Mojo: Armed Watchers

mojo/edk/system/watcher_unittest.cc

Index: mojo/edk/system/watcher_unittest.cc
diff --git a/mojo/edk/system/watcher_unittest.cc b/mojo/edk/system/watcher_unittest.cc
new file mode 100644
index 0000000000000000000000000000000000000000..a3f27424321d55a6e225da26dce44dff3ec9c1f3
--- /dev/null
+++ b/mojo/edk/system/watcher_unittest.cc
@@ -0,0 +1,1590 @@
+// Copyright 2017 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 <stdint.h>
+
+#include <map>
+#include <memory>
+
+#include "base/bind.h"
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/ptr_util.h"
+#include "base/synchronization/waitable_event.h"
+#include "base/threading/platform_thread.h"
+#include "base/threading/simple_thread.h"
+#include "base/time/time.h"
+#include "mojo/edk/test/mojo_test_base.h"
+#include "mojo/public/c/system/data_pipe.h"
+#include "mojo/public/c/system/types.h"
+#include "mojo/public/c/system/watcher.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace mojo {
+namespace edk {
+namespace {
+
+using WatcherTest = test::MojoTestBase;
+
+class WatchHelper {
+ public:
+  using ContextCallback =
+      base::Callback<void(MojoResult, MojoHandleSignalsState)>;
+
+  WatchHelper() {}
+  ~WatchHelper() {}
+
+  MojoResult CreateWatcher(MojoHandle* handle) {
+    return MojoCreateWatcher(&Notify, handle);
+  }
+
+  uintptr_t CreateContext(const ContextCallback& callback) {
+    return CreateContextWithCancel(callback, base::Closure());
+  }
+
+  uintptr_t CreateContextWithCancel(const ContextCallback& callback,
+                                    const base::Closure& cancel_callback) {
+    auto context = base::MakeUnique<NotificationContext>(callback);
+    NotificationContext* raw_context = context.get();
+    raw_context->SetCancelCallback(base::Bind(
+        [](std::unique_ptr<NotificationContext> context,
+           const base::Closure& cancel_callback) {
+          if (cancel_callback)
+            cancel_callback.Run();
+        },
+        base::Passed(&context), cancel_callback));
+    return reinterpret_cast<uintptr_t>(raw_context);
+  }
+
+ private:
+  class NotificationContext {
+   public:
+    explicit NotificationContext(const ContextCallback& callback)
+        : callback_(callback) {}
+
+    ~NotificationContext() {}
+
+    void SetCancelCallback(const base::Closure& cancel_callback) {
+      cancel_callback_ = cancel_callback;
+    }
+
+    void Notify(MojoResult result, MojoHandleSignalsState state) {
+      if (result == MOJO_RESULT_CANCELLED)
+        cancel_callback_.Run();
+      else
+        callback_.Run(result, state);
+    }
+
+   private:
+    const ContextCallback callback_;
+    base::Closure cancel_callback_;
+
+    DISALLOW_COPY_AND_ASSIGN(NotificationContext);
+  };
+
+  static void Notify(uintptr_t context,
+                     MojoResult result,
+                     MojoHandleSignalsState state,
+                     MojoWatcherNotificationFlags flags) {
+    reinterpret_cast<NotificationContext*>(context)->Notify(result, state);
+  }
+
+  DISALLOW_COPY_AND_ASSIGN(WatchHelper);
+};
+
+class ThreadedRunner : public base::SimpleThread {
+ public:
+  explicit ThreadedRunner(const base::Closure& callback)
+      : SimpleThread("ThreadedRunner"), callback_(callback) {}
+  ~ThreadedRunner() override {}
+
+  void Run() override { callback_.Run(); }
+
+ private:
+  const base::Closure callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(ThreadedRunner);
+};
+
+void ExpectNoNotification(uintptr_t context,
+                          MojoResult result,
+                          MojoHandleSignalsState state,
+                          MojoWatcherNotificationFlags flags) {
+  NOTREACHED();
+}
+
+void ExpectOnlyCancel(uintptr_t context,
+                      MojoResult result,
+                      MojoHandleSignalsState state,
+                      MojoWatcherNotificationFlags flags) {
+  EXPECT_EQ(result, MOJO_RESULT_CANCELLED);
+}
+
+TEST_F(WatcherTest, InvalidArguments) {
+  EXPECT_EQ(MOJO_RESULT_INVALID_ARGUMENT,
+            MojoCreateWatcher(&ExpectNoNotification, nullptr));
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, MojoCreateWatcher(&ExpectNoNotification, &w));
+
+  // Try to watch unwatchable handles.
