Cache VC incoming revisions

* lisp/vc/vc-hooks.el (vc-file-setprop): Use cl-pushnew.
(vc--repo-setprop, vc--repo-getprop, vc--repo-clearprops): New
functions.
* lisp/vc/vc.el (vc--incoming-revision): Cache incoming
revisions.
* src/fns.c (Fput): State that VALUE is returned.

lisp/progmodes/project.el

@@ -576,6 +576,8 @@ project backend implementation of `project-external-roots'.")
 
 See `project-vc-extra-root-markers' for the marker value format.")
 
+;; FIXME: Should perhaps use `vc--repo-*prop' functions
+;;        (after promoting those to public).  --spwhitton
 (defun project-try-vc (dir)
   ;; FIXME: Learn to invalidate when the value changes:
   ;; `project-vc-merge-submodules' or `project-vc-extra-root-markers'.

lisp/vc/vc-hooks.el

@@ -228,24 +228,26 @@ VC commands are globally reachable under the prefix \\[vc-prefix-map]:
 (defmacro vc-error-occurred (&rest body)
   `(condition-case nil (progn ,@body nil) (error t)))
 
-;; We need a notion of per-file properties because the version
-;; control state of a file is expensive to derive --- we compute
-;; them when the file is initially found, keep them up to date
-;; during any subsequent VC operations, and forget them when
-;; the buffer is killed.
+;; We need a notion of per-file properties because the version control
+;; state of a file is expensive to derive -- we compute it when the file
+;; is initially found, keep them up to date during any subsequent VC
+;; operations, and forget them when the buffer is killed.
+;;
+;; In addition we store some whole-repository properties keyed to the
+;; repository root.  We invalidate/update these during VC operations,
+;; but there isn't a point analagous to the killing of a buffer at which
+;; we clear them all out, like there is for per-file properties.
 
 (defvar vc-file-prop-obarray (obarray-make 17)
-  "Obarray for per-file properties.")
+  "Obarray for VC per-file and per-repository properties.")
 
 (defvar vc-touched-properties nil)
 
 (defun vc-file-setprop (file property value)
   "Set per-file VC PROPERTY for FILE to VALUE."
-  (if (and vc-touched-properties
-	   (not (memq property vc-touched-properties)))
-      (setq vc-touched-properties (append (list property)
-					  vc-touched-properties)))
-  (put (intern (expand-file-name file) vc-file-prop-obarray) property value))
+  (cl-pushnew property vc-touched-properties)
+  (put (intern (expand-file-name file) vc-file-prop-obarray)
+       property value))
 
 (defun vc-file-getprop (file property)
   "Get per-file VC PROPERTY for FILE."
@@ -257,6 +259,18 @@ VC commands are globally reachable under the prefix \\[vc-prefix-map]:
       (kill-local-variable 'vc-parent-buffer))
   (setplist (intern (expand-file-name file) vc-file-prop-obarray) nil))
 
+(defun vc--repo-setprop (property value)
+  "Set per-repository VC PROPERTY to VALUE and return the value."
+  (vc-file-setprop (vc-root-dir) property value))
+
+(defun vc--repo-getprop (property)
+  "Get per-repository VC PROPERTY."
+  (vc-file-getprop (vc-root-dir) property))
+
+(defun vc--repo-clearprops ()
+  "Clear all VC whole-repository properties."
+  (vc-file-clearprops (vc-root-dir)))
+
 
 ;; We keep properties on each symbol naming a backend as follows:
 ;;  * `vc-functions': an alist mapping vc-FUNCTION to vc-BACKEND-FUNCTION.

lisp/vc/vc.el

@@ -4015,7 +4015,28 @@ The command prompts for the branch whose change log to show."
                     'vc-remote-location-history)))
 
 (defun vc--incoming-revision (backend &optional upstream-location refresh)
-  (or (vc-call-backend backend 'incoming-revision upstream-location refresh)
+  ;; Some backends don't support REFRESH and so always behave as though
+  ;; REFRESH is non-nil.  This is not just for a lack of implementation
+  ;; in Emacs; for example, Mercurial repositories don't store any
+  ;; representation of the incoming revision between running commands.
+  ;;
+  ;; Fetching the incoming revision is often slow, and in many cases the
+  ;; last known incoming revision will serve perfectly well.  For
+  ;; example, when finding revisions that are outgoing, the last known
+  ;; incoming revision is fine except for the rare case in which someone
+  ;; else cherry-picks the very same commits that you have outstanding,
+  ;; and pushes them.  Given this, we implement our own caching.
+  (or (and (not refresh)
+           (cdr (assoc upstream-location
+                       (vc--repo-getprop 'vc-incoming-revision))))
+      (let ((res (vc-call-backend backend 'incoming-revision
+                                  upstream-location refresh)))
+        (if-let* ((alist (vc--repo-getprop 'vc-incoming-revision)))
+            (setf (alist-get upstream-location alist nil nil #'equal)
+                  res)
+          (vc--repo-setprop 'vc-incoming-revision
+                            `((,upstream-location . ,res))))
+        res)
       (user-error "No incoming revision -- local-only branch?")))
 
 ;;;###autoload

src/fns.c

@@ -2702,8 +2702,8 @@ plist_put (Lisp_Object plist, Lisp_Object prop, Lisp_Object val)
 }
 
 DEFUN ("put", Fput, Sput, 3, 3, 0,
-       doc: /* Store SYMBOL's PROPNAME property with value VALUE.
-It can be retrieved with `(get SYMBOL PROPNAME)'.  */)
+       doc: /* Store SYMBOL's PROPNAME property with value VALUE and return that value.
+It can later be retrieved with `(get SYMBOL PROPNAME)'.  */)
   (Lisp_Object symbol, Lisp_Object propname, Lisp_Object value)
 {
   CHECK_SYMBOL (symbol);