Bug 1358988 - Write Marionette port to preference when binding; r=whimboo

Write the allocated port to the marionette.port preference when binding
the TCP socket server to the requested port.

If the socket is bound to port 0, this instructs the system to atomically
allocate a free port in the ephemeral range.  Writing the resolved port
to a preference will make it possible to communicate this port number
to the client, removing any chance of a race condition occurring by the
client looking for a free port (binding and releasing) for the server.

MozReview-Commit-ID: JwyG2G4YQmX

--HG--
extra : rebase_source : 98e38660813c0868f3358183825ad08392e62d37

testing/marionette/server.js

@@ -32,6 +32,7 @@ this.server = {};
 const PROTOCOL_VERSION = 3;
 
 const PREF_CONTENT_LISTENER = "marionette.contentListener";
+const PREF_PORT = "marionette.port";
 const PREF_RECOMMENDED = "marionette.prefs.recommended";
 
 // Marionette sets preferences recommended for automation when it starts,
@@ -273,6 +274,7 @@ server.TCPListener = class {
    */
   constructor (port) {
     this.port = port;
+    this.socket = null;
     this.conns = new Set();
     this.nextConnID = 0;
     this.alive = false;
@@ -283,7 +285,7 @@ server.TCPListener = class {
   /**
    * Function produces a GeckoDriver.
    *
-   * Determines application nameto initialise the driver with.
+   * Determines application name to initialise the driver with.
    *
    * @return {GeckoDriver}
    *     A driver instance.
@@ -303,6 +305,13 @@ server.TCPListener = class {
     this._acceptConnections = value;
   }
 
+  /**
+   * Bind this listener to |port| and start accepting incoming socket
+   * connections on |onSocketAccepted|.
+   *
+   * The marionette.port preference will be populated with the value
+   * of |this.port|.
+   */
   start () {
     if (this.alive) {
       return;
@@ -321,8 +330,10 @@ server.TCPListener = class {
 
     const flags = KeepWhenOffline | LoopbackOnly;
     const backlog = 1;
-    this.listener = new ServerSocket(this.port, flags, backlog);
-    this.listener.asyncListen(this);
+    this.socket = new ServerSocket(this.port, flags, backlog);
+    this.socket.asyncListen(this);
+    this.port = this.socket.port;
+    Preferences.set(PREF_PORT, this.port);
 
     this.alive = true;
     this._acceptConnections = true;
@@ -333,20 +344,18 @@ server.TCPListener = class {
       return;
     }
 
+    this._acceptConnections = false;
+
+    this.socket.close();
+    this.socket = null;
+
     for (let k of this.alteredPrefs) {
       logger.debug(`Resetting recommended pref ${k}`);
       Preferences.reset(k);
     }
-    this.closeListener();
 
     this.alteredPrefs.clear();
     this.alive = false;
-    this._acceptConnections = false;
-  }
-
-  closeListener () {
-    this.listener.close();
-    this.listener = null;
   }
 
   onSocketAccepted (serverSocket, clientSocket) {