Add handshake for the inner/outer xpc connections of the browser repl

Prior to this commit, the iframe connection just hopes that the
roundtrip to the server plus 50ms is enough for the outer xpc connection
to get established. This is the reason the browser repl occasionally
fails to connect.

This commit adds a handshake to that interaction, ensuring that both
connections are established prior to sending any work to the outer.

Note that it waits for the handshake to complete prior to sending the
:ready message to the server. This is slower than sending to both the
server and the outer simultaneously, then forcing the inner to wait for
an ack from the outer. However, the code is significantly simpler this
way. We can revisit if it's an issue.

Author: potetm

.gitignore

@@ -13,6 +13,7 @@ closure
 *out
 .lein*
 /pom.xml
+*.iml
 .repl*
 *.swp
 *.zip

src/main/cljs/clojure/browser/repl.cljs

@@ -97,7 +97,31 @@
   "Start the REPL server connection."
   [url]
   (if-let [repl-connection (net/xpc-connection)]
-    (let [connection (net/xhr-connection)]
+    (let [connection (net/xhr-connection)
+          repl-connected? (atom false)
+          try-handshake (fn try-handshake []
+                          (when-not @repl-connected?
+                            (net/transmit repl-connection
+                                          :start-handshake
+                                          nil)
+                            ;; In case we miss, try again. Parent will only
+                            ;; ack once.
+                            (js/setTimeout try-handshake
+                                           10)))]
+      (net/connect repl-connection
+                   try-handshake)
+
+      (net/register-service repl-connection
+        :ack-handshake
+        (fn [_]
+          (when-not @repl-connected?
+            (reset! repl-connected? true)
+            ;; Now that we're connected to the parent, we can start talking to
+            ;; the server.
+            (send-result connection
+                         url
+                         (wrap-message :ready "ready")))))
+
       (event/listen connection
         :success
         (fn [e]
@@ -115,12 +139,7 @@
       (net/register-service repl-connection
         :print
         (fn [data]
-          (send-print url (wrap-message :print data))))
-
-      (net/connect repl-connection
-        (constantly nil))
-
-      (js/setTimeout #(send-result connection url (wrap-message :ready "ready")) 1000))
+          (send-print url (wrap-message :print data)))))
     (js/alert "No 'xpc' param provided to child iframe.")))
 
 (def load-queue nil)
@@ -189,10 +208,21 @@
   connection is made, the REPL will evaluate forms in the context of
   the document that called this function."
   [repl-server-url]
-  (let [repl-connection
+  (let [connected? (atom false)
+        repl-connection
         (net/xpc-connection
           {:peer_uri repl-server-url})]
     (swap! xpc-connection (constantly repl-connection))
+    (net/register-service repl-connection
+      :start-handshake
+      (fn [_]
+        ;; Child will keep retrying, but we only want
+        ;; to ack once.
+        (when-not @connected?
+          (reset! connected? true)
+          (net/transmit repl-connection
+                        :ack-handshake
+                        nil))))
     (net/register-service repl-connection
       :evaluate-javascript
       (fn [js]