+  EXPECT_EQ(MOJO_RESULT_INVALID_ARGUMENT,
+            MojoWatch(w, w, MOJO_HANDLE_SIGNAL_READABLE, 0));
+  MojoHandle buffer_handle = CreateBuffer(42);
+  EXPECT_EQ(MOJO_RESULT_INVALID_ARGUMENT,
+            MojoWatch(w, buffer_handle, MOJO_HANDLE_SIGNAL_READABLE, 0));
+
+  // Try to cancel a watch on an invalid watcher handle.
+  EXPECT_EQ(MOJO_RESULT_INVALID_ARGUMENT, MojoCancelWatch(buffer_handle, 0));
+
+  // Try to arm an invalid handle.
+  EXPECT_EQ(
+      MOJO_RESULT_INVALID_ARGUMENT,
+      MojoArmWatcher(MOJO_HANDLE_INVALID, nullptr, nullptr, nullptr, nullptr));
+  EXPECT_EQ(MOJO_RESULT_INVALID_ARGUMENT,
+            MojoArmWatcher(buffer_handle, nullptr, nullptr, nullptr, nullptr));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(buffer_handle));
+
+  // Try to arm with a non-null count but at least one null output buffer.
+  uint32_t num_ready_contexts = 1;
+  uintptr_t ready_context;
+  MojoResult ready_result;
+  MojoHandleSignalsState ready_state;
+  EXPECT_EQ(MOJO_RESULT_INVALID_ARGUMENT,
+            MojoArmWatcher(w, &num_ready_contexts, nullptr, &ready_result,
+                           &ready_state));
+  EXPECT_EQ(MOJO_RESULT_INVALID_ARGUMENT,
+            MojoArmWatcher(w, &num_ready_contexts, &ready_context, nullptr,
+                           &ready_state));
+  EXPECT_EQ(MOJO_RESULT_INVALID_ARGUMENT,
+            MojoArmWatcher(w, &num_ready_contexts, &ready_context,
+                           &ready_result, nullptr));
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+}
+
+TEST_F(WatcherTest, WatchMessagePipeReadable) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  int num_expected_notifications = 1;
+  const uintptr_t readable_a_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, int* expected_count, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_GT(*expected_count, 0);
+        *expected_count -= 1;
+
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        event->Signal();
+      },
+      &event, &num_expected_notifications));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  const char kMessage1[] = "hey hey hey hey";
+  const char kMessage2[] = "i said hey";
+  const char kMessage3[] = "what's goin' on?";
+
+  // Writing to |b| multiple times should notify exactly once.
+  WriteMessage(b, kMessage1);
+  WriteMessage(b, kMessage2);
+  event.Wait();
+
+  // This also shouldn't fire a notification; the watcher is still disarmed.
+  WriteMessage(b, kMessage3);
+
+  // Arming should fail with relevant information.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = kMaxReadyContexts;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(readable_a_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+
+  // Flush the three messages from above.
+  EXPECT_EQ(kMessage1, ReadMessage(a));
+  EXPECT_EQ(kMessage2, ReadMessage(a));
+  EXPECT_EQ(kMessage3, ReadMessage(a));
+
+  // Now we can rearm the watcher.
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+}
+
+TEST_F(WatcherTest, CloseWatchedMessagePipeHandle) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  const uintptr_t readable_a_context = helper.CreateContextWithCancel(
+      WatchHelper::ContextCallback(),
+      base::Bind([](base::WaitableEvent* event) { event->Signal(); }, &event));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+
+  // Test that closing a watched handle fires an appropriate notification, even
+  // when the watcher is unarmed.
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+  event.Wait();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+}
+
+TEST_F(WatcherTest, CloseWatchedMessagePipeHandlePeer) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  const uintptr_t readable_a_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION, result);
+        event->Signal();
+      },
+      &event));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+
+  // Test that closing a watched handle's peer with an armed watcher fires an
+  // appropriate notification.
