Implement THROTTLED priority semantics.

net/base/network_throttle_manager_impl_unittest.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "net/base/network_throttle_manager_impl.h"
+
+#include <memory>
+
+#include "base/bind.h"
+#include "base/callback.h"
+#include "base/callback_helpers.h"
+#include "base/memory/scoped_vector.h"
+#include "base/run_loop.h"
+#include "base/test/simple_test_tick_clock.h"
+#include "base/test/test_message_loop.h"
+#include "net/base/request_priority.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace net {
+
+namespace {
+
+#include "testing/gtest/include/gtest/gtest.h"
+
+const int kInitialAgeHorizonForUncountedRequests =
+    (NetworkThrottleManagerImpl::kInitialMedianInMs *
+     NetworkThrottleManagerImpl::kMedianLifetimeMultiple);
+
+// Must be greater than the corresponding fudge factor in
+// network_throttle_manager_impl.cc.
+const int kAgeHorizonFudgeFactor = 20;
+
+// Test fixture for throttle manager tests.
+
+// Note that the manager owned and managed by this fixture has a clock
+// that is set to base::TimeTicks::Now() (which value is also exposed
+// via an accessor) on creation but does not change without
+// intervention by tests (to make the tests more predictable).
+//
+// HOWEVER, also note that that manager uses the base::Timer class, which
+// uses the system clock, which isn't affected by the setting of the
+// test fixture clock.  So test should be written to a) avoid situations
+// in which the manager's timer will actually go off based on the system
+// clock, and b) call ConditionallyTriggerTimerForTesting() (which does
+// evaluate the manager's clock) when timer based tests are necessary.
+class NetworkThrottleManagerTest : public testing::Test,
+                                   NetworkThrottleManager::ThrottleDelegate {
+ public:
+  NetworkThrottleManagerTest()
+      : clock_(new base::SimpleTestTickClock),
+        now_(base::TimeTicks::Now()),
+        throttle_state_change_count_(0),
+        last_throttle_to_change_state_(nullptr),
+        throttle_manager_(new NetworkThrottleManagerImpl) {
+    clock_->SetNowTicks(now_);
+    throttle_manager_->SetTickClockForTesting(
+        std::unique_ptr<base::TickClock>(clock_));
+  }
+
+ protected:
+  enum ExpectedThrottleBlockState { BLOCKED, UNBLOCKED };
+
+  base::TimeTicks now() { return now_; }
+  NetworkThrottleManagerImpl* throttle_manager() {
+    return throttle_manager_.get();
+  }
+
+  // Set the offset of the test clock from now_.
+  void SetClockDelta(base::TimeDelta time_delta) {
+    clock_->SetNowTicks(now_ + time_delta);
+  }
+
+  // Throttle creation
+  std::unique_ptr<NetworkThrottleManager::Throttle> CreateThrottle(
+      net::RequestPriority priority,
+      ExpectedThrottleBlockState throttle_state) {
+    std::unique_ptr<NetworkThrottleManager::Throttle> throttle(
+        throttle_manager_->CreateThrottle(this, priority, false));
+    EXPECT_EQ(throttle_state == BLOCKED, throttle->IsBlocked());
+    return throttle;
+  }
+  std::unique_ptr<NetworkThrottleManager::Throttle>
+  CreateThrottleIgnoringLimits(net::RequestPriority priority) {
+    std::unique_ptr<NetworkThrottleManager::Throttle> throttle(
+        throttle_manager_->CreateThrottle(this, priority, true));
+    EXPECT_FALSE(throttle->IsBlocked());
+    return throttle;
+  }
+
+  // Throttle state change information.
+  int throttle_state_change_count() { return throttle_state_change_count_; }
+  NetworkThrottleManager::Throttle* last_throttle_to_change_state() {
+    return last_throttle_to_change_state_;
+  }
+
+  // Setting a callback to be invoked when a throttle's state changes.
+  void SetThrottleStateChangedCallback(const base::Closure& callback) {
+    throttle_state_changed_callback_ = callback;
+  }
+
+ private:
+  // NetworkThrottleManager::Delegate
+  void OnThrottleUnblocked(
+      NetworkThrottleManager::Throttle* throttle) override {
+    ++throttle_state_change_count_;
+    last_throttle_to_change_state_ = throttle;
+    if (!throttle_state_changed_callback_.is_null())
+      base::ResetAndReturn(&throttle_state_changed_callback_).Run();
+  }
+
+  base::SimpleTestTickClock* clock_;
+  base::TimeTicks now_;
+  int throttle_state_change_count_;
+  NetworkThrottleManager::Throttle* last_throttle_to_change_state_;
+  std::unique_ptr<NetworkThrottleManagerImpl> throttle_manager_;
+  base::Closure throttle_state_changed_callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(NetworkThrottleManagerTest);
+};
+
+// Check to confirm that all created throttles at priorities other than
+// THROTTLED start unblocked.
