Provide function for asking vc about project root

3 files changed, 30 insertions, 4 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index a1da41a5695..28b9c79079e 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,5 +1,13 @@
 2014-04-20  Daniel Colascione  <dancol@dancol.org>
 
+        * vc/vc.el (vc-root-dir): New public autoloaded function for
+        generically finding the current VC root.
+        * vc/vc-hooks.el (vc-not-supported): New error.
+        (vc-call-backend): Signal `vc-not-supported' instead of generic
+        error.
+
+2014-04-20  Daniel Colascione  <dancol@dancol.org>
+
         * emacs-lisp/cl-macs.el (cl-the): Make `cl-the' assert its type
         argument.
         (cl--const-expr-val): cl--const-expr-val should macroexpand its
diff --git a/lisp/vc/vc-hooks.el b/lisp/vc/vc-hooks.el
index 1cd297ae43a..ce7f2c8f4ed 100644
--- a/lisp/vc/vc-hooks.el
+++ b/lisp/vc/vc-hooks.el
@@ -190,6 +190,11 @@ individually should stay local."
 (make-variable-buffer-local 'vc-mode)
 (put 'vc-mode 'permanent-local t)
 
+;;; We signal this error when we try to do something a VC backend
+;;; doesn't support.  Two arguments: the method that's not supported
+;;; and the backend
+(define-error 'vc-not-supported "VC method not implemented for backend")
+
 (defun vc-mode (&optional _arg)
   ;; Dummy function for C-h m
   "Version Control minor mode.
@@ -268,10 +273,10 @@ It is usually called via the `vc-call' macro."
       (setq f (vc-find-backend-function backend function-name))
       (push (cons function-name f) (get backend 'vc-functions)))
     (cond
-     ((null f)
-      (error "Sorry, %s is not implemented for %s" function-name backend))
-     ((consp f) (apply (car f) (cdr f) args))
-     (t         (apply f args)))))
+      ((null f)
+       (signal 'vc-not-supported (list function-name backend)))
+      ((consp f)        (apply (car f) (cdr f) args))
+      (t                (apply f args)))))
 
 (defmacro vc-call (fun file &rest args)
   "A convenience macro for calling VC backend functions.
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 305cc6b561e..5491d67e700 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -1879,6 +1879,19 @@ saving the buffer."
          (called-interactively-p 'interactive))))))
 
 ;;;###autoload
+(defun vc-root-dir ()
+  "Return the root directory for the current VC tree.
+Return nil if the root directory cannot be identified."
+  (let ((backend (vc-deduce-backend)))
+    (if backend
+        (condition-case err
+            (vc-call-backend backend 'root default-directory)
+          (vc-not-supported
+           (unless (eq (cadr err) 'root)
+             (signal (car err) (cdr err)))
+           nil)))))
+
+;;;###autoload
 (defun vc-revision-other-window (rev)
   "Visit revision REV of the current file in another window.
 If the current file is named `F', the revision is named `F.~REV~'.