sensors: Do nothing in Sensor.start() when the document is not fully …

…active.

When a sensor is created on e.g. an iframe that is later removed from
its parent via removeChild(), we end up in a situation where a sensor
instance did get created but which does not have a valid
ExecutionContext by the time start() is invoked.

Check for a valid ExecutionContext when start() is called and bail out
early if it is null. w3c/sensors#415 tracks
handling non-fully active documents from a spec perspective; once that
one is fixed we should probably throw an error in this case rather than
silently doing nothing.

Bug: 1289924
Change-Id: I2b033252d93347ba7c91385bdb510b69b8298aa2

generic-sensor/generic-sensor-iframe-tests.sub.js

@@ -155,4 +155,32 @@ function run_generic_sensor_iframe_tests(sensorName) {
     iframe.parentNode.removeChild(iframe);
     window.focus();
   }, `${sensorName}: losing a document's frame with an active sensor does not crash`);
+
+  sensor_test(async t => {
+    assert_implements(sensorName in self, `${sensorName} is not supported.`);
+    const iframe = document.createElement('iframe');
+    iframe.allow = featurePolicies.join(';') + ';';
+    iframe.src = 'https://{{host}}:{{ports[https][0]}}/generic-sensor/resources/iframe_sensor_handler.html';
+
+    // Create sensor in the iframe (we do not care whether this is a
+    // cross-origin nested context in this test).
+    const iframeLoadWatcher = new EventWatcher(t, iframe, 'load');
+    document.body.appendChild(iframe);
+    await iframeLoadWatcher.wait_for('load');
+
+    // The purpose of this message is to initialize the mock backend in the
+    // iframe. We are not going to use the sensor created there.
+    await send_message_to_iframe(iframe, {command: 'create_sensor',
+                                          type: sensorName});
+
+    const iframeSensor = new iframe.contentWindow[sensorName]();
+    assert_not_equals(iframeSensor, null);
+
+    // Remove iframe from main document. |iframeSensor| no longer has a
+    // non-null browsing context. Calling start() should probably throw an
+    // error when called from a non-fully active document, but that depends on
+    // https://github.com/w3c/sensors/issues/415
+    iframe.parentNode.removeChild(iframe);
+    iframeSensor.start();
+  }, `${sensorName}: calling start() in a non-fully active document does not crash`);
 }