Subject: [PATCH] `find-change-log' fails in *derived* Diff modes

From: "Aaron S. Hawley" <[email-masked]>
Date: Tue, 26 Oct 2010 15:47:43 -0400

Running `C-x 4 a' (add-change-log-entry-other-window) in a derived mode
of Diff mode (psvn-diff-mode, magit-diff-mode, and so on) does not open
the correct ChangeLog file.  The following patch fixes the problem.

--- lisp/vc/add-log.el~	2010-09-21 14:11:28.216630100 -0400
+++ lisp/vc/add-log.el	2010-10-25 16:46:49.808297800 -0400
@@ -701,7 +701,7 @@
 Optional arg BUFFER-FILE overrides `buffer-file-name'."
   ;; If we are called from a diff, first switch to the source buffer;
   ;; in order to respect buffer-local settings of change-log-default-name, etc.
-  (with-current-buffer (let ((buff (if (eq major-mode 'diff-mode)
+  (with-current-buffer (let ((buff (if (derived-mode-p 'diff-mode)
 				       (car (ignore-errors
 					     (diff-find-source-location))))))
 			 (if (buffer-live-p buff) buff

Perhaps, more of the cases of Emacs's Lisp that have (eq major-mode ...)
should be converted to (derived-mode-p ...).  Obviously, they'll need
to be studied on a case-by-case basis since there might be cases when
only the exact major-mode is intended to be matched.

Thanks for Emacs,
/a