+TEST_F(NetworkThrottleManagerTest, AllUnthrottled) {
+  for (int i = MINIMUM_PRIORITY; i <= MAXIMUM_PRIORITY; ++i) {
+    if (i == THROTTLED)
+      continue;
+    CreateThrottle(static_cast<RequestPriority>(i), UNBLOCKED);
+  }
+}
+
+// Check for basic semantics around the new THROTTLED level.
+TEST_F(NetworkThrottleManagerTest, ThrottledBlocking) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle5(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  EXPECT_EQ(0, throttle_state_change_count());
+
+  throttle1.reset();
+  base::RunLoop().RunUntilIdle();  // Allow posttasks to run.
+  EXPECT_EQ(1, throttle_state_change_count());
+  EXPECT_EQ(throttle3.get(), last_throttle_to_change_state());
+
+  EXPECT_FALSE(throttle3->IsBlocked());
+  EXPECT_TRUE(throttle4->IsBlocked());
+  EXPECT_TRUE(throttle5->IsBlocked());
+
+  throttle2.reset();
+  base::RunLoop().RunUntilIdle();  // Allow posttasks to run.
+  EXPECT_EQ(2, throttle_state_change_count());
+  EXPECT_EQ(throttle4.get(), last_throttle_to_change_state());
+
+  EXPECT_FALSE(throttle3->IsBlocked());
+  EXPECT_FALSE(throttle4->IsBlocked());
+  EXPECT_TRUE(throttle5->IsBlocked());
+}
+
+// Check that THROTTLED semantics are dependent on all outstanding requests.
+TEST_F(NetworkThrottleManagerTest, ThrottledBlockingMultiPriority) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(HIGHEST, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(LOW, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle5(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  EXPECT_EQ(0, throttle_state_change_count());
+
+  throttle1.reset();
+  base::RunLoop().RunUntilIdle();  // Allow posttasks to run.
+  EXPECT_EQ(0, throttle_state_change_count());
+  EXPECT_FALSE(throttle3->IsBlocked());
+  EXPECT_TRUE(throttle4->IsBlocked());
+  EXPECT_TRUE(throttle5->IsBlocked());
+
+  throttle2.reset();
+  base::RunLoop().RunUntilIdle();  // Allow posttasks to run.
+  EXPECT_EQ(1, throttle_state_change_count());
+  EXPECT_EQ(throttle4.get(), last_throttle_to_change_state());
+
+  EXPECT_FALSE(throttle3->IsBlocked());
+  EXPECT_FALSE(throttle4->IsBlocked());
+  EXPECT_TRUE(throttle5->IsBlocked());
+
+  throttle3.reset();
+  base::RunLoop().RunUntilIdle();  // Allow posttasks to run.
+  EXPECT_EQ(2, throttle_state_change_count());
+  EXPECT_EQ(throttle5.get(), last_throttle_to_change_state());
+
+  EXPECT_FALSE(throttle4->IsBlocked());
+  EXPECT_FALSE(throttle5->IsBlocked());
+}
+
+// Check that a SetPriority() away from THROTTLED results in unblocking
+// and an upcall.
+TEST_F(NetworkThrottleManagerTest, ThrottledSetPriority) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  EXPECT_EQ(0, throttle_state_change_count());
+
+  throttle3->SetPriority(LOW);
+  EXPECT_EQ(1, throttle_state_change_count());
+  EXPECT_EQ(throttle3.get(), last_throttle_to_change_state());
+  EXPECT_FALSE(throttle3->IsBlocked());
+  EXPECT_TRUE(throttle4->IsBlocked());
+}
+
+void ResetThrottles(bool* function_called,
+                    ScopedVector<NetworkThrottleManager::Throttle> throttles) {
+  *function_called = true;
+  // All pointers in the vector should be deleted on exit.
+}
+
+// Check that tearing down all elements in the NTM on a SetPriority
+// upcall doesn't create any problems.
