Perform asynchronous connection if possible

When using native GnuTLS, we can now connect asynchronously, without
blocking Emacs if the remote server is slow to accept the connection
(or just times out).

Such connections are now identical to "network" (i.e. TLS-less)
connections, so I reverted jabber-starttls-connect to its previous
state: it is now used exclusively to connect using gnutls-cli external
processes.

jabber-conn.el

@@ -88,8 +88,14 @@ or later."
   :group 'jabber-conn)
 
 (defvar jabber-connect-methods
-  '((network jabber-network-connect jabber-network-send)
-    (starttls jabber-starttls-connect jabber-network-send)
+  `((network jabber-network-connect jabber-network-send)
+    (starttls
+     ,(if (and (fboundp 'gnutls-available-p)
+	       (gnutls-available-p))
+	  ;; With "native" TLS, we can use a normal connection.
+	  'jabber-network-connect
+	'jabber-starttls-connect)
+     jabber-network-send)
     (ssl jabber-ssl-connect jabber-ssl-send)
     (virtual jabber-virtual-connect jabber-virtual-send))
   "Alist of connection methods and functions.
@@ -128,7 +134,60 @@ If we can't find SRV records, use standard defaults."
 Send a message of the form (:connected CONNECTION) to FSM if
 connection succeeds.  Send a message :connection-failed if
 connection fails."
-  ;; XXX: asynchronous connection
+  (cond
+   ((featurep 'make-network-process '(:nowait t))
+    ;; We can connect asynchronously!
+    (jabber-network-connect-async fsm server network-server port))
+   (t
+    ;; Connecting to the server will block Emacs.
+    (jabber-network-connect-sync fsm server network-server port))))
+
+(defun jabber-network-connect-async (fsm server network-server port)
+  ;; Get all potential targets...
+  (lexical-let ((targets (jabber-srv-targets server network-server port))
+		(fsm fsm))
+    ;; ...and connect to them one after another, asynchronously, until
+    ;; connection succeeds.
+    (labels
+	((connect
+	  (target remaining-targets)
+	  (make-network-process
+	   :name "jabber"
+	   :buffer (generate-new-buffer jabber-process-buffer)
+	   :host (car target) :service (cdr target)
+	   :coding 'utf-8
+	   :nowait t
+	   :sentinel
+	   (lexical-let ((target target) (remaining-targets remaining-targets))
+	     (lambda (connection status)
+	       (cond
+		((string-match "^open" status)
+		 ;; This mustn't be `fsm-send-sync', because the FSM
+		 ;; needs to change the sentinel, which cannot be done
+		 ;; from inside the sentinel.
+		 (fsm-send fsm (list :connected connection)))
+		((string-match "^failed" status)
+		 (message "Couldn't connect to %s:%s" (car target) (cdr target))
+		 (delete-process connection)
+		 (if remaining-targets
+		     (progn
+		       (message
+			"Connecting to %s:%s..."
+			(caar remaining-targets) (cdar remaining-targets))
+		       (connect (car remaining-targets) (cdr remaining-targets)))
+		   (fsm-send fsm :connection-failed)))
+		((string-match "^deleted" status)
+		 ;; This happens when we delete a process in the
+		 ;; "failed" case above.
+		 nil)
+		(t
+		 (message "Unknown sentinel status `%s'" status))))))))
+      (message "Connecting to %s:%s..." (caar targets) (cdar targets))
+      (connect (car targets) (cdr targets)))))
+
+(defun jabber-network-connect-sync (fsm server network-server port)
+  ;; This code will AFAIK only be used on Windows.  Apologies in
+  ;; advance for any bit rot...
   (let ((coding-system-for-read 'utf-8)
 	(coding-system-for-write 'utf-8)
 	(targets (jabber-srv-targets server network-server port)))
@@ -205,25 +264,15 @@ connection fails."
   (process-send-string connection "\n"))
 
 (defun jabber-starttls-connect (fsm server network-server port)
-  "Connect via GnuTLS to a Jabber Server.
+  "Connect via an external GnuTLS process to a Jabber Server.
 Send a message of the form (:connected CONNECTION) to FSM if
 connection succeeds.  Send a message :connection-failed if
 connection fails."
   (let ((coding-system-for-read 'utf-8)
 	(coding-system-for-write 'utf-8)
-	(targets (jabber-srv-targets server network-server port))
-	(connect-function
-	 (cond
-	  ((and (fboundp 'gnutls-available-p)
-		(gnutls-available-p))
-	   (lambda (buffer host port)
-	     (open-network-stream "jabber" buffer host port)))
-	  ((fboundp 'starttls-open-stream)
-	   (lambda (buffer host port)
-	     (starttls-open-stream "jabber" buffer host port)))
-	  (t
-	   (error "Neither native GnuTLS nor starttls.el available")))))
-    
+	(targets (jabber-srv-targets server network-server port)))
+    (unless (fboundp 'starttls-open-stream)
+      (error "starttls.el not available"))
     (catch 'connected
       (dolist (target targets)
 	(condition-case e
@@ -231,10 +280,11 @@ connection fails."
 		  connection)
 	      (unwind-protect
 		  (setq connection
-			(funcall connect-function
-				 process-buffer
-				 (car target)
-				 (cdr target)))
+			(starttls-open-stream
+			 "jabber"
+			 process-buffer
+			 (car target)
+			 (cdr target)))
 		(unless (or connection jabber-debug-keep-process-buffers)
 		  (kill-buffer process-buffer)))
 	      (when connection