New user option: vc-allow-async-diff

Centralize the control for whether 'vc-diff' is asynchronous
across the backends, while also allowing 'vc-BACKEND-diff' to be
called asynchronously when needed.  Previously in bug#21969.

* lisp/vc/vc.el (vc-allow-async-diff): New user option.

* lisp/vc/vc.el (vc-version-diff, vc-diff, vc-diff-mergebase)
(vc-root-diff): Obey it.

* lisp/vc/log-view.el (log-view-diff-common): Same.

* lisp/vc/vc-annotate.el
(vc-annotate-show-diff-revision-at-line-internal): Same.

* lisp/vc/vc-git.el (vc-git-diff): Follow the ASYNC argument.

* lisp/vc/vc-hg.el (vc-hg-diff): Same.

* lisp/vc/vc-rcs.el (vc-rcs-diff): Same.

lisp/vc/log-view.el

@@ -637,6 +637,7 @@ considered file(s)."
     (log-view-diff-common beg end t)))
 
 (defun log-view-diff-common (beg end &optional whole-changeset)
+  (defvar vc-allow-async-diff)
   (let* ((to (log-view-current-tag beg))
          (fr-entry (log-view-current-entry end))
          (fr (cadr fr-entry)))
@@ -650,15 +651,16 @@ considered file(s)."
                     (point))))
       (setq fr (vc-call-backend log-view-vc-backend 'previous-revision nil fr)))
     (vc-diff-internal
-     t (list log-view-vc-backend
-             ;; The value passed here should follow what
-             ;; `vc-deduce-fileset' returns.  If we want to see the
-             ;; diff for all the files in the changeset, pass NIL for
-             ;; the file list.
-             (unless whole-changeset
-               (if log-view-per-file-logs
-                   (list (log-view-current-file))
-                 log-view-vc-fileset)))
+     vc-allow-async-diff
+     (list log-view-vc-backend
+           ;; The value passed here should follow what
+           ;; `vc-deduce-fileset' returns.  If we want to see the
+           ;; diff for all the files in the changeset, pass NIL for
+           ;; the file list.
+           (unless whole-changeset
+             (if log-view-per-file-logs
+                 (list (log-view-current-file))
+               log-view-vc-fileset)))
      fr to)))
 
 (provide 'log-view)

lisp/vc/vc-annotate.el

@@ -594,6 +594,7 @@ the file in question, search for the log entry required and move point."
 	     (car rev-at-line) t 1)))))))
 
 (defun vc-annotate-show-diff-revision-at-line-internal (filediff)
+  (defvar vc-allow-async-diff)
   (if (not (derived-mode-p 'vc-annotate-mode))
       (message "Cannot be invoked outside of a vc annotate buffer")
     (let* ((rev-at-line (vc-annotate-extract-revision-at-line))
@@ -606,7 +607,7 @@ the file in question, search for the log entry required and move point."
 	      (vc-call-backend vc-annotate-backend 'previous-revision
                                (if filediff fname nil) rev))
 	(vc-diff-internal
-         t
+         vc-allow-async-diff
          ;; The value passed here should follow what
          ;; `vc-deduce-fileset' returns.
          (list vc-annotate-backend

lisp/vc/vc-git.el

@@ -1805,7 +1805,7 @@ This requires git 1.8.4 or later, for the \"-L\" option of \"git log\"."
                                 samp coding-system-for-read t)))
     (setq coding-system-for-read 'undecided)))
 
-(defun vc-git-diff (files &optional rev1 rev2 buffer _async)
+(defun vc-git-diff (files &optional rev1 rev2 buffer async)
   "Get a difference report using Git between two revisions of FILES."
   (let (process-file-side-effects
         (command "diff-tree"))
@@ -1817,7 +1817,7 @@ This requires git 1.8.4 or later, for the \"-L\" option of \"git log\"."
       (unless rev1 (setq rev1 "HEAD")))
     (if vc-git-diff-switches
         (apply #'vc-git-command (or buffer "*vc-diff*")
-	       1 ; bug#21969
+               (if async 'async 1)
                files
                command
                "--exit-code"

lisp/vc/vc-hg.el

@@ -525,7 +525,7 @@ This requires hg 4.4 or later, for the \"-L\" option of \"hg log\"."
               (cons 'vc-hg-region-history-font-lock-keywords
                     (cdr font-lock-defaults))))
 