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+  event.Wait();
+
+  // And now arming should fail with correct information about |a|'s state.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = kMaxReadyContexts;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(readable_a_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION, ready_results[0]);
+  EXPECT_TRUE(ready_states[0].satisfied_signals &
+              MOJO_HANDLE_SIGNAL_PEER_CLOSED);
+  EXPECT_FALSE(ready_states[0].satisfiable_signals &
+               MOJO_HANDLE_SIGNAL_READABLE);
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+}
+
+TEST_F(WatcherTest, WatchDataPipeConsumerReadable) {
+  constexpr size_t kTestPipeCapacity = 64;
+  MojoHandle producer, consumer;
+  CreateDataPipe(&producer, &consumer, kTestPipeCapacity);
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  int num_expected_notifications = 1;
+  const uintptr_t readable_consumer_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, int* expected_count, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_GT(*expected_count, 0);
+        *expected_count -= 1;
+
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        event->Signal();
+      },
+      &event, &num_expected_notifications));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(w, consumer, MOJO_HANDLE_SIGNAL_READABLE,
+                                      readable_consumer_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  const char kMessage1[] = "hey hey hey hey";
+  const char kMessage2[] = "i said hey";
+  const char kMessage3[] = "what's goin' on?";
+
+  // Writing to |producer| multiple times should notify exactly once.
+  WriteData(producer, kMessage1);
+  WriteData(producer, kMessage2);
+  event.Wait();
+
+  // This also shouldn't fire a notification; the watcher is still disarmed.
+  WriteData(producer, kMessage3);
+
+  // Arming should fail with relevant information.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = kMaxReadyContexts;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(readable_consumer_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+
+  // Flush the three messages from above.
+  EXPECT_EQ(kMessage1, ReadData(consumer, sizeof(kMessage1) - 1));
+  EXPECT_EQ(kMessage2, ReadData(consumer, sizeof(kMessage2) - 1));
+  EXPECT_EQ(kMessage3, ReadData(consumer, sizeof(kMessage3) - 1));
+
+  // Now we can rearm the watcher.
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(producer));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(consumer));
+}
+
+TEST_F(WatcherTest, WatchDataPipeConsumerNewDataReadable) {
+  constexpr size_t kTestPipeCapacity = 64;
+  MojoHandle producer, consumer;
+  CreateDataPipe(&producer, &consumer, kTestPipeCapacity);
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  int num_new_data_notifications = 0;
+  const uintptr_t new_data_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, int* notification_count, MojoResult result,
+         MojoHandleSignalsState state) {
+        *notification_count += 1;
+
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        event->Signal();
+      },
+      &event, &num_new_data_notifications));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, consumer, MOJO_HANDLE_SIGNAL_NEW_DATA_READABLE,
+                      new_data_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  const char kMessage1[] = "hey hey hey hey";
+  const char kMessage2[] = "i said hey";
+  const char kMessage3[] = "what's goin' on?";
+
+  // Writing to |producer| multiple times should notify exactly once.
+  WriteData(producer, kMessage1);
+  WriteData(producer, kMessage2);
+  event.Wait();
+
+  // This also shouldn't fire a notification; the watcher is still disarmed.
+  WriteData(producer, kMessage3);
+
+  // Arming should fail with relevant information.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = kMaxReadyContexts;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(new_data_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+
+  // Attempt to read more data than is available. Should fail but clear the
+  // NEW_DATA_READABLE signal.
+  char large_buffer[512];
+  uint32_t large_read_size = 512;
+  EXPECT_EQ(MOJO_RESULT_OUT_OF_RANGE,
+            MojoReadData(consumer, large_buffer, &large_read_size,
+                         MOJO_READ_DATA_FLAG_ALL_OR_NONE));
+
+  // Attempt to arm again. Should succeed.
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  // Write more data. Should notify.
+  event.Reset();
+  WriteData(producer, kMessage1);
+  event.Wait();
+
+  // Reading some data should clear NEW_DATA_READABLE again so we can rearm.
+  EXPECT_EQ(kMessage1, ReadData(consumer, sizeof(kMessage1) - 1));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  EXPECT_EQ(2, num_new_data_notifications);
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(producer));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(consumer));
+}
+
+TEST_F(WatcherTest, WatchDataPipeProducerWritable) {
+  constexpr size_t kTestPipeCapacity = 8;
+  MojoHandle producer, consumer;
+  CreateDataPipe(&producer, &consumer, kTestPipeCapacity);
+
+  // Half the capacity of the data pipe.