+TEST_F(NetworkThrottleManagerTest, ThrottleTeardown) {
+  ScopedVector<NetworkThrottleManager::Throttle> throttles;
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle_temporary;
+
+  throttles.push_back(std::unique_ptr<NetworkThrottleManager::Throttle>(
+      CreateThrottle(THROTTLED, UNBLOCKED)));
+  throttles.push_back(std::unique_ptr<NetworkThrottleManager::Throttle>(
+      CreateThrottle(THROTTLED, UNBLOCKED)));
+
+  // Note that if there is more than one throttle blocked, then the
+  // number of throttle state changes is dependent on destruction order.
+  // So only one blocked throttle is created.
+  throttle_temporary = CreateThrottle(THROTTLED, BLOCKED);
+  NetworkThrottleManager::Throttle* throttle3 = throttle_temporary.get();
+  throttles.push_back(std::move(throttle_temporary));
+
+  bool callback_called(false);
+  SetThrottleStateChangedCallback(
+      base::Bind(&ResetThrottles, &callback_called, base::Passed(&throttles)));
+
+  EXPECT_EQ(0, throttle_state_change_count());
+
+  throttle3->SetPriority(LOW);
+
+  // If the test is functioning as expected, throttle3 now points to
+  // a deleted object and can no longer be indirected through.
+
+  EXPECT_TRUE(callback_called);
+  EXPECT_EQ(1, throttle_state_change_count());
+  EXPECT_EQ(throttle3, last_throttle_to_change_state());
+}
+
+// Note that this routine is dependent on priority setting *not* resulting in
+// destruction of any throttle and should only be used in tests where that is
+// true.
+void SetAllToPriority(
+    RequestPriority priority,
+    std::vector<NetworkThrottleManager::Throttle*> throttles) {
+  for (size_t i = 0; i < throttles.size(); ++i)
+    throttles[i]->SetPriority(priority);
+}
+
+// Check that modifying all the priorities of the allocated throttles in
+// the callback works properly.
+TEST_F(NetworkThrottleManagerTest, ThrottlePriorityReset) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  std::vector<NetworkThrottleManager::Throttle*> throttles;
+  throttles.push_back(throttle1.get());
+  throttles.push_back(throttle2.get());
+  throttles.push_back(throttle3.get());
+
+  SetThrottleStateChangedCallback(
+      base::Bind(&SetAllToPriority, MEDIUM, base::Passed(&throttles)));
+
+  EXPECT_EQ(0, throttle_state_change_count());
+  throttle3->SetPriority(HIGHEST);
+
+  // Expected result: throttles 1-3 @ medium priority (the callback should
+  // have overridden the priority setting above), only throttle 4 blocked
+  // (throttle3 should have been unblocked by either of the priority changes),
+  // and one state changes (the unblocking).
+  EXPECT_EQ(MEDIUM, throttle1->Priority());
+  EXPECT_EQ(MEDIUM, throttle2->Priority());
+  EXPECT_EQ(MEDIUM, throttle3->Priority());
+  EXPECT_EQ(THROTTLED, throttle4->Priority());
+  EXPECT_FALSE(throttle1->IsBlocked());
+  EXPECT_FALSE(throttle2->IsBlocked());
+  EXPECT_FALSE(throttle3->IsBlocked());
+  EXPECT_TRUE(throttle4->IsBlocked());
+  EXPECT_EQ(1, throttle_state_change_count());
+}
+
+// Check that modifying the priority of a request from a non-THROTTLED
+// value to THROTTLED causes no change in behavior.
+TEST_F(NetworkThrottleManagerTest, ThrottlePriorityResetToThrottled) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(LOW, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  EXPECT_EQ(0, throttle_state_change_count());
+  throttle3->SetPriority(THROTTLED);
+  EXPECT_EQ(0, throttle_state_change_count());
+
+  EXPECT_FALSE(throttle1->IsBlocked());
+  EXPECT_FALSE(throttle2->IsBlocked());
+  EXPECT_FALSE(throttle3->IsBlocked());
+  EXPECT_TRUE(throttle4->IsBlocked());
+
+  EXPECT_EQ(THROTTLED, throttle1->Priority());
+  EXPECT_EQ(THROTTLED, throttle2->Priority());
+  EXPECT_EQ(THROTTLED, throttle3->Priority());
+  EXPECT_EQ(THROTTLED, throttle4->Priority());
+}
+
+// Confirm that old requests don't count against the limit.