-(defun vc-hg-diff (files &optional oldvers newvers buffer _async)
+(defun vc-hg-diff (files &optional oldvers newvers buffer async)
   "Get a difference report using hg between two revisions of FILES."
   (let* ((firstfile (car files))
          (working (and firstfile (vc-working-revision firstfile))))
@@ -535,7 +535,7 @@ This requires hg 4.4 or later, for the \"-L\" option of \"hg log\"."
       (setq oldvers working))
     (apply #'vc-hg-command
 	   (or buffer "*vc-diff*")
-           nil ; bug#21969
+           (if async 'async 1)
            files "diff"
            (append
             (vc-switches 'hg 'diff)

lisp/vc/vc-rcs.el

@@ -554,11 +554,10 @@ files beneath it."
     (vc-rcs-print-log-cleanup))
   (when limit 'limit-unsupported))
 
-(defun vc-rcs-diff (files &optional oldvers newvers buffer _async)
+(defun vc-rcs-diff (files &optional oldvers newvers buffer async)
   "Get a difference report using RCS between two sets of files."
   (apply #'vc-do-command (or buffer "*vc-diff*")
-	 ;; The repo is local, so this is fast anyway.
-	 1 ; bug#21969
+         (if async 'async 1)
 	 "rcsdiff" (vc-expand-dirs files 'RCS)
          (append (list "-q"
                        (and oldvers (concat "-r" oldvers))

lisp/vc/vc.el

@@ -876,6 +876,14 @@ if the local changes in the file have not been found and displayed yet."
                  (const :tag "Yes" t))
   :version "22.1")
 
+(defcustom vc-allow-async-diff nil
+  "Non-nil to allow asynchronous diff process.
+Enabling this means the buffer will be displayed before the diff is
+generated, and so might only say \"No changes ...\"."
+  :type '(choice (const :tag "No" nil)
+                 (const :tag "Yes" t))
+  :version "31.1")
+
 ;;;###autoload
 (defcustom vc-checkout-hook nil
   "Normal hook (list of functions) run after checking out a file.
@@ -2352,7 +2360,7 @@ state of each file in the fileset."
     (error "Not a valid revision range"))
   ;; Yes, it's painful to call (vc-deduce-fileset) again.  Alas, the
   ;; placement rules for (interactive) don't actually leave us a choice.
-  (vc-diff-internal t (vc-deduce-fileset t) rev1 rev2
+  (vc-diff-internal vc-allow-async-diff (vc-deduce-fileset t) rev1 rev2
 		    (called-interactively-p 'interactive)))
 
 ;;;###autoload
@@ -2367,7 +2375,7 @@ state of each file in the fileset."
     (error "Not a valid revision range"))
   (vc--with-backend-in-rootdir "VC root-diff"
     (let ((default-directory rootdir))
-      (vc-diff-internal t (list backend (list rootdir)) rev1 rev2
+      (vc-diff-internal vc-allow-async-diff (list backend (list rootdir)) rev1 rev2
                         (called-interactively-p 'interactive)))))
 
 ;;;###autoload
@@ -2385,7 +2393,7 @@ Optional argument FILESET, if non-nil, overrides the fileset."
       (call-interactively 'vc-version-diff)
     (let ((fileset (or fileset (vc-deduce-fileset t))))
       (vc-buffer-sync-fileset fileset not-essential)
-      (vc-diff-internal t fileset nil nil
+      (vc-diff-internal vc-allow-async-diff fileset nil nil
 			(called-interactively-p 'interactive)))))
 
 (defun vc-buffer-sync-fileset (fileset &optional not-essential missing-in-dirs)
@@ -2437,8 +2445,9 @@ The merge base is a common ancestor between REV1 and REV2 revisions."
   (vc--with-backend-in-rootdir "VC root-diff"
     (let ((default-directory rootdir)
           (rev1 (vc-call-backend backend 'mergebase rev1 rev2)))
-      (vc-diff-internal t (list backend (list rootdir)) rev1 rev2
-                        (called-interactively-p 'interactive)))))
+      (vc-diff-internal
+       vc-allow-async-diff (list backend (list rootdir)) rev1 rev2
+       (called-interactively-p 'interactive)))))
 
 (declare-function ediff-load-version-control "ediff" (&optional silent))
 (declare-function ediff-vc-internal "ediff-vers"
@@ -2514,7 +2523,7 @@ saving the buffer."
       (let ((default-directory rootdir)
             (fileset `(,backend (,rootdir))))
         (vc-buffer-sync-fileset fileset not-essential)
-        (vc-diff-internal t fileset nil nil
+        (vc-diff-internal vc-allow-async-diff fileset nil nil
                           (called-interactively-p 'interactive))))))
 
 ;;;###autoload