Mitigate access to messages on slow IMAP servers.

* org-gnus.el (org-gnus-nnimap-query-article-no-from-file): New
customization variable.
(org-gnus-nnimap-cached-article-number): New function.
(org-gnus-follow-link): Try to fetch cached article number of
message-id.

Some IMAP servers (e.g. Courier) are slow when searching for a message
by its message id header field.  Because article numbers in IMAP
mailboxes are persistent UIDs, we can try to look up the UID of a IMAP
message in Gnus' cache for the mailbox in question and skip the slow
search on the server.

The problem with slow server was reported by Sébastien Vauban and the
patch is based on the work of Tassilo Horn.

lisp/org-gnus.el

@@ -54,12 +54,40 @@ negates this setting for the duration of the command."
   :group 'org-link-store
   :type 'boolean)
 
+(defcustom org-gnus-nnimap-query-article-no-from-file t
+  "If non-nil, `org-gnus-follow-link' will try to translate
+Message-Ids to article numbers by querying the .overview file.
+Normally, this translation is done by querying the IMAP server,
+which is usually very fast.  Unfortunately, some (maybe badly
+configured) IMAP servers don't support this operation quickly.
+So if following a link to a Gnus article takes ages, try setting
+this variable to `t'."
+  :group 'org-link-store
+  :type 'boolean)
+
+
 ;; Install the link type
 (org-add-link-type "gnus" 'org-gnus-open)
 (add-hook 'org-store-link-functions 'org-gnus-store-link)
 
 ;; Implementation
 
+(defun org-gnus-nnimap-cached-article-number (group server message-id)
+  "Return cached article number (uid) of message in GROUP on SERVER.
+MESSAGE-ID is the message-id header field that identifies the
+message.  If the uid is not cached, return nil."
+  (with-temp-buffer
+    (let ((nov (nnimap-group-overview-filename group server)))
+      (when (file-exists-p nov)
+	(mm-insert-file-contents nov)
+	(set-buffer-modified-p nil)
+	(goto-char (point-min))
+	(catch 'found
+	  (while (search-forward message-id nil t)
+	    (let ((hdr (split-string (thing-at-point 'line) "\t")))
+	      (if (string= (nth 4 hdr) message-id)
+		  (throw 'found (nth 0 hdr))))))))))
+
 (defun org-gnus-group-link (group)
   "Create a link to the Gnus group GROUP.
 If GROUP is a newsgroup and `org-gnus-prefer-web-links' is
@@ -171,7 +199,9 @@ If `org-store-link' was called with a prefix arg the meaning of
   (cond ((and group article)
 	 (gnus-activate-group group t)
 	 (condition-case nil
-	     (let ((backend (car (gnus-find-method-for-group group))))
+	     (let* ((method (gnus-find-method-for-group group))
+		    (backend (car method))
+		    (server (cadr method)))
 	       (cond
 		((eq backend 'nndoc)
 		 (if (gnus-group-read-group t nil group)
@@ -181,6 +211,12 @@ If `org-store-link' was called with a prefix arg the meaning of
 		(t
 		 (let ((articles 1)
 		       group-opened)
+		   (when (and (eq backend 'nnimap)
+			      org-gnus-nnimap-query-article-no-from-file)
+		     (setq article
+			   (or (org-gnus-nnimap-cached-article-number
+				(nth 1 (split-string group ":"))
+				server (concat "<" article ">")) article)))
 		   (while (and (not group-opened)
 			       ;; stop on integer overflows
 			       (> articles 0))