ui: Ensure an EventSource isn't opened if closed before first tick (#5703)

EventSources will wait for 1 tick before 'opening'. There is always the
chance that the EventSource is '.close()'ed before that tick. We
therefore check the 'readyState' before opening the EventSource

ui-v2/app/utils/dom/event-source/callable.js

@@ -41,11 +41,15 @@ export default function(
               return P.resolve();
             }
           : source;
-      this.readyState = 0; // connecting
+      this.readyState = 0; // CONNECTING
       P.resolve()
         .then(() => {
+          // if we are already closed, don't do anything
+          if (this.readyState !== 0) {
+            return;
+          }
           this.readyState = 1; // open
-          // ...that the connection _was just_ opened
+          // the connection _was just_ opened
           this.dispatchEvent({ type: 'open' });
           return run(this, configuration, isClosed);
         })
@@ -63,8 +67,13 @@ export default function(
     }
     close() {
       // additional readyState 3 = CLOSING
-      if (this.readyState !== 2) {
+      switch (this.readyState) {
-        this.readyState = 3;
+        case 0: // CONNECTING
+        case 2: // CLOSED
+          this.readyState = 2; // CLOSED
+          break;
+        default:
+          this.readyState = 3; // CLOSING
       }
     }
   };

ui-v2/tests/integration/utils/dom/event-source/callable-test.js

@@ -81,4 +81,19 @@ module('Integration | Utility | dom/event-source/callable', function(hooks) {
       });
     });
   });
+  test("it can be closed before the first tick, and therefore doesn't run", function(assert) {
+    assert.expect(3);
+    const EventSource = domEventSourceCallable(EventTarget);
+    const listener = this.stub();
+    const source = new EventSource();
+    source.close();
+    assert.equal(source.readyState, 2);
+    source.addEventListener('open', function(e) {
+      listener();
+    });
+    return Promise.resolve().then(function() {
+      assert.notOk(listener.called);
+      assert.equal(source.readyState, 2);
+    });
+  });
 });