Implement shutdown API in Glean singleton

Author: brizental

glean/src/core/glean.ts

@@ -413,6 +413,27 @@ class Glean {
     });
   }
 
+  /**
+   * Finishes executing all pending tasks
+   * and shutsdown both Glean's dispatcher and ping uploader.
+   *
+   * # Important
+   *
+   * This is irreversible.
+   * Only a restart will return Glean back to an idle state.
+   *
+   * @returns A promise which resolves once the shutdown is complete.
+   */
+  static async shutdown(): Promise<void> {
+    // Order here matters!
+    //
+    // The main dispatcher needs to be shutdown first,
+    // because some of its tasks may enqueue new tasks on the ping uploader dispatcher
+    // and we want these uploading tasks to also be executed prior to complete shutdown.
+    await Context.dispatcher.shutdown();
+    await Glean.pingUploader.shutdown();
+  }
+
   /**
    * Sets the current environment.
    *

glean/tests/unit/core/glean.spec.ts

@@ -18,6 +18,7 @@ import TestPlatform from "../../../src/platform/test";
 import Plugin from "../../../src/plugins";
 import { Lifetime } from "../../../src/core/metrics/lifetime";
 import { Context } from "../../../src/core/context";
+import { DispatcherState } from "../../../src/core/dispatcher";
 
 class MockPlugin extends Plugin<typeof CoreEvents["afterPingCollection"]> {
   constructor() {
@@ -504,4 +505,25 @@ describe("Glean", function() {
 
     assert.strictEqual(TestPlatform.name, Glean.platform.name);
   });
+
+  it("shutdown correctly shutsdown dispatcher and ping uploader", async function () {
+    await Glean.shutdown();
+    assert.strictEqual(Context.dispatcher["state"], DispatcherState.Shutdown);
+    assert.strictEqual(Glean["pingUploader"]["dispatcher"]["state"], DispatcherState.Shutdown);
+  });
+
+  it("shutdown allows all pending pings to be sent before shutting down uploader", async function() {
+    const postSpy = sandbox.spy(Glean.platform.uploader, "post");
+
+    // `setUploadEnabled` is a task dispatched on the main dispatcher,
+    // inside this task an uploading task is dispatched to the ping uploader dispatcher.
+    //
+    // We want to be sure the ping uploader dispatcher is not shutdown
+    // before it can execute this final task.
+    Glean.setUploadEnabled(false);
+    await Glean.shutdown();
+
+    assert.strictEqual(postSpy.callCount, 1);
+    assert.ok(postSpy.getCall(0).args[0].indexOf(DELETION_REQUEST_PING_NAME) !== -1);
+  });
 });