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diff --git a/toolkit/modules/tests/xpcshell/test_DeferredTask.js b/toolkit/modules/tests/xpcshell/test_DeferredTask.js
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+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * This file tests the DeferredTask.sys.mjs module.
+ */
+
+// Globals
+
+ChromeUtils.defineESModuleGetters(this, {
+  DeferredTask: "resource://gre/modules/DeferredTask.sys.mjs",
+});
+
+/**
+ * Due to the nature of this module, most of the tests are time-dependent.  All
+ * the timeouts are designed to occur at multiples of this granularity value,
+ * in milliseconds, that should be high enough to prevent intermittent failures,
+ * but low enough to prevent an excessive overall test execution time.
+ */
+const T = 100;
+
+const originalIdleDispatch = DeferredTask.prototype._startIdleDispatch;
+function replaceIdleDispatch(handleIdleDispatch) {
+  DeferredTask.prototype._startIdleDispatch = function(callback, timeout) {
+    handleIdleDispatch(callback, timeout);
+  };
+}
+function restoreIdleDispatch() {
+  DeferredTask.prototype.idleDispatch = originalIdleDispatch;
+}
+
+/**
+ * Waits for the specified timeout before resolving the returned promise.
+ */
+function promiseTimeout(aTimeoutMs) {
+  return new Promise(resolve => {
+    do_timeout(aTimeoutMs, resolve);
+  });
+}
+
+// Tests
+
+/**
+ * Creates a simple DeferredTask and executes it once.
+ */
+add_test(function test_arm_simple() {
+  new DeferredTask(run_next_test, 10).arm();
+});
+
+/**
+ * Checks that the delay set for the task is respected.
+ */
+add_test(function test_arm_delay_respected() {
+  let executed1 = false;
+  let executed2 = false;
+
+  new DeferredTask(function() {
+    executed1 = true;
+    Assert.ok(!executed2);
+  }, 1 * T).arm();
+
+  new DeferredTask(function() {
+    executed2 = true;
+    Assert.ok(executed1);
+    run_next_test();
+  }, 2 * T).arm();
+});
+
+/**
+ * Checks that calling "arm" again does not introduce further delay.
+ */
+add_test(function test_arm_delay_notrestarted() {
+  let executed = false;
+
+  // Create a task that will run later.
+  let deferredTask = new DeferredTask(() => {
+    executed = true;
+  }, 4 * T);
+  deferredTask.arm();
+
+  // Before the task starts, call "arm" again.
+  do_timeout(2 * T, () => deferredTask.arm());
+
+  // The "arm" call should not have introduced further delays.
+  do_timeout(5 * T, function() {
+    Assert.ok(executed);
+    run_next_test();
+  });
+});
+
+/**
+ * Checks that a task runs only once when armed multiple times synchronously.
+ */
+add_test(function test_arm_coalesced() {
+  let executed = false;
+
+  let deferredTask = new DeferredTask(function() {
+    Assert.ok(!executed);
+    executed = true;
+    run_next_test();
+  }, 50);
+
+  deferredTask.arm();
+  deferredTask.arm();
+});
+
+/**
+ * Checks that a task runs only once when armed multiple times synchronously,
+ * even when it has been created with a delay of zero milliseconds.
+ */
+add_test(function test_arm_coalesced_nodelay() {
+  let executed = false;
+
+  let deferredTask = new DeferredTask(function() {
+    Assert.ok(!executed);
+    executed = true;
+    run_next_test();
+  }, 0);
+
+  deferredTask.arm();
+  deferredTask.arm();
+});
+
+/**
+ * Checks that a task can be armed again while running.
+ */
+add_test(function test_arm_recursive() {
+  let executed = false;
+
+  let deferredTask = new DeferredTask(function() {
+    if (!executed) {
+      executed = true;
+      deferredTask.arm();
+    } else {
+      run_next_test();
+    }
+  }, 50);
+
+  deferredTask.arm();
+});
+
+/**
+ * Checks that calling "arm" while an asynchronous task is running waits until
+ * the task is finished before restarting the delay.
+ */
+add_test(function test_arm_async() {
+  let finishedExecution = false;
+  let finishedExecutionAgain = false;
+
+  // Create a task that will run later.
+  let deferredTask = new DeferredTask(async function() {
+    await promiseTimeout(4 * T);
+    if (!finishedExecution) {
+      finishedExecution = true;
+    } else if (!finishedExecutionAgain) {
+      finishedExecutionAgain = true;
+    }
+  }, 2 * T);
+  deferredTask.arm();
+
+  // While the task is running, call "arm" again.  This will result in a wait
+  // of 2*T until the task finishes, then another 2*T for the normal task delay
+  // specified on construction.