+  const char kTestData[] = "aaaa";
+  static_assert((sizeof(kTestData) - 1) * 2 == kTestPipeCapacity,
+                "Invalid test data for this test.");
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  int num_expected_notifications = 1;
+  const uintptr_t writable_producer_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, int* expected_count, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_GT(*expected_count, 0);
+        *expected_count -= 1;
+
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        event->Signal();
+      },
+      &event, &num_expected_notifications));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(w, producer, MOJO_HANDLE_SIGNAL_WRITABLE,
+                                      writable_producer_context));
+
+  // The producer is already writable, so arming should fail with relevant
+  // information.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = kMaxReadyContexts;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(writable_producer_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+  EXPECT_TRUE(ready_states[0].satisfied_signals & MOJO_HANDLE_SIGNAL_WRITABLE);
+
+  // Write some data, but don't fill the pipe yet. Arming should fail again.
+  WriteData(producer, kTestData);
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(writable_producer_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+  EXPECT_TRUE(ready_states[0].satisfied_signals & MOJO_HANDLE_SIGNAL_WRITABLE);
+
+  // Write more data, filling the pipe to capacity. Arming should succeed now.
+  WriteData(producer, kTestData);
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  // Now read from the pipe, making the producer writable again. Should notify.
+  EXPECT_EQ(kTestData, ReadData(consumer, sizeof(kTestData) - 1));
+  event.Wait();
+
+  // Arming should fail again.
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(writable_producer_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+  EXPECT_TRUE(ready_states[0].satisfied_signals & MOJO_HANDLE_SIGNAL_WRITABLE);
+
+  // Fill the pipe once more and arm the watcher. Should succeed.
+  WriteData(producer, kTestData);
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(producer));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(consumer));
+};
+
+TEST_F(WatcherTest, CloseWatchedDataPipeConsumerHandle) {
+  constexpr size_t kTestPipeCapacity = 8;
+  MojoHandle producer, consumer;
+  CreateDataPipe(&producer, &consumer, kTestPipeCapacity);
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  const uintptr_t readable_consumer_context = helper.CreateContextWithCancel(
+      WatchHelper::ContextCallback(),
+      base::Bind([](base::WaitableEvent* event) { event->Signal(); }, &event));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(w, consumer, MOJO_HANDLE_SIGNAL_READABLE,
+                                      readable_consumer_context));
+
+  // Closing the consumer should fire a cancellation notification.
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(consumer));
+  event.Wait();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(producer));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+}
+
+TEST_F(WatcherTest, CloseWatcherDataPipeConsumerHandlePeer) {
+  constexpr size_t kTestPipeCapacity = 8;
+  MojoHandle producer, consumer;
+  CreateDataPipe(&producer, &consumer, kTestPipeCapacity);
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  const uintptr_t readable_consumer_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION, result);
+        event->Signal();
+      },
+      &event));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(w, consumer, MOJO_HANDLE_SIGNAL_READABLE,
+                                      readable_consumer_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  // Closing the producer should fire a notification for an unsatisfiable watch.
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(producer));
+  event.Wait();
+
+  // Now attempt to rearm and expect appropriate error feedback.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = kMaxReadyContexts;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(readable_consumer_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION, ready_results[0]);
+  EXPECT_FALSE(ready_states[0].satisfiable_signals &
+               MOJO_HANDLE_SIGNAL_READABLE);
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(consumer));
+}
+
+TEST_F(WatcherTest, CloseWatchedDataPipeProducerHandle) {
+  constexpr size_t kTestPipeCapacity = 8;
+  MojoHandle producer, consumer;
+  CreateDataPipe(&producer, &consumer, kTestPipeCapacity);
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  const uintptr_t writable_producer_context = helper.CreateContextWithCancel(
+      WatchHelper::ContextCallback(),
+      base::Bind([](base::WaitableEvent* event) { event->Signal(); }, &event));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(w, producer, MOJO_HANDLE_SIGNAL_WRITABLE,
+                                      writable_producer_context));
+
+  // Closing the consumer should fire a cancellation notification.
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(producer));
+  event.Wait();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(consumer));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+}
+
+TEST_F(WatcherTest, CloseWatchedDataPipeProducerHandlePeer) {
+  constexpr size_t kTestPipeCapacity = 8;
+  MojoHandle producer, consumer;
+  CreateDataPipe(&producer, &consumer, kTestPipeCapacity);
+
+  const char kTestMessageFullCapacity[] = "xxxxxxxx";
+  static_assert(sizeof(kTestMessageFullCapacity) - 1 == kTestPipeCapacity,
+                "Invalid test message size for this test.");
+
+  // Make the pipe unwritable initially.
