bug#18535: tramp alters `vc-handled-backends'

[Emilio Lopes]

`tramp-sh-handle-vc-registered' can permanently alter the value of
`vc-handled-backends' causing Emacs to stop recognizing version
controlled files as such.  This happened to me a couple of times after
having pressed `C-g' while some `tramp' operation was apparently
hanging.

Here is a patch:

=== modified file 'lisp/ChangeLog'
--- lisp/ChangeLog    2014-09-16 13:36:04 +0000
+++ lisp/ChangeLog    2014-09-23 09:27:10 +0000
@@ -1,4 +1,9 @@
+2014-09-23  Emilio C. Lopes  <eclig@gmx.net>
+
+    * net/tramp-sh.el (tramp-sh-handle-vc-registered): use `unwind-protect'
+    to avoid altering `vc-handled-backends' permanently.
+
 2014-09-22  Kan-Ru Chen  <kanru@kanru.info>

    * window.el (fit-window-to-buffer): When counting buffer width,
    count the whole visible buffer.  Correctly convert the body-height
=== modified file 'lisp/net/tramp-sh.el'
--- lisp/net/tramp-sh.el    2014-09-08 15:52:04 +0000
+++ lisp/net/tramp-sh.el    2014-09-23 09:27:10 +0000
@@ -3456,30 +3456,33 @@
     ;; calls shall be answered from the file cache.  We unset
     ;; `process-file-side-effects' and `remote-file-name-inhibit-cache'
     ;; in order to keep the cache.
-    (let ((vc-handled-backends vc-handled-backends)
+    (let ((vc-handled-backends-saved vc-handled-backends)
           remote-file-name-inhibit-cache process-file-side-effects)
       ;; Reduce `vc-handled-backends' in order to minimize process calls.
-      (when (and (memq 'Bzr vc-handled-backends)
-             (boundp 'vc-bzr-program)
-             (not (with-tramp-connection-property v vc-bzr-program
-                (tramp-find-executable
-                 v vc-bzr-program (tramp-get-remote-path v)))))
-        (setq vc-handled-backends (delq 'Bzr vc-handled-backends)))
-      (when (and (memq 'Git vc-handled-backends)
-             (boundp 'vc-git-program)
-             (not (with-tramp-connection-property v vc-git-program
-                (tramp-find-executable
-                 v vc-git-program (tramp-get-remote-path v)))))
-        (setq vc-handled-backends (delq 'Git vc-handled-backends)))
-      (when (and (memq 'Hg vc-handled-backends)
-             (boundp 'vc-hg-program)
-             (not (with-tramp-connection-property v vc-hg-program
-                (tramp-find-executable
-                 v vc-hg-program (tramp-get-remote-path v)))))
-        (setq vc-handled-backends (delq 'Hg vc-handled-backends)))
-      ;; Run.
-      (ignore-errors
-        (tramp-run-real-handler 'vc-registered (list file))))))))
+          (unwind-protect
+              (progn
+                (when (and (memq 'Bzr vc-handled-backends)
+                           (boundp 'vc-bzr-program)
+                           (not (with-tramp-connection-property v
vc-bzr-program
+                                  (tramp-find-executable
+                                   v vc-bzr-program
(tramp-get-remote-path v)))))
+                  (setq vc-handled-backends (delq 'Bzr vc-handled-backends)))
+                (when (and (memq 'Git vc-handled-backends)
+                           (boundp 'vc-git-program)
+                           (not (with-tramp-connection-property v
vc-git-program
+                                  (tramp-find-executable
+                                   v vc-git-program
(tramp-get-remote-path v)))))
+                  (setq vc-handled-backends (delq 'Git vc-handled-backends)))
+                (when (and (memq 'Hg vc-handled-backends)
+                           (boundp 'vc-hg-program)
+                           (not (with-tramp-connection-property v vc-hg-program
+                                  (tramp-find-executable
+                                   v vc-hg-program
(tramp-get-remote-path v)))))
+                  (setq vc-handled-backends (delq 'Hg vc-handled-backends)))
+                ;; Run.
+                (ignore-errors
+                  (tramp-run-real-handler 'vc-registered (list file))))
+            (setq vc-handled-backends vc-handled-backends-saved)))))))

 ;;;###tramp-autoload
 (defun tramp-sh-file-name-handler (operation &rest args)