+TEST_F(NetworkThrottleManagerTest, DontCountAgedRequests) {
+  const int age_in_days_of_old_throttles = 4;
+
+  // Confirm default median and timing means that 4 days is long enough ago
+  // to be aged out.
+  EXPECT_GT(age_in_days_of_old_throttles * 24 * 60 * 60 * 1000,
+            kInitialAgeHorizonForUncountedRequests);
+
+  SetClockDelta(-base::TimeDelta::FromDays(age_in_days_of_old_throttles));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(IDLE, UNBLOCKED));
+
+  SetClockDelta(base::TimeDelta());
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(LOW, UNBLOCKED));
+
+  // First throttled request should not be blocked.
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+
+  // Second should be.
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle5(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  // Destroying the old requests should not result in any upcalls.
+  EXPECT_EQ(0, throttle_state_change_count());
+  throttle1.reset();
+  base::RunLoop().RunUntilIdle();  // Allow posttasks to run.
+  EXPECT_EQ(0, throttle_state_change_count());
+  throttle2.reset();
+  base::RunLoop().RunUntilIdle();  // Allow posttasks to run.
+  EXPECT_EQ(0, throttle_state_change_count());
+
+  // But destroying a new request should result in a state change.
+  throttle3.reset();
+  base::RunLoop().RunUntilIdle();  // Allow posttasks to run.
+  EXPECT_EQ(1, throttle_state_change_count());
+  EXPECT_EQ(throttle5.get(), last_throttle_to_change_state());
+}
+
+// Confirm that a slew of throttles of a specific age will shift the
+// median for determining "aged requests" to that age.
+TEST_F(NetworkThrottleManagerTest, ShiftMedian) {
+  // Setup two throttles of age *just short* of aging out; confirm
+  // they result in blocked THROTTLED requests.
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(IDLE, UNBLOCKED));
+  SetClockDelta(base::TimeDelta::FromMilliseconds(
+      kInitialAgeHorizonForUncountedRequests - 1));
+  EXPECT_FALSE(throttle_manager()->ConditionallyTriggerTimerForTesting());
+
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  throttle1.reset();
+  throttle2.reset();
+  throttle3.reset();
+  base::RunLoop().RunUntilIdle();  // Allow posttasks to run.
+
+  // Create 100 throttles and destroy them, effectively with lifetime zero.
+  // This should substantially decrease the median age estimate.
+  SetClockDelta(base::TimeDelta());
+  for (int i = 0; i < 100; ++i) {
+    std::unique_ptr<NetworkThrottleManager::Throttle> tmp(
+        CreateThrottle(IDLE, UNBLOCKED));
+  }
+
+  // Clear out any possible leftover timer by setting the clock to a point
+  // in the future at which it will definitely go off, and triggering it.
+  SetClockDelta(base::TimeDelta::FromMilliseconds(
+      2 * kInitialAgeHorizonForUncountedRequests + kAgeHorizonFudgeFactor));
+  throttle_manager()->ConditionallyTriggerTimerForTesting();
+
+  // The identical test above should no longer result in blocked throttles.
+  SetClockDelta(base::TimeDelta());
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle5(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle6(
+      CreateThrottle(IDLE, UNBLOCKED));
+  SetClockDelta(base::TimeDelta::FromMilliseconds(
+      kInitialAgeHorizonForUncountedRequests - 1));
+  EXPECT_TRUE(throttle_manager()->ConditionallyTriggerTimerForTesting());
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle7(
+      CreateThrottle(THROTTLED, UNBLOCKED));
+}
+
+// Confirm that just "aging out" requests will result in unblocking
+// blocked requests.
+TEST_F(NetworkThrottleManagerTest, AgeInvalidThrottles) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  EXPECT_EQ(0, throttle_state_change_count());
+  SetClockDelta(base::TimeDelta::FromMilliseconds(
+      kInitialAgeHorizonForUncountedRequests + kAgeHorizonFudgeFactor));
+  EXPECT_TRUE(throttle_manager()->ConditionallyTriggerTimerForTesting());
+  EXPECT_EQ(1, throttle_state_change_count());
+  EXPECT_EQ(throttle3.get(), last_throttle_to_change_state());
+  EXPECT_FALSE(throttle3->IsBlocked());
+}
+
+// Confirm that if throttles are unblocked and made active by all
+// existing outstanding throttles aging out, they will also eventually
+// age out and let new throttles through.
