VC: Deprecate log-incoming and log-outgoing backend functions

* lisp/vc/vc-git.el (vc-git-incoming-revision):
Inline vc-git--fetch-incoming.
(vc-git--fetch-incoming, vc-git-log-incoming)
(vc-git-log-outgoing): Delete.
* lisp/vc/vc-hg.el (vc-hg-mergebase):
* lisp/vc/vc.el (vc-default-log-incoming)
(vc-default-log-outgoing): New functions.
* lisp/vc/vc.el:
* etc/NEWS: Document the deprecation.

etc/NEWS

@@ -1809,6 +1809,12 @@ This replaces and generalizes the old 'vc-annotate-parent-rev'.
 ---
 *** vc-dav.el is now obsolete.
 
+---
+*** The 'log-incoming' and 'log-outgoing' functions are deprecated.
+Backend authors should implement the 'incoming-revision' and 'mergebase'
+backend functions instead.  These are jointly sufficient to support the
+'C-x v I' and 'C-x v O' commands.
+
 ** Diff mode
 
 +++

lisp/vc/vc-bzr.el

@@ -816,6 +816,7 @@ If LIMIT is non-nil, show no more than this many entries."
       (indent-region (match-end 0) (point-max) 2)
       (buffer-substring (match-end 0) (point-max)))))
 
+;; FIXME: Implement `vc-bzr-mergebase' and then delete this.
 (defun vc-bzr-log-incoming (buffer remote-location)
   (apply #'vc-bzr-command "missing" buffer 'async nil
 	 (list "--theirs-only" (and (not (string-empty-p remote-location))
@@ -832,6 +833,7 @@ If LIMIT is non-nil, show no more than this many entries."
     (and (re-search-forward "^revision-id: " nil t)
          (buffer-substring (point) (pos-eol)))))
 
+;; FIXME: Implement `vc-bzr-mergebase' and then delete this.
 (defun vc-bzr-log-outgoing (buffer remote-location)
   (apply #'vc-bzr-command "missing" buffer 'async nil
 	 (list "--mine-only" (and (not (string-empty-p remote-location))

lisp/vc/vc-git.el

@@ -70,8 +70,7 @@
 ;; - get-change-comment (files rev)                OK
 ;; HISTORY FUNCTIONS
 ;; * print-log (files buffer &optional shortlog start-revision limit)   OK
-;; * log-outgoing (buffer remote-location)         OK
+;; * incoming-revision (remote-location)           OK
-;; * log-incoming (buffer remote-location)         OK
 ;; - log-search (buffer pattern)                   OK
 ;; - log-view-mode ()                              OK
 ;; - show-log-entry (revision)                     OK
@@ -1577,41 +1576,12 @@ If LIMIT is a revision string, use it as an end-revision."
                   (list "-p"))
 		'("--")))))))
 
-(defun vc-git-log-outgoing (buffer remote-location)
+(defun vc-git-incoming-revision (remote-location)
-  (vc-setup-buffer buffer)
-  (apply #'vc-git-command buffer 'async nil
-         `("log"
-           "--no-color" "--graph" "--decorate" "--date=short"
-           ,(format "--pretty=tformat:%s" (car vc-git-root-log-format))
-           "--abbrev-commit"
-           ,@(ensure-list vc-git-shortlog-switches)
-           ,(concat (if (string-empty-p remote-location)
-	                "@{upstream}"
-	              remote-location)
-	            "..HEAD"))))
-
-(defun vc-git--fetch-incoming (remote-location)
   (vc-git-command nil 0 nil "fetch"
                   (and (not (string-empty-p remote-location))
                        ;; Extract remote from "remote/branch".
                        (replace-regexp-in-string "/.*" ""
-                                                 remote-location))))
+                                                 remote-location)))
-
-(defun vc-git-log-incoming (buffer remote-location)
-  (vc-setup-buffer buffer)
-  (vc-git--fetch-incoming remote-location)
-  (apply #'vc-git-command buffer 'async nil
-         `("log"
-           "--no-color" "--graph" "--decorate" "--date=short"
-           ,(format "--pretty=tformat:%s" (car vc-git-root-log-format))
-           "--abbrev-commit"
-           ,@(ensure-list vc-git-shortlog-switches)
-           ,(concat "HEAD.." (if (string-empty-p remote-location)
-			         "@{upstream}"
-		               remote-location)))))
-
-(defun vc-git-incoming-revision (remote-location)
-  (vc-git--fetch-incoming remote-location)
   (ignore-errors              ; in order to return nil if no such branch
     (with-output-to-string
       (vc-git-command standard-output 0 nil

lisp/vc/vc-hg.el

@@ -1458,6 +1458,7 @@ This runs the command \"hg summary\"."
          (nreverse result))
        "\n"))))
 
