>From 0e0c14b0d8ff8582301eab65c91120a52aad5597 Mon Sep 17 00:00:00 2001
From: Davis Herring <address@hidden>
Date: Sun, 12 Jun 2016 05:37:49 -0600
Subject: [PATCH 4/6] Snap `vc-parent-buffer' chains

This avoids having to search the tree and check for loops.
* lisp/vc/vc-dispatcher.el (vc-get-parent-buffer, vc-set-parent-buffer):
New functions.
(vc-setup-buffer, vc-start-logentry): Use `vc-set-parent-buffer'.
* lisp/vc/vc.el (vc-ensure-vc-buffer, vc-find-revision): Use both.
---
 lisp/vc/vc-dispatcher.el |   22 ++++++++++++++++------
 lisp/vc/vc.el            |    9 ++-------
 2 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/lisp/vc/vc-dispatcher.el b/lisp/vc/vc-dispatcher.el
index a551542..f39d517 100644
--- a/lisp/vc/vc-dispatcher.el
+++ b/lisp/vc/vc-dispatcher.el
@@ -166,6 +166,20 @@ Another is that undo information is not kept."
             (insert s)
             (set-marker (process-mark p) (point))))))))
 
+(defun vc-get-parent-buffer ()
+  "Return the current buffer or its parent if it has one."
+  (if (buffer-live-p vc-parent-buffer) vc-parent-buffer (current-buffer)))
+(defun vc-set-parent-buffer (buf &optional silent)
+  "Set current buffer's parent to BUF (or its parent if it has one).
+Do nothing if that is the current buffer.
+Set the mode line name as well unless SILENT."
+  (setq buf (with-current-buffer buf (vc-get-parent-buffer)))
+  (unless (eq buf (current-buffer))
+    (set (make-local-variable 'vc-parent-buffer) buf)
+    (unless silent
+      (set (make-local-variable 'vc-parent-buffer-name)
+           (concat " from " (buffer-name buf))))))
+
 (defun vc-setup-buffer (buf)
   "Prepare BUF for executing a slave command and make it current."
   (let ((camefrom (current-buffer))
@@ -178,9 +192,7 @@ Another is that undo information is not kept."
       ;; want any of its output to appear from now on.
       (when oldproc (delete-process oldproc)))
     (kill-all-local-variables)
-    (set (make-local-variable 'vc-parent-buffer) camefrom)
-    (set (make-local-variable 'vc-parent-buffer-name)
-	 (concat " from " (buffer-name camefrom)))
+    (vc-set-parent-buffer camefrom)
     (setq default-directory olddir)
     (let ((buffer-undo-list t)
           (inhibit-read-only t))
@@ -662,9 +674,7 @@ BACKEND, if non-nil, specifies a VC backend for the Log Edit buffer."
     (if (and comment (not initial-contents))
 	(set-buffer (get-buffer-create logbuf))
       (pop-to-buffer (get-buffer-create logbuf)))
-    (set (make-local-variable 'vc-parent-buffer) parent)
-    (set (make-local-variable 'vc-parent-buffer-name)
-	 (concat " from " (buffer-name vc-parent-buffer)))
+    (vc-set-parent-buffer parent)
     (vc-log-edit files mode backend)
     (make-local-variable 'vc-log-after-operation-hook)
     (when after-hook
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 343cd70..bb5cdd5 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -1069,12 +1069,7 @@ BEWARE: this function may change the current buffer."
    ((derived-mode-p 'vc-dir-mode)
     (set-buffer (find-file-noselect (vc-dir-current-file))))
    (t
-    (while (and vc-parent-buffer
-                (buffer-live-p vc-parent-buffer)
-		;; Avoid infinite looping when vc-parent-buffer and
-		;; current buffer are the same buffer.
- 		(not (eq vc-parent-buffer (current-buffer))))
-      (set-buffer vc-parent-buffer))
+    (set-buffer (vc-get-parent-buffer))
     (if (not buffer-file-name)
 	(error "Buffer %s is not associated with a file" (buffer-name))
       (unless (vc-backend buffer-file-name)
@@ -1978,7 +1973,7 @@ Use BACKEND as the VC backend if specified."
       (with-current-buffer result-buf
 	;; Set the parent buffer so that things like
 	;; C-x v g, C-x v l, ... etc work.
-	(set (make-local-variable 'vc-parent-buffer) filebuf))
+	(vc-set-parent-buffer filebuf t))
       result-buf)))
 
 ;; Header-insertion code
-- 
1.7.1