Bug 1663941 - Do not call ResourceWatcher onAvailable callback with empty arrays of ressources r=ochameau

Differential Revision: https://phabricator.services.mozilla.com/D89609

devtools/shared/resources/resource-watcher.js

@@ -551,11 +551,12 @@ class ResourceWatcher {
   }
 
   async _forwardCachedResources(resourceTypes, onAvailable) {
-    await onAvailable(
-      this._cache.filter(resource =>
-        resourceTypes.includes(resource.resourceType)
-      )
+    const cachedResources = this._cache.filter(resource =>
+      resourceTypes.includes(resource.resourceType)
     );
+    if (cachedResources.length > 0) {
+      await onAvailable(cachedResources);
+    }
   }
 
   /**

devtools/shared/resources/tests/browser_resources_client_caching.js

@@ -275,6 +275,55 @@ async function testIgnoreExistingResources(isFirstListenerIgnoreExisting) {
   await client.close();
 }
 
+add_task(async function() {
+  info("Test that onAvailable is not called with an empty resources array");
+
+  const tab = await addTab(TEST_URI);
+
+  const {
+    client,
+    resourceWatcher,
+    targetList,
+  } = await initResourceWatcherAndTarget(tab);
+
+  info("Register first listener to get all available resources");
+  const availableResources = [];
+  let onAvailableCallCount = 0;
+  const onAvailable = resources => {
+    ok(
+      resources.length > 0,
+      "onAvailable is called with a non empty resources array"
+    );
+    availableResources.push(...resources);
+    onAvailableCallCount++;
+  };
+
+  await resourceWatcher.watchResources(
+    [ResourceWatcher.TYPES.CONSOLE_MESSAGE],
+    { onAvailable }
+  );
+  is(availableResources.length, 0, "availableResources array is empty");
+  is(onAvailableCallCount, 0, "onAvailable was never called");
+
+  info("Add messages as console message");
+  await logMessages(tab.linkedBrowser, ["expected message"]);
+
+  await waitUntil(() => availableResources.length === 1);
+  is(availableResources.length, 1, "availableResources array has one item");
+  is(onAvailableCallCount, 1, "onAvailable was called only once");
+  is(
+    availableResources[0].message.arguments[0],
+    "expected message",
+    "onAvailable was called with the expected resource"
+  );
+
+  resourceWatcher.unwatchResources([ResourceWatcher.TYPES.CONSOLE_MESSAGE], {
+    onAvailable,
+  });
+  await targetList.destroy();
+  await client.close();
+});
+
 function assertContents(resources, expectedMessages) {
   is(
     resources.length,