+;; FIXME: Resolve issue with `vc-hg-mergebase' and then delete this.
 (defun vc-hg-log-incoming (buffer remote-location)
   (vc-setup-buffer buffer)
   (vc-hg-command buffer 1 nil "incoming" "-n"
@@ -1475,12 +1476,23 @@ This runs the command \"hg summary\"."
     (and (not (string-empty-p output))
          output)))
 
+;; FIXME: Resolve issue with `vc-hg-mergebase' and then delete this.
 (defun vc-hg-log-outgoing (buffer remote-location)
   (vc-setup-buffer buffer)
   (vc-hg-command buffer 1 nil "outgoing" "-n"
                  (and (not (string-empty-p remote-location))
 		      remote-location)))
 
+;; FIXME: This works only when both rev1 and rev2 have already been pulled.
+;;        That means it can't do the work
+;;        `vc-default-log-incoming' and `vc-default-log-outgoing' need it to do.
+(defun vc-hg-mergebase (rev1 &optional rev2)
+  (or (vc-hg--run-log "{node}"
+                      (format "last(ancestors(%s) and ancestors(%s))"
+                              rev1 (or rev2 "tip"))
+                      nil)
+      (error "No common ancestor for merge base")))
+
 (defvar vc-hg-error-regexp-alist
   '(("^M \\(.+\\)" 1 nil nil 0))
   "Value of `compilation-error-regexp-alist' in *vc-hg* buffers.")

lisp/vc/vc.el

@@ -347,15 +347,17 @@
 ;;   revision shown, rather than the working revision, which is normally
 ;;   the case).  Not all backends support this.
 ;;
-;; * log-outgoing (buffer remote-location)
+;; - log-outgoing (buffer remote-location) (DEPRECATED)
 ;;
 ;;   Insert in BUFFER the revision log for the changes that will be
 ;;   sent when performing a push operation to REMOTE-LOCATION.
+;;   Deprecated: implement incoming-revision and mergebase instead.
 ;;
-;; * log-incoming (buffer remote-location)
+;; - log-incoming (buffer remote-location) (DEPRECATED)
 ;;
 ;;   Insert in BUFFER the revision log for the changes that will be
 ;;   received when performing a pull operation from REMOTE-LOCATION.
+;;   Deprecated: implement incoming-revision and mergebase instead.
 ;;
 ;; * incoming-revision (remote-location)
 ;;
@@ -3252,6 +3254,16 @@ In some version control systems REMOTE-LOCATION can be a remote branch name."
     (vc-incoming-outgoing-internal backend (or remote-location "")
                                    "*vc-incoming*" 'log-incoming)))
 
+(defun vc-default-log-incoming (_backend buffer remote-location)
+  (vc--with-backend-in-rootdir ""
+    (let ((incoming (or (vc-call-backend backend
+                                         'incoming-revision
+                                         remote-location)
+                        (user-error "No incoming revision -- local-only branch?"))))
+      (vc-call-backend backend 'print-log (list rootdir) buffer t
+                       (vc-call-backend backend 'mergebase incoming)
+                       incoming))))
+
 ;;;###autoload
 (defun vc-log-outgoing (&optional remote-location)
   "Show log of changes that will be sent with a push operation to REMOTE-LOCATION.
@@ -3264,6 +3276,16 @@ In some version control systems REMOTE-LOCATION can be a remote branch name."
     (vc-incoming-outgoing-internal backend (or remote-location "")
                                    "*vc-outgoing*" 'log-outgoing)))
 
+(defun vc-default-log-outgoing (_backend buffer remote-location)
+  (vc--with-backend-in-rootdir ""
+    (let ((incoming (or (vc-call-backend backend
+                                         'incoming-revision
+                                         remote-location)
+                        (user-error "No incoming revision -- local-only branch?"))))
+      (vc-call-backend backend 'print-log (list rootdir) buffer t
+                       (vc-call-backend backend 'mergebase incoming)
+                       ""))))
+
 ;;;###autoload
 (defun vc-log-search (pattern)
   "Search the VC log of changes for PATTERN and show log of matching changes.

test/lisp/vc/vc-tests/vc-tests.el

@@ -69,8 +69,8 @@
 ;; HISTORY FUNCTIONS
 ;;
 ;; * print-log (files buffer &optional shortlog start-revision limit)
-;; * log-outgoing (backend remote-location)
+;; - log-outgoing (backend remote-location)
-;; * log-incoming (backend remote-location)
+;; - log-incoming (backend remote-location)
 ;; - log-view-mode ()
 ;; - show-log-entry (revision)
 ;; - comment-history (file)