+  do_timeout(4 * T, function() {
+    Assert.ok(deferredTask.isRunning);
+    Assert.ok(!finishedExecution);
+    deferredTask.arm();
+  });
+
+  // This will fail in case the task was started without waiting 2*T after it
+  // has finished.
+  do_timeout(7 * T, function() {
+    Assert.ok(!deferredTask.isRunning);
+    Assert.ok(finishedExecution);
+  });
+
+  // This is in the middle of the second execution.
+  do_timeout(10 * T, function() {
+    Assert.ok(deferredTask.isRunning);
+    Assert.ok(!finishedExecutionAgain);
+  });
+
+  // Wait enough time to verify that the task was executed as expected.
+  do_timeout(13 * T, function() {
+    Assert.ok(!deferredTask.isRunning);
+    Assert.ok(finishedExecutionAgain);
+    run_next_test();
+  });
+});
+
+/**
+ * Checks that an armed task can be disarmed.
+ */
+add_test(function test_disarm() {
+  // Create a task that will run later.
+  let deferredTask = new DeferredTask(function() {
+    do_throw("This task should not run.");
+  }, 2 * T);
+  deferredTask.arm();
+
+  // Disable execution later, but before the task starts.
+  do_timeout(1 * T, () => deferredTask.disarm());
+
+  // Wait enough time to verify that the task did not run.
+  do_timeout(3 * T, run_next_test);
+});
+
+/**
+ * Checks that calling "disarm" allows the delay to be restarted.
+ */
+add_test(function test_disarm_delay_restarted() {
+  let executed = false;
+
+  let deferredTask = new DeferredTask(() => {
+    executed = true;
+  }, 4 * T);
+  deferredTask.arm();
+
+  do_timeout(2 * T, function() {
+    deferredTask.disarm();
+    deferredTask.arm();
+  });
+
+  do_timeout(5 * T, function() {
+    Assert.ok(!executed);
+  });
+
+  do_timeout(7 * T, function() {
+    Assert.ok(executed);
+    run_next_test();
+  });
+});
+
+/**
+ * Checks that calling "disarm" while an asynchronous task is running does not
+ * prevent the task to finish.
+ */
+add_test(function test_disarm_async() {
+  let finishedExecution = false;
+
+  let deferredTask = new DeferredTask(async function() {
+    deferredTask.arm();
+    await promiseTimeout(2 * T);
+    finishedExecution = true;
+  }, 1 * T);
+  deferredTask.arm();
+
+  do_timeout(2 * T, function() {
+    Assert.ok(deferredTask.isRunning);
+    Assert.ok(deferredTask.isArmed);
+    Assert.ok(!finishedExecution);
+    deferredTask.disarm();
+  });
+
+  do_timeout(4 * T, function() {
+    Assert.ok(!deferredTask.isRunning);
+    Assert.ok(!deferredTask.isArmed);
+    Assert.ok(finishedExecution);
+    run_next_test();
+  });
+});
+
+/**
+ * Checks that calling "arm" immediately followed by "disarm" while an
+ * asynchronous task is running does not cause it to run again.
+ */
+add_test(function test_disarm_immediate_async() {
+  let executed = false;
+
+  let deferredTask = new DeferredTask(async function() {
+    Assert.ok(!executed);
+    executed = true;
+    await promiseTimeout(2 * T);
+  }, 1 * T);
+  deferredTask.arm();
+
+  do_timeout(2 * T, function() {
+    Assert.ok(deferredTask.isRunning);
+    Assert.ok(!deferredTask.isArmed);
+    deferredTask.arm();
+    deferredTask.disarm();
+  });
+
+  do_timeout(4 * T, function() {
+    Assert.ok(executed);
+    Assert.ok(!deferredTask.isRunning);
+    Assert.ok(!deferredTask.isArmed);
+    run_next_test();
+  });
+});
+
+/**
+ * Checks the isArmed and isRunning properties with a synchronous task.
+ */
+add_test(function test_isArmed_isRunning() {
+  let deferredTask = new DeferredTask(function() {
+    Assert.ok(deferredTask.isRunning);
+    Assert.ok(!deferredTask.isArmed);
+    deferredTask.arm();
+    Assert.ok(deferredTask.isArmed);
+    deferredTask.disarm();
+    Assert.ok(!deferredTask.isArmed);
+    run_next_test();
+  }, 50);
+
+  Assert.ok(!deferredTask.isArmed);
+  deferredTask.arm();
+  Assert.ok(deferredTask.isArmed);
+  Assert.ok(!deferredTask.isRunning);
+});
+
+/**
+ * Checks that task execution is delayed when the idle task has no deadline.
