Buffer RPC calls made before port is connected

Fix a race condition where inter-frame RPC calls would silently fail if made
before the MessagePort discovery process had completed. Fix the issue by queuing
calls in PortRPC and then dispatching them once a port is connected. This is
similar to how MessagePort buffers messages until they are consumed by calling
`start`.

src/shared/port-rpc.js

@@ -80,6 +80,13 @@ export class PortRPC {
     this._callbacks = new Map();
 
     this._listeners = new ListenerCollection();
+
+    /**
+     * Method and arguments of pending RPC calls made before a port was connected.
+     *
+     * @type {Array<[CallMethod, any[]]>}
+     */
+    this._pendingCalls = [];
   }
 
   /**
@@ -108,6 +115,11 @@ export class PortRPC {
       this._handle(/** @type {MessageEvent} */ (event))
     );
     port.start();
+
+    for (let [method, args] of this._pendingCalls) {
+      this.call(method, ...args);
+    }
+    this._pendingCalls = [];
   }
 
   /**
@@ -123,6 +135,9 @@ export class PortRPC {
   /**
    * Send an RPC request via the connected port.
    *
+   * If this client is not yet connected to a port, the call will be queued and
+   * sent when {@link connect} is called.
+   *
    * If the final argument in `args` is a function, it is treated as a callback
    * which is invoked with the response in the form of (error, result) arguments.
    *
@@ -130,8 +145,9 @@ export class PortRPC {
    * @param {any[]} args
    */
   call(method, ...args) {
-    // TODO - What should happen here if the port is not connected? Buffer
-    // method calls until the port is connected?
+    if (!this._port) {
+      this._pendingCalls.push([method, args]);
+    }
 
     if (!this._port || this._destroyed) {
       return;

src/shared/test/port-rpc-test.js

@@ -156,4 +156,22 @@ describe('PortRPC', () => {
       rpc.on('foo', () => {});
     }, 'Cannot add a method handler after a port is connected');
   });
+
+  it('should queue RPC requests made before port is connected', async () => {
+    const { port1, port2 } = new MessageChannel();
+    const sender = new PortRPC();
+
+    const receiver = new PortRPC();
+    const testMethod = sinon.stub();
+    receiver.on('test', testMethod);
+    receiver.connect(port2);
+
+    sender.call('test', 'first', 'call');
+    sender.call('test', 'second', 'call');
+    sender.connect(port1);
+
+    await waitForMessageDelivery();
+    assert.calledWith(testMethod, 'first', 'call');
+    assert.calledWith(testMethod, 'second', 'call');
+  });
 });