Refactor VC merging to fix a layer violation.

* vc/vc.el, vc/vc-cvs.el, vc/vc-rcs.el, vc/vc-svn.el: The 'merge'
backend method of RCS/CVS/SVN is now 'merge-file', to contrast with
'merge-branch'. Prompting for merge revisions is pushed down to the
back ends; this fixes a layering violation that caused bad behavior
with SVN.

lisp/ChangeLog

@@ -5,6 +5,12 @@
 
 2014-12-01  Eric S. Raymond  <esr@snark.thyrsus.com>
 
+	* vc/vc.el, vc/vc-cvs.el, vc/vc-rcs.el, vc/vc-svn.el: The 'merge'
+	backend method of RCS/CVS/SVN is now 'merge-file', to contrast with
+	'merge-branch'. Prompting for merge revisions is pushed down to
+	the back ends; this fixes a layering violation that caused bad
+	behavior with SVN.
+
 	* vc/vc.el, vc-hooks.el, and all backends: API simplification;
 	vc-stay-local-p and repository-hostname are no longer public
 	methods. Only the CVS and SVN backends used these, and the SVN

lisp/vc/vc-cvs.el

@@ -440,6 +440,35 @@ REV is the revision to check out."
       ;; Make the file read-only by switching off all w-bits
       (set-file-modes file (logand (file-modes file) 3950)))))
 
+(defun vc-cvs-merge-file (file)
+  "Accept a file merge request, prompting for revisions."
+  (let* ((first-revision
+        (vc-read-revision
+         (concat "Merge " file
+                 " from branch or revision "
+                 "(default news on current branch): ")
+         (list file)
+         'CVS))
+        second-revision
+        status)
+    (cond
+     ((string= first-revision "")
+      (setq status (vc-cvs-merge-news file)))
+     (t
+      (if (not (vc-branch-p first-revision))
+         (setq second-revision
+               (vc-read-revision
+                "Second revision: "
+                (list file) 'CVS nil
+                (concat (vc-branch-part first-revision) ".")))
+       ;; We want to merge an entire branch.  Set revisions
+       ;; accordingly, so that vc-cvs-merge understands us.
+       (setq second-revision first-revision)
+       ;; first-revision must be the starting point of the branch
+       (setq first-revision (vc-branch-part first-revision)))
+      (setq status (vc-cvs-merge file first-revision second-revision))))
+    status))
+
 (defun vc-cvs-merge (file first-revision &optional second-revision)
   "Merge changes into current working copy of FILE.
 The changes are between FIRST-REVISION and SECOND-REVISION."

lisp/vc/vc-rcs.el

@@ -486,6 +486,31 @@ revert all registered files beneath it."
 		   (concat (if (eq (vc-state file) 'edited) "-u" "-r")
 			   (vc-working-revision file)))))
 
+(defun vc-rcs-merge-file (file)
+  "Accept a file merge request, prompting for revisions."
+  (let* ((first-revision
+        (vc-read-revision
+         (concat "Merge " file " from branch or revision: ")
+         (list file)
+         'RCS))
+        second-revision)
+    (cond
+     ((string= first-revision "")
+      (error "A starting RCS revision is required"))
+     (t
+      (if (not (vc-branch-p first-revision))
+         (setq second-revision
+               (vc-read-revision
+                "Second RCS revision: "
+                (list file) 'RCS nil
+                (concat (vc-branch-part first-revision) ".")))
+       ;; We want to merge an entire branch.  Set revisions
+       ;; accordingly, so that vc-rcs-merge understands us.
+       (setq second-revision first-revision)
+       ;; first-revision must be the starting point of the branch
+       (setq first-revision (vc-branch-part first-revision)))))
+    (vc-rcs-merge file first-revision second-revision)))
+
 (defun vc-rcs-merge (file first-version &optional second-version)
   "Merge changes into current working copy of FILE.
 The changes are between FIRST-VERSION and SECOND-VERSION."

lisp/vc/vc-svn.el

@@ -379,6 +379,29 @@ FILE is a file wildcard, relative to the root directory of DIRECTORY."
   (unless contents-done
     (vc-svn-command nil 0 file "revert")))
 