+  WriteData(producer, kTestMessageFullCapacity);
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  const uintptr_t writable_producer_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION, result);
+        event->Signal();
+      },
+      &event));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(w, producer, MOJO_HANDLE_SIGNAL_WRITABLE,
+                                      writable_producer_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  // Closing the consumer should fire a notification for an unsatisfiable watch,
+  // as the full data pipe can never be read from again and is therefore
+  // permanently full and unwritable.
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(consumer));
+  event.Wait();
+
+  // Now attempt to rearm and expect appropriate error feedback.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = kMaxReadyContexts;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(writable_producer_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION, ready_results[0]);
+  EXPECT_FALSE(ready_states[0].satisfiable_signals &
+               MOJO_HANDLE_SIGNAL_WRITABLE);
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(producer));
+}
+
+TEST_F(WatcherTest, ArmWithNoWatches) {
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, MojoCreateWatcher(&ExpectNoNotification, &w));
+  EXPECT_EQ(MOJO_RESULT_NOT_FOUND,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+}
+
+TEST_F(WatcherTest, WatchDuplicateContext) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, MojoCreateWatcher(&ExpectOnlyCancel, &w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, 0));
+  EXPECT_EQ(MOJO_RESULT_ALREADY_EXISTS,
+            MojoWatch(w, b, MOJO_HANDLE_SIGNAL_READABLE, 0));
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+}
+
+TEST_F(WatcherTest, CancelUnknownWatch) {
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, MojoCreateWatcher(&ExpectNoNotification, &w));
+  EXPECT_EQ(MOJO_RESULT_NOT_FOUND, MojoCancelWatch(w, 1234));
+}
+
+TEST_F(WatcherTest, ArmWithWatchAlreadySatisfied) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, MojoCreateWatcher(&ExpectOnlyCancel, &w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(w, a, MOJO_HANDLE_SIGNAL_WRITABLE, 0));
+
+  // |a| is always writable, so we can never arm this watcher.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = kMaxReadyContexts;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(0u, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+  EXPECT_TRUE(ready_states[0].satisfied_signals & MOJO_HANDLE_SIGNAL_WRITABLE);
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+}
+
+TEST_F(WatcherTest, ArmWithWatchAlreadyUnsatisfiable) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, MojoCreateWatcher(&ExpectOnlyCancel, &w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, 0));
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+
+  // |b| is closed and never wrote any messages, so |a| won't be readable again.
+  // MojoArmWatcher() should fail, incidcating as much.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = kMaxReadyContexts;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(0u, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION, ready_results[0]);
+  EXPECT_TRUE(ready_states[0].satisfied_signals &
+              MOJO_HANDLE_SIGNAL_PEER_CLOSED);
+  EXPECT_FALSE(ready_states[0].satisfiable_signals &
+               MOJO_HANDLE_SIGNAL_READABLE);
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+}
+
+TEST_F(WatcherTest, MultipleWatches) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  base::WaitableEvent a_event(base::WaitableEvent::ResetPolicy::MANUAL,
+                              base::WaitableEvent::InitialState::NOT_SIGNALED);
+  base::WaitableEvent b_event(base::WaitableEvent::ResetPolicy::MANUAL,
+                              base::WaitableEvent::InitialState::NOT_SIGNALED);
+  WatchHelper helper;
+  int num_a_notifications = 0;
+  int num_b_notifications = 0;
+  auto notify_callback =
+      base::Bind([](base::WaitableEvent* event, int* notification_count,
+                    MojoResult result, MojoHandleSignalsState state) {
+        *notification_count += 1;
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        event->Signal();
+      });
+  uintptr_t readable_a_context = helper.CreateContext(
+      base::Bind(notify_callback, &a_event, &num_a_notifications));
+  uintptr_t readable_b_context = helper.CreateContext(
+      base::Bind(notify_callback, &b_event, &num_b_notifications));
+
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  // Add two independent watch contexts to watch for |a| or |b| readability.
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, b, MOJO_HANDLE_SIGNAL_READABLE, readable_b_context));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  const char kMessage1[] = "things are happening";
+  const char kMessage2[] = "ok. ok. ok. ok.";
+  const char kMessage3[] = "plz wake up";
+
+  // Writing to |b| should signal |a|'s watch.
