'C-x v v' on a diff buffer commits it as a patch (bug#52349)

* lisp/vc/diff-mode.el (diff-vc-deduce-fileset): New function.

* lisp/vc/log-edit.el (log-edit-diff-patch): New function.

* lisp/vc/vc-dispatcher.el (vc-log-edit): Set log-edit-diff-function
to log-edit-diff-patch when vc-patch-string is non-nil.
(vc-start-logentry): New optional arg 'patch-string'.
Set buffer-local 'vc-patch-string' to 'patch-string'.
(vc-dispatcher-browsing): Add (derived-mode-p 'diff-mode).

* lisp/vc/vc-git.el (vc-git-checkin-patch): New function.
(vc-git-checkin): When vc-git-patch-string is non-nil,
use `git apply --cached` to add the patch to the index,
then commit the staged changes.

* lisp/vc/vc.el: New backend function 'checkin-patch'.
(vc-deduce-fileset-1): Call diff-vc-deduce-fileset in diff-mode.
(vc-next-action): For model 'patch' call vc-checkin with the
diff buffer string.
(vc-checkin): New optional arg 'patch-string'.
Call backend function 'checkin-patch' when 'patch-string' is non-nil.
Call vc-start-logentry with 'patch-string'.
(vc-diff-patch-string): New function.

6 files changed, 91 insertions, 12 deletions

diff --git a/etc/NEWS b/etc/NEWS
index f337381dd44..b27f0760d12 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1580,6 +1580,12 @@ info node.  This command only works for the Emacs and Emacs Lisp manuals.
 This command marks files based on a regexp.  If given a prefix
 argument, unmark instead.
 
+*** 'C-x v v' on a diff buffer commits it as a patch.
+You can create a diff buffer by e.g. 'C-x v D' ('vc-root-diff'),
+then remove unnecessary hunks, and commit only part of your changes
+by typing 'C-x v v' in that diff buffer.  Currently this works only
+with Git.
+
 ---
 *** 'C-x v v' on an unregistered file will now use the most specific backend.
 Previously, if you had an SVN-covered "~/" directory, and a Git-covered
diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index 6b30de3cb37..a01943437c1 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -2928,6 +2928,15 @@ hunk text is not found in the source file."
         (forward-line 1)))
     (nreverse props)))
 
+;;;###autoload
+(defun diff-vc-deduce-fileset ()
+  (let ((backend (vc-responsible-backend default-directory))
+        files)
+    (save-excursion
+      (goto-char (point-min))
+      (while (progn (diff-file-next) (not (eobp)))
+        (push (diff-find-file-name nil t) files)))
+    (list backend (nreverse files) nil nil 'patch)))
 
 (defun diff--filter-substring (str)
   (when diff-font-lock-prettify
diff --git a/lisp/vc/log-edit.el b/lisp/vc/log-edit.el
index e958673fea8..4a94553b214 100644
--- a/lisp/vc/log-edit.el
+++ b/lisp/vc/log-edit.el
@@ -664,6 +664,12 @@ comment history, see `log-edit-comment-ring', and hides `log-edit-files-buf'."
       (indent-rigidly (point) (point-max)
 		      (- log-edit-common-indent common)))))
 
+(defvar vc-patch-string)
+
+(autoload 'vc-diff-patch-string "vc")
+(defun log-edit-diff-patch ()
+  (vc-diff-patch-string vc-patch-string))
+
 (defun log-edit-show-diff ()
   "Show the diff for the files to be committed."
   (interactive)
diff --git a/lisp/vc/vc-dispatcher.el b/lisp/vc/vc-dispatcher.el
index e2a490092b5..df5bf1cfa69 100644
--- a/lisp/vc/vc-dispatcher.el
+++ b/lisp/vc/vc-dispatcher.el
@@ -624,6 +624,8 @@ NOT-URGENT means it is ok to continue if the user says not to save."
 
 (declare-function log-edit-empty-buffer-p "log-edit" ())
 
+(defvar vc-patch-string)
+
 (defun vc-log-edit (fileset mode backend)
   "Set up `log-edit' for use on FILE."
   (setq default-directory
@@ -653,15 +655,17 @@ NOT-URGENT means it is ok to continue if the user says not to save."
                       (mapcar
                        (lambda (file) (file-relative-name file root))
                        fileset))))
-	      (log-edit-diff-function . vc-diff)
+	      (log-edit-diff-function
+               . ,(if vc-patch-string 'log-edit-diff-patch 'vc-diff))
 	      (log-edit-vc-backend . ,backend)
-	      (vc-log-fileset . ,fileset))
+	      (vc-log-fileset . ,fileset)
+	      (vc-patch-string . ,vc-patch-string))
 	    nil
 	    mode)
   (set-buffer-modified-p nil)
   (setq buffer-file-name nil))
 