+TEST_F(NetworkThrottleManagerTest, NewlyUnblockedThrottlesAlsoAge) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle5(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle6(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  // Age the first two throttles out of the outstanding, which should
+  // result in the next two throttles becoming unblocked (and in the
+  // oustanding list).  (The internal implementation will zero out
+  // the outstanding queue and then add in the two new unblocked throttles.)
+  EXPECT_EQ(0, throttle_state_change_count());
+  SetClockDelta(base::TimeDelta::FromMilliseconds(
+      kInitialAgeHorizonForUncountedRequests + kAgeHorizonFudgeFactor));
+  EXPECT_TRUE(throttle_manager()->ConditionallyTriggerTimerForTesting());
+  EXPECT_EQ(2, throttle_state_change_count());
+  EXPECT_FALSE(throttle3->IsBlocked());
+  EXPECT_FALSE(throttle4->IsBlocked());
+
+  // Age the next two throttles out of the outstanding queue, which
+  // should result in the next two throttles becoming unblocked (and
+  // in the oustanding list).  This will only happen if a timer was properly
+  // set in the above age process as the oustanding queue went through
+  // the empty state.
+  SetClockDelta(base::TimeDelta::FromMilliseconds(
+      2 * (kInitialAgeHorizonForUncountedRequests + kAgeHorizonFudgeFactor)));
+  EXPECT_TRUE(throttle_manager()->ConditionallyTriggerTimerForTesting());
+  EXPECT_EQ(4, throttle_state_change_count());
+  EXPECT_FALSE(throttle5->IsBlocked());
+  EXPECT_FALSE(throttle6->IsBlocked());
+}
+
+// Confirm that throttles that are blocked for a while and then
+// unblocked don't "age out".
+TEST_F(NetworkThrottleManagerTest, AgeBlockedThrottles) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle5(
+      CreateThrottle(THROTTLED, BLOCKED));
+
+  EXPECT_EQ(0, throttle_state_change_count());
+  SetClockDelta(base::TimeDelta::FromMilliseconds(
+      kInitialAgeHorizonForUncountedRequests + kAgeHorizonFudgeFactor));
+  EXPECT_TRUE(throttle_manager()->ConditionallyTriggerTimerForTesting());
+
+  // If blocked throttles aged out, all three throttles should have been
+  // unblocked.  If not, only the two replacing the IDLE throttles should
+  // have.
+  EXPECT_EQ(2, throttle_state_change_count());
+}
+
+// Confirm that deleting old throttles before they age out doesn't
+// interfere with the aging out of more recent throttles.
+TEST_F(NetworkThrottleManagerTest, DeletionAgingInterference) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(THROTTLED, BLOCKED));
+  EXPECT_EQ(0, throttle_state_change_count());
+
+  SetClockDelta(base::TimeDelta::FromMilliseconds(
+      kInitialAgeHorizonForUncountedRequests / 2));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle5(
+      CreateThrottle(IDLE, UNBLOCKED));
+  EXPECT_FALSE(throttle_manager()->ConditionallyTriggerTimerForTesting());
+  EXPECT_EQ(0, throttle_state_change_count());
+
+  throttle1.reset();
+  throttle2.reset();
+  EXPECT_FALSE(throttle_manager()->ConditionallyTriggerTimerForTesting());
+  EXPECT_EQ(0, throttle_state_change_count());
+
+  SetClockDelta(base::TimeDelta::FromMilliseconds(
+      (3 * kInitialAgeHorizonForUncountedRequests / 2 +
+       2 * kAgeHorizonFudgeFactor)));
+  EXPECT_TRUE(throttle_manager()->ConditionallyTriggerTimerForTesting());
+  EXPECT_EQ(1, throttle_state_change_count());
+  EXPECT_EQ(throttle3.get(), last_throttle_to_change_state());
+  EXPECT_FALSE(throttle3->IsBlocked());
+}
+
+// Confirm that "ignore_limits" boolean is respected.
+TEST_F(NetworkThrottleManagerTest, IgnoreLimits) {
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle1(
+      CreateThrottle(HIGHEST, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle2(
+      CreateThrottle(LOW, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle3(
+      CreateThrottle(IDLE, UNBLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle4(
+      CreateThrottle(THROTTLED, BLOCKED));
+  std::unique_ptr<NetworkThrottleManager::Throttle> throttle5(
+      CreateThrottleIgnoringLimits(THROTTLED));
+}
+
+}  // namespace
+
+}  // namespace net