+  WriteMessage(b, kMessage1);
+  a_event.Wait();
+  a_event.Reset();
+
+  // Subsequent messages on |b| should not trigger another notification.
+  WriteMessage(b, kMessage2);
+  WriteMessage(b, kMessage3);
+
+  // Messages on |a| also shouldn't trigger |b|'s notification, since the
+  // watcher should be disarmed by now.
+  WriteMessage(a, kMessage1);
+  WriteMessage(a, kMessage2);
+  WriteMessage(a, kMessage3);
+
+  // Arming should fail. Since we only ask for at most one context's information
+  // that's all we should get back. Which one we get is unspecified.
+  constexpr size_t kMaxReadyContexts = 10;
+  uint32_t num_ready_contexts = 1;
+  uintptr_t ready_contexts[kMaxReadyContexts];
+  MojoResult ready_results[kMaxReadyContexts];
+  MojoHandleSignalsState ready_states[kMaxReadyContexts];
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_TRUE(ready_contexts[0] == readable_a_context ||
+              ready_contexts[0] == readable_b_context);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+  EXPECT_TRUE(ready_states[0].satisfied_signals & MOJO_HANDLE_SIGNAL_WRITABLE);
+
+  // Now try arming again, verifying that both contexts are returned.
+  num_ready_contexts = kMaxReadyContexts;
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(2u, num_ready_contexts);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[1]);
+  EXPECT_TRUE(ready_states[0].satisfied_signals & MOJO_HANDLE_SIGNAL_WRITABLE);
+  EXPECT_TRUE(ready_states[1].satisfied_signals & MOJO_HANDLE_SIGNAL_WRITABLE);
+  EXPECT_TRUE((ready_contexts[0] == readable_a_context &&
+               ready_contexts[1] == readable_b_context) ||
+              (ready_contexts[0] == readable_b_context &&
+               ready_contexts[1] == readable_a_context));
+
+  // Flush out the test messages so we should be able to successfully rearm.
+  EXPECT_EQ(kMessage1, ReadMessage(a));
+  EXPECT_EQ(kMessage2, ReadMessage(a));
+  EXPECT_EQ(kMessage3, ReadMessage(a));
+  EXPECT_EQ(kMessage1, ReadMessage(b));
+  EXPECT_EQ(kMessage2, ReadMessage(b));
+  EXPECT_EQ(kMessage3, ReadMessage(b));
+
+  // Add a watch which is always satisfied, so we can't arm. Arming should fail
+  // with only this new watch's information.
+  uintptr_t writable_c_context = helper.CreateContext(base::Bind(
+      [](MojoResult result, MojoHandleSignalsState state) { NOTREACHED(); }));
+  MojoHandle c, d;
+  CreateMessagePipe(&c, &d);
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, c, MOJO_HANDLE_SIGNAL_WRITABLE, writable_c_context));
+  num_ready_contexts = kMaxReadyContexts;
+  EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION,
+            MojoArmWatcher(w, &num_ready_contexts, ready_contexts,
+                           ready_results, ready_states));
+  EXPECT_EQ(1u, num_ready_contexts);
+  EXPECT_EQ(writable_c_context, ready_contexts[0]);
+  EXPECT_EQ(MOJO_RESULT_OK, ready_results[0]);
+  EXPECT_TRUE(ready_states[0].satisfied_signals & MOJO_HANDLE_SIGNAL_WRITABLE);
+
+  // Cancel the new watch and arming should succeed once again.
+  EXPECT_EQ(MOJO_RESULT_OK, MojoCancelWatch(w, writable_c_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(c));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(d));
+}
+
+TEST_F(WatcherTest, NotifyOtherFromNotificationCallback) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  static const char kTestMessageToA[] = "hello a";
+  static const char kTestMessageToB[] = "hello b";
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  WatchHelper helper;
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  uintptr_t readable_a_context = helper.CreateContext(base::Bind(
+      [](MojoHandle w, MojoHandle a, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        EXPECT_EQ("hello a", ReadMessage(a));
+
+        // Re-arm the watcher and signal |b|.