-(defun vc-start-logentry (files comment initial-contents msg logbuf mode action &optional after-hook backend)
+(defun vc-start-logentry (files comment initial-contents msg logbuf mode action &optional after-hook backend patch-string)
   "Accept a comment for an operation on FILES.
 If COMMENT is nil, pop up a LOGBUF buffer, emit MSG, and set the
 action on close to ACTION.  If COMMENT is a string and
@@ -673,7 +677,8 @@ empty comment.  Remember the file's buffer in `vc-parent-buffer'
 \(current one if no file).  Puts the log-entry buffer in major mode
 MODE, defaulting to `log-edit-mode' if MODE is nil.
 AFTER-HOOK specifies the local value for `vc-log-after-operation-hook'.
-BACKEND, if non-nil, specifies a VC backend for the Log Edit buffer."
+BACKEND, if non-nil, specifies a VC backend for the Log Edit buffer.
+PATCH-STRING is a patch to check in."
   (let ((parent
          (if (vc-dispatcher-browsing)
              ;; If we are called from a directory browser, the parent buffer is
@@ -688,6 +693,8 @@ BACKEND, if non-nil, specifies a VC backend for the Log Edit buffer."
     (setq-local vc-parent-buffer parent)
     (setq-local vc-parent-buffer-name
                 (concat " from " (buffer-name vc-parent-buffer)))
+    (when patch-string
+      (setq-local vc-patch-string patch-string))
     (vc-log-edit files mode backend)
     (make-local-variable 'vc-log-after-operation-hook)
     (when after-hook
@@ -753,7 +760,8 @@ the buffer contents as a comment."
 (defun vc-dispatcher-browsing ()
   "Are we in a directory browser buffer?"
   (or (derived-mode-p 'vc-dir-mode)
-      (derived-mode-p 'dired-mode)))
+      (derived-mode-p 'dired-mode)
+      (derived-mode-p 'diff-mode)))
 
 ;; These are unused.
 ;; (defun vc-dispatcher-in-fileset-p (fileset)
diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 46a486a46c3..7395253745e 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -53,7 +53,8 @@
 ;; - responsible-p (file)                          OK
 ;; - receive-file (file rev)                       NOT NEEDED
 ;; - unregister (file)                             OK
-;; * checkin (files rev comment)                   OK
+;; * checkin (files comment rev)                   OK
+;; - checkin-patch (patch-string comment)          OK
 ;; * find-revision (file rev buffer)               OK
 ;; * checkout (file &optional rev)                 OK
 ;; * revert (file &optional contents-done)         OK
@@ -914,6 +915,12 @@ If toggling on, also insert its message into the buffer."
   "Major mode for editing Git log messages.
 It is based on `log-edit-mode', and has Git-specific extensions.")
 
+(defvar vc-git-patch-string nil)
+
+(defun vc-git-checkin-patch (patch-string comment)
+  (let ((vc-git-patch-string patch-string))
+    (vc-git-checkin nil comment)))
+
 (defun vc-git-checkin (files comment &optional _rev)
   (let* ((file1 (or (car files) default-directory))
          (root (vc-git-root file1))
@@ -936,12 +943,21 @@ It is based on `log-edit-mode', and has Git-specific extensions.")
           (if (eq system-type 'windows-nt)
               (let ((default-directory (file-name-directory file1)))
                 (make-nearby-temp-file "git-msg")))))
+    (when vc-git-patch-string
+      (unless (zerop (vc-git-command nil t nil "diff" "--cached" "--quiet"))
+        (user-error "Index not empty"))
+      (let ((patch-file (make-temp-file "git-patch")))
+        (with-temp-file patch-file
+          (insert vc-git-patch-string))
+        (unwind-protect
+            (vc-git-command nil 0 patch-file "apply" "--cached")
+          (delete-file patch-file))))
     (cl-flet ((boolean-arg-fn
                (argument)
                (lambda (value) (when (equal value "yes") (list argument)))))
       ;; When operating on the whole tree, better pass "-a" than ".", since "."
       ;; fails when we're committing a merge.
-      (apply #'vc-git-command nil 0 (if only files)
+      (apply #'vc-git-command nil 0 (if (and only (not vc-git-patch-string)) files)
              (nconc (if msg-file (list "commit" "-F"
                                        (file-local-name msg-file))
                       (list "commit" "-m"))
@@ -959,7 +975,8 @@ It is based on `log-edit-mode', and has Git-specific extensions.")
                           (write-region (car args) nil msg-file))
                         (setq args (cdr args)))
                       args)
-		    (if only (list "--only" "--") '("-a")))))
+                    (unless vc-git-patch-string
+                      (if only (list "--only" "--") '("-a"))))))
     (if (and msg-file (file-exists-p msg-file)) (delete-file msg-file))))
 