+(defun vc-svn-merge-file (file)
+  "Accept a file merge request, prompting for revisions."
+  (let* ((first-revision
+        (vc-read-revision
+         (concat "Merge " file
+                 " from SVN revision "
+                 "(default news on current branch): ")
+         (list file)
+         'SVN))
+        second-revision
+        status)
+    (cond
+     ((string= first-revision "")
+      (setq status (vc-svn-merge-news file)))
+     (t
+      (setq second-revision
+           (vc-read-revision
+            "Second SVN revision: "
+            (list file) 'SVN nil
+            first-revision))
+      (setq status (vc-svn-merge file first-revision second-revision))))
+    status))
+
 (defun vc-svn-merge (file first-version &optional second-version)
   "Merge changes into current working copy of FILE.
 The changes are between FIRST-VERSION and SECOND-VERSION."

lisp/vc/vc.el

@@ -289,10 +289,11 @@
 ;;   'cancel-version' and took a single file arg, not a list of
 ;;   files.)
 ;;
-;; - merge (file rev1 rev2)
+;; - merge-file (file rev1 rev2)
 ;;
-;;   Merge the changes between REV1 and REV2 into the current working file
-;;   (for non-distributed VCS).
+;;   Merge the changes between REV1 and REV2 into the current working
+;;   file (for non-distributed VCS).  It is expected that with an
+;;   empty first revision this will behave like the merge-news method.
 ;;
 ;; - merge-branch ()
 ;;
@@ -594,6 +595,11 @@
 ;;   RCS has setting the initial revision been even possible, let alone
 ;;   sane.
 ;;
+;; - The backend operation for non-distributed VCSes formerly called
+;;   "merge" is now "merge-file" (to contrast with merge-branch), and
+;;   does its own prompting for revisions.  (This fixes a layer violation
+;;   that produced bad behavior under SVN.)
+;;
 ;;   workfile-unchanged-p is no longer a public back-end method.  It
 ;;   was redundant with vc-state and usually implemented with a trivial
 ;;   call to it.  A few older back ends retain versions for internal use in
@@ -2060,42 +2066,17 @@ changes from the current branch."
       (vc-buffer-sync)
       (dolist (file files)
 	(let* ((state (vc-state file))
-	       first-revision second-revision status)
+	       status)
 	  (cond
 	   ((stringp state)	;; Locking VCses only
 	    (error "File %s is locked by %s" file state))
 	   ((not (vc-editable-p file))
 	    (vc-checkout file t)))
-	  (setq first-revision
-		(vc-read-revision
-		 (concat "Merge " file
-			 " from branch or revision "
-			 "(default news on current branch): ")
-		 (list file)
-		 backend))
-	  (cond
-	   ((string= first-revision "")
-	    (setq status (vc-call-backend backend 'merge-news file)))
-	   (t
-	    (if (not (vc-branch-p first-revision))
-		(setq second-revision
-		      (vc-read-revision
-		       "Second revision: "
-		       (list file) backend nil
-		       ;; FIXME: This is CVS/RCS/SCCS specific.
-		       (concat (vc-branch-part first-revision) ".")))
-	      ;; We want to merge an entire branch.  Set revisions
-	      ;; accordingly, so that vc-BACKEND-merge understands us.
-	      (setq second-revision first-revision)
-	      ;; first-revision must be the starting point of the branch
-	      (setq first-revision (vc-branch-part first-revision)))
-	    (setq status (vc-call-backend backend 'merge file
-					  first-revision second-revision))))
+	  (setq status (vc-call-backend backend 'merge-file file))
 	  (vc-maybe-resolve-conflicts file status "WORKFILE" "MERGE SOURCE"))))
      (t
       (error "Sorry, merging is not implemented for %s" backend)))))
 
-
 (defun vc-maybe-resolve-conflicts (file status &optional _name-A _name-B)
   (vc-resynch-buffer file t (not (buffer-modified-p)))
   (if (zerop status) (message "Merge successful")