+        EXPECT_EQ(MOJO_RESULT_OK,
+                  MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+        WriteMessage(a, kTestMessageToB);
+      },
+      w, a));
+
+  uintptr_t readable_b_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, MojoHandle w, MojoHandle b,
+         MojoResult result, MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        EXPECT_EQ(kTestMessageToB, ReadMessage(b));
+        EXPECT_EQ(MOJO_RESULT_OK,
+                  MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+        event->Signal();
+      },
+      &event, w, b));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, b, MOJO_HANDLE_SIGNAL_READABLE, readable_b_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  // Send a message to |a|. The relevant watch context should be notified, and
+  // should in turn send a message to |b|, waking up the other context. The
+  // second context signals |event|.
+  WriteMessage(b, kTestMessageToA);
+  event.Wait();
+}
+
+TEST_F(WatcherTest, NotifySelfFromNotificationCallback) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  static const char kTestMessageToA[] = "hello a";
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  WatchHelper helper;
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  int expected_notifications = 10;
+  uintptr_t readable_a_context = helper.CreateContext(base::Bind(
+      [](int* expected_count, MojoHandle w, MojoHandle a, MojoHandle b,
+         base::WaitableEvent* event, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        EXPECT_EQ("hello a", ReadMessage(a));
+
+        EXPECT_GT(*expected_count, 0);
+        *expected_count -= 1;
+        if (*expected_count == 0) {
+          event->Signal();
+          return;
+        } else {
+          // Re-arm the watcher and signal |a| again.
+          EXPECT_EQ(MOJO_RESULT_OK,
+                    MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+          WriteMessage(b, kTestMessageToA);
+        }
+      },
+      &expected_notifications, w, a, b, &event));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  // Send a message to |a|. When the watch above is notified, it will rearm and
+  // send another message to |a|. This will happen until
+  // |expected_notifications| reaches 0.
+  WriteMessage(b, kTestMessageToA);
+  event.Wait();
+}
+
+TEST_F(WatcherTest, ImplicitCancelOtherFromNotificationCallback) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  MojoHandle c, d;
+  CreateMessagePipe(&c, &d);
+
+  static const char kTestMessageToA[] = "hi a";
+  static const char kTestMessageToC[] = "hi c";
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  WatchHelper helper;
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  uintptr_t readable_a_context = helper.CreateContextWithCancel(
+      base::Bind([](MojoResult result, MojoHandleSignalsState state) {
+        NOTREACHED();
+      }),
+      base::Bind([](base::WaitableEvent* event) { event->Signal(); }, &event));
+
+  uintptr_t readable_c_context = helper.CreateContext(base::Bind(
+      [](MojoHandle w, MojoHandle a, MojoHandle b, MojoHandle c,
+         MojoResult result, MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        EXPECT_EQ(kTestMessageToC, ReadMessage(c));
+
+        // Now rearm the watcher.
+        EXPECT_EQ(MOJO_RESULT_OK,
+                  MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+        // Must result in exactly ONE notification on the above context, for
+        // CANCELLED only. Because we cannot dispatch notifications until the
+        // stack unwinds, and because we must never dispatch non-cancellation
+        // notifications for a handle once it's been closed, we must be certain
+        // that cancellation due to closure preemptively invalidates any
+        // pending non-cancellation notifications queued on the current
+        // RequestContext, such as the one resulting from the WriteMessage here.
+        WriteMessage(b, kTestMessageToA);
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+
+        // Rearming should be fine since |a|'s watch should already be
+        // implicitly cancelled (even though the notification will not have
+        // been invoked yet.)
+        EXPECT_EQ(MOJO_RESULT_OK,
+                  MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+        // Nothing interesting should happen as a result of this.
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+      },
+      w, a, b, c));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, c, MOJO_HANDLE_SIGNAL_READABLE, readable_c_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  WriteMessage(d, kTestMessageToC);
+  event.Wait();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(c));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(d));
+}
+
+TEST_F(WatcherTest, ExplicitCancelOtherFromNotificationCallback) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  MojoHandle c, d;
+  CreateMessagePipe(&c, &d);
+
+  static const char kTestMessageToA[] = "hi a";
+  static const char kTestMessageToC[] = "hi c";
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  WatchHelper helper;
+  MojoHandle w;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  uintptr_t readable_a_context = helper.CreateContext(base::Bind(
+      [](MojoResult result, MojoHandleSignalsState state) { NOTREACHED(); }));
+
+  uintptr_t readable_c_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, uintptr_t readable_a_context, MojoHandle w,
+         MojoHandle a, MojoHandle b, MojoHandle c, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        EXPECT_EQ(kTestMessageToC, ReadMessage(c));
+
+        // Now rearm the watcher.