 (defun vc-git-find-revision (file rev buffer)
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 85a96a29fa3..88139fe13d8 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -247,6 +247,11 @@
 ;;   revision argument is only supported with some older VCSes, like
 ;;   RCS and CVS, and is otherwise silently ignored.
 ;;
+;; - checkin-patch (patch-string comment)
+;;
+;;   Commit a single patch PATCH-STRING to this backend, bypassing
+;;   the changes in filesets.  COMMENT is used as a check-in comment.
+;;
 ;; * find-revision (file rev buffer)
 ;;
 ;;   Fetch revision REV of file FILE and put it into BUFFER.
@@ -1102,6 +1107,8 @@ BEWARE: this function may change the current buffer."
       (vc-dir-deduce-fileset state-model-only-files))
      ((derived-mode-p 'dired-mode)
       (dired-vc-deduce-fileset state-model-only-files not-state-changing))
+     ((derived-mode-p 'diff-mode)
+      (diff-vc-deduce-fileset))
      ((setq backend (vc-backend buffer-file-name))
       (if state-model-only-files
 	(list backend (list buffer-file-name)
@@ -1114,7 +1121,8 @@ BEWARE: this function may change the current buffer."
            (or (buffer-file-name vc-parent-buffer)
 				(with-current-buffer vc-parent-buffer
 				  (or (derived-mode-p 'vc-dir-mode)
-				      (derived-mode-p 'dired-mode)))))
+				      (derived-mode-p 'dired-mode)
+				      (derived-mode-p 'diff-mode)))))
       (progn                  ;FIXME: Why not `with-current-buffer'? --Stef.
 	(set-buffer vc-parent-buffer)
 	(vc-deduce-fileset-1 not-state-changing allow-unregistered state-model-only-files)))
@@ -1230,6 +1238,8 @@ with, using the most specific one."
       (error "Fileset files are missing, so cannot be operated on"))
      ((eq state 'ignored)
       (error "Fileset files are ignored by the version-control system"))
+     ((eq model 'patch)
+      (vc-checkin files backend nil nil nil (buffer-string)))
      ((or (null state) (eq state 'unregistered))
       (cond (verbose
              (let ((backend (vc-read-backend "Backend to register to: ")))
@@ -1615,13 +1625,14 @@ Type \\[vc-next-action] to check in changes.")
      ".\n")
     (message "Please explain why you stole the lock.  Type C-c C-c when done.")))
 
-(defun vc-checkin (files backend &optional comment initial-contents rev)
+(defun vc-checkin (files backend &optional comment initial-contents rev patch-string)
   "Check in FILES. COMMENT is a comment string; if omitted, a
 buffer is popped up to accept a comment.  If INITIAL-CONTENTS is
 non-nil, then COMMENT is used as the initial contents of the log
 entry buffer.
 The optional argument REV may be a string specifying the new revision
 level (only supported for some older VCSes, like RCS and CVS).
+The optional argument PATCH-STRING is a string to check in as a patch.
 
 Runs the normal hooks `vc-before-checkin-hook' and `vc-checkin-hook'."
   (run-hooks 'vc-before-checkin-hook)
@@ -1643,7 +1654,9 @@ Runs the normal hooks `vc-before-checkin-hook' and `vc-checkin-hook'."
        ;; vc-checkin-switches, but 'the' local buffer is
        ;; not a well-defined concept for filesets.
        (progn
-         (vc-call-backend backend 'checkin files comment rev)
+         (if patch-string
+             (vc-call-backend backend 'checkin-patch patch-string comment)
+           (vc-call-backend backend 'checkin files comment rev))
          (mapc #'vc-delete-automatic-version-backups files))
        `((vc-state . up-to-date)
          (vc-checkout-time . ,(file-attribute-modification-time
@@ -1651,7 +1664,8 @@ Runs the normal hooks `vc-before-checkin-hook' and `vc-checkin-hook'."
          (vc-working-revision . nil)))
      (message "Checking in %s...done" (vc-delistify files)))
    'vc-checkin-hook
-   backend))
+   backend
+   patch-string))
 
 ;;; Additional entry points for examining version histories
 
@@ -1779,6 +1793,25 @@ objects, and finally killing buffer ORIGINAL."
 (defvar vc-diff-added-files nil
   "If non-nil, diff added files by comparing them to /dev/null.")
 
+(defvar vc-patch-string nil)
+
+(defun vc-diff-patch-string (patch-string)
+  "Report diffs to be committed from the patch.
+Like `vc-diff-internal' but uses PATCH-STRING to display
+in the output buffer."
+  (let ((buffer "*vc-diff*"))
+    (vc-setup-buffer buffer)
+    (let ((buffer-undo-list t)
+          (inhibit-read-only t))
+      (insert patch-string))
+    (setq buffer-read-only t)
+    (diff-mode)
+    (setq-local diff-vc-backend (vc-responsible-backend default-directory))
+    (setq-local revert-buffer-function (lambda (_ _) (vc-diff-patch-string)))
+    (setq-local vc-patch-string patch-string)
+    (pop-to-buffer (current-buffer))
+    (vc-run-delayed (vc-diff-finish (current-buffer) nil))))
+
 (defun vc-diff-internal (async vc-fileset rev1 rev2 &optional verbose buffer)
   "Report diffs between two revisions of a fileset.
 Output goes to the buffer BUFFER, which defaults to *vc-diff*.