diff --git a/lisp/vc/diff.el b/lisp/vc/diff.el
index 8b4ff7929692c99c3e40b178729d3ac883dfba7e..0fc0d2e3f733ae82192fc8f11c42414202c0bbc9 100644 (file)
--- a/lisp/vc/diff.el
+++ b/lisp/vc/diff.el
@@ -114,6 +114,13 @@ specified in the variable `diff-switches' are passed to the diff command."
           tempfile))
     (file-local-copy file-or-buf)))
 
+(defvar diff-use-labels 'check
+  "Whether `diff-command' understands the \"--label\" option.
+Possible values are:
+  t     -- yes, it does
+  nil   -- no, it does not
+  check -- try to probe whether it does")
+
 (defun diff-no-select (old new &optional switches no-async buf)
   ;; Noninteractive helper for creating and reverting diff buffers
   (unless (bufferp new) (setq new (expand-file-name new)))
@@ -121,6 +128,11 @@ specified in the variable `diff-switches' are passed to the diff command."
   (or switches (setq switches diff-switches)) ; If not specified, use default.
   (unless (listp switches) (setq switches (list switches)))
   (or buf (setq buf (get-buffer-create "*Diff*")))
+  (when (eq 'check diff-use-labels)
+    (setq diff-use-labels
+         (with-temp-buffer
+           (when (ignore-errors (call-process diff-command nil t nil "--help"))
+             (if (search-backward "--label" nil t) t)))))
   (let* ((old-alt (diff-file-local-copy old))
         (new-alt (diff-file-local-copy new))
         (command
@@ -130,11 +142,14 @@ specified in the variable `diff-switches' are passed to the diff command."
                       ,@switches
                        ,@(mapcar #'shell-quote-argument
                                  (nconc
-                                  (when (or old-alt new-alt)
-                                    (list "-L" (if (stringp old)
-                                                   old (prin1-to-string old))
-                                          "-L" (if (stringp new)
-                                                   new (prin1-to-string new))))
+                                  (and (or old-alt new-alt)
+                                      (eq diff-use-labels t)
+                                      (list "--label"
+                                            (if (stringp old) old
+                                              (prin1-to-string old))
+                                            "--label"
+                                            (if (stringp new) new
+                                              (prin1-to-string new))))
                                   (list (or old-alt old)
                                         (or new-alt new)))))
                     " "))