+        EXPECT_EQ(MOJO_RESULT_OK,
+                  MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+        // Should result in no notifications on the above context, because the
+        // watch will have been cancelled by the time the notification callback
+        // can execute.
+        WriteMessage(b, kTestMessageToA);
+        WriteMessage(b, kTestMessageToA);
+        EXPECT_EQ(MOJO_RESULT_OK, MojoCancelWatch(w, readable_a_context));
+
+        // Rearming should be fine now.
+        EXPECT_EQ(MOJO_RESULT_OK,
+                  MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+        // Nothing interesting should happen as a result of these.
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+
+        event->Signal();
+      },
+      &event, readable_a_context, w, a, b, c));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, c, MOJO_HANDLE_SIGNAL_READABLE, readable_c_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  WriteMessage(d, kTestMessageToC);
+  event.Wait();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(c));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(d));
+}
+
+TEST_F(WatcherTest, NestedCancellation) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  MojoHandle c, d;
+  CreateMessagePipe(&c, &d);
+
+  static const char kTestMessageToA[] = "hey a";
+  static const char kTestMessageToC[] = "hey c";
+  static const char kTestMessageToD[] = "hey d";
+
+  // This is a tricky test. It establishes a watch on |b| using one watcher and
+  // watches on |c| and |d| using another watcher.
+  //
+  // A message is written to |d| to wake up |c|'s watch, and the notification
+  // handler for that event does the following:
+  //   1. Writes to |a| to eventually wake up |b|'s watcher.
+  //   2. Rearms |c|'s watcher.
+  //   3. Writes to |d| to eventually wake up |c|'s watcher again.
+  //
+  // Meanwhile, |b|'s watch notification handler cancels |c|'s watch altogether
+  // before writing to |c| to wake up |d|.
+  //
+  // The net result should be that |c|'s context only gets notified once (from
+  // the first write to |d| above) and everyone else gets notified as expected.
+
+  MojoHandle b_watcher;
+  MojoHandle cd_watcher;
+  WatchHelper helper;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&b_watcher));
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&cd_watcher));
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  uintptr_t readable_d_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, MojoHandle d, MojoResult result,
+         MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        EXPECT_EQ(kTestMessageToD, ReadMessage(d));
+        event->Signal();
+      },
+      &event, d));
+
+  static int num_expected_c_notifications = 1;
+  uintptr_t readable_c_context = helper.CreateContext(base::Bind(
+      [](MojoHandle cd_watcher, MojoHandle a, MojoHandle c, MojoHandle d,
+         MojoResult result, MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        EXPECT_GT(num_expected_c_notifications--, 0);
+
+        // Trigger an eventual |readable_b_context| notification.
+        WriteMessage(a, kTestMessageToA);
+
+        EXPECT_EQ(kTestMessageToC, ReadMessage(c));
+        EXPECT_EQ(MOJO_RESULT_OK, MojoArmWatcher(cd_watcher, nullptr, nullptr,
+                                                 nullptr, nullptr));
+
+        // Trigger another eventual |readable_c_context| notification.
+        WriteMessage(d, kTestMessageToC);
+      },
+      cd_watcher, a, c, d));
+
+  uintptr_t readable_b_context = helper.CreateContext(base::Bind(
+      [](MojoHandle cd_watcher, uintptr_t readable_c_context, MojoHandle c,
+         MojoResult result, MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK,
+                  MojoCancelWatch(cd_watcher, readable_c_context));
+
+        EXPECT_EQ(MOJO_RESULT_OK, MojoArmWatcher(cd_watcher, nullptr, nullptr,
+                                                 nullptr, nullptr));
+
+        WriteMessage(c, kTestMessageToD);
+      },
+      cd_watcher, readable_c_context, c));
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(b_watcher, b, MOJO_HANDLE_SIGNAL_READABLE,
+                                      readable_b_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(cd_watcher, c, MOJO_HANDLE_SIGNAL_READABLE,
+                      readable_c_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(cd_watcher, d, MOJO_HANDLE_SIGNAL_READABLE,
+                      readable_d_context));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(b_watcher, nullptr, nullptr, nullptr, nullptr));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(cd_watcher, nullptr, nullptr, nullptr, nullptr));
+
+  WriteMessage(d, kTestMessageToC);
+  event.