+ */
+add_test(function test_idle_without_deadline() {
+  let idleStarted = false;
+  let executed = false;
+
+  // When idleDispatch is not passed a deadline/timeout, let it take a while.
+  replaceIdleDispatch((callback, timeout) => {
+    Assert.ok(!idleStarted);
+    idleStarted = true;
+    do_timeout(timeout || 2 * T, callback);
+  });
+
+  let deferredTask = new DeferredTask(function() {
+    Assert.ok(!executed);
+    executed = true;
+  }, 1 * T);
+  deferredTask.arm();
+
+  do_timeout(2 * T, () => {
+    Assert.ok(idleStarted);
+    Assert.ok(!executed);
+  });
+
+  do_timeout(4 * T, () => {
+    Assert.ok(executed);
+    restoreIdleDispatch();
+    run_next_test();
+  });
+});
+
+/**
+ * Checks that the third parameter can be used to enforce an execution deadline.
+ */
+add_test(function test_idle_deadline() {
+  let idleStarted = false;
+  let executed = false;
+
+  // Let idleDispatch wait until the deadline.
+  replaceIdleDispatch((callback, timeout) => {
+    Assert.ok(!idleStarted);
+    idleStarted = true;
+    do_timeout(timeout || 0, callback);
+  });
+
+  let deferredTask = new DeferredTask(
+    function() {
+      Assert.ok(!executed);
+      executed = true;
+    },
+    1 * T,
+    2 * T
+  );
+  deferredTask.arm();
+
+  // idleDispatch is expected to be called after 1 * T,
+  // the task is expected to be executed after 1 * T + 2 * T.
+  do_timeout(2 * T, () => {
+    Assert.ok(idleStarted);
+    Assert.ok(!executed);
+  });
+
+  do_timeout(4 * T, () => {
+    Assert.ok(executed);
+    restoreIdleDispatch();
+    run_next_test();
+  });
+});
+
+/**
+ * Checks that the "finalize" method executes a synchronous task.
+ */
+add_test(function test_finalize() {
+  let executed = false;
+  let timePassed = false;
+  let idleStarted = false;
+  let finalized = false;
+
+  // Let idleDispatch take longer.
+  replaceIdleDispatch((callback, timeout) => {
+    Assert.ok(!idleStarted);
+    idleStarted = true;
+    do_timeout(T, callback);
+  });
+
+  let deferredTask = new DeferredTask(function() {
+    Assert.ok(!timePassed);
+    executed = true;
+  }, 2 * T);
+  deferredTask.arm();
+
+  do_timeout(1 * T, () => {
+    timePassed = true;
+    Assert.ok(finalized);
+    Assert.ok(!idleStarted);
+  });
+
+  // This should trigger the immediate execution of the task.
+  deferredTask.finalize().then(function() {
+    finalized = true;
+    Assert.ok(executed);
+  });
+
+  // idleDispatch was originally supposed to start at 2 * T,
+  // so if the task did not run at T * 3, then the finalization was successful.
+  do_timeout(3 * T, () => {
+    // Because the timer was canceled, the idle task shouldn't even start.
+    Assert.ok(!idleStarted);
+    restoreIdleDispatch();
+    run_next_test();
+  });
+});
+
+/**
+ * Checks that the "finalize" method executes the task again from start to
+ * finish in case it is already running.
+ */
+add_test(function test_finalize_executes_entirely() {
+  let executed = false;
+  let executedAgain = false;
+  let timePassed = false;
+
+  let deferredTask = new DeferredTask(async function() {
+    // The first time, we arm the timer again and set up the finalization.
+    if (!executed) {
+      deferredTask.arm();
+      Assert.ok(deferredTask.isArmed);
+      Assert.ok(deferredTask.isRunning);
+
+      deferredTask.finalize().then(function() {
+        // When we reach this point, the task must be finished.
+        Assert.ok(executedAgain);
+        Assert.ok(!timePassed);
+        Assert.ok(!deferredTask.isArmed);
+        Assert.ok(!deferredTask.isRunning);
+        run_next_test();
+      });
+
+      // The second execution triggered by the finalization waits 1*T for the
+      // current task to finish (see the timeout below), but then it must not
+      // wait for the 2*T specified on construction as normal task delay.  The
+      // second execution will finish after the timeout below has passed again,
+      // for a total of 2*T of wait time.
+      do_timeout(3 * T, () => {
+        timePassed = true;
+      });
+    }
+
+    await promiseTimeout(1 * T);
+
+    // Just before finishing, indicate if we completed the second execution.
+    if (executed) {
+      Assert.ok(deferredTask.isRunning);
+      executedAgain = true;
+    } else {
+      executed = true;
+    }
+  }, 2 * T);
+
+  deferredTask.arm();
+});