Re: vc-modify-change-comment for "modern" backend fix

["Alfred M. Szmidt" <ams@gnu.org>]

What about something like this, I've tested it briefly with a
vc-fossil-get-log-entry, and with RCS and it seems to behave as it
does previously -- though slightly confused by what it does for RCS
(it inserts the header, but not the commit?).

diff --git a/lisp/vc/log-view.el b/lisp/vc/log-view.el
index 415b1564ed..93e38bf3d2 100644
--- a/lisp/vc/log-view.el
+++ b/lisp/vc/log-view.el
@@ -521,17 +520,6 @@
 (defun log-view-extract-comment ()
-  "Parse comment from around the current point in the log."
-  (save-excursion
-    (let (st en (backend (vc-backend (log-view-current-file))))
-      (log-view-end-of-defun)
-      (cond ((eq backend 'SVN)
-	     (forward-line -1)))
-      (setq en (point))
-      (or (log-view-current-entry nil t)
-          (throw 'beginning-of-buffer nil))
-      (cond ((memq backend '(SCCS RCS CVS SVN))
-	     (forward-line 2))
-	    ((eq backend 'Hg)
-	     (forward-line 4)
-	     (re-search-forward "summary: *" nil t)))
-      (setq st (point))
-      (buffer-substring st en))))
+  "Extract the commit message for the current file."
+  (vc-call-backend log-view-vc-backend
+                   'get-log-entry
+                   (log-view-current-file)
+                   (log-view-current-tag)))
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 3508f684c4..01ff7db356 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -359,6 +359,11 @@
 ;;   and make sure it is displayed in the buffer's window.  The default
 ;;   implementation of this function works for RCS-style logs.
 ;;
+;; - get-log-entry (file revision)
+;;
+;;   Return a string containing the log entry for FILE at revision
+;;   REVISION.
+
 ;; - comment-history (file)
 ;;
 ;;   Return a string containing all log entries that were made for FILE.
@@ -671,12 +676,7 @@
 ;; - The git backend supports amending, but in a different
 ;;   way (press `C-c C-e' in log-edit buffer, when making a new commit).
 ;;
-;; - Second, `log-view-modify-change-comment' doesn't seem to support
-;;   modern backends at all because `log-view-extract-comment'
-;;   unconditionally calls `log-view-current-file'.  This should be easy to
-;;   fix.
-;;
-;; - Third, doing message editing in log-view might be a natural way to go
+;; - Second, doing message editing in log-view might be a natural way to go
 ;;   about it, but editing any but the last commit (and even it, if it's
 ;;   been pushed) is a dangerous operation in Git, which we shouldn't make
 ;;   too easy for users to perform.
@@ -3158,6 +3158,23 @@ vc-default-show-log-entry
   (with-no-warnings
    (log-view-goto-rev rev)))
 
+(defun vc-default-get-log-entry (_backend _file _revision)
+  (save-excursion
+    (let (st en (backend (vc-backend (log-view-current-file))))
+      (log-view-end-of-defun)
+      (cond ((eq backend 'SVN)
+             (forward-line -1)))
+      (setq en (point))
+      (or (log-view-current-entry nil t)
+          (throw 'beginning-of-buffer nil))
+      (cond ((memq backend '(SCCS RCS CVS SVN))
+             (forward-line 2))
+            ((eq backend 'Hg)
+             (forward-line 4)
+             (re-search-forward "summary: *" nil t)))
+      (setq st (point))
+      (buffer-substring st en))))
+
 (defun vc-default-comment-history (backend file)
   "Return a string with all log entries stored in BACKEND for FILE."
   (when (vc-find-backend-function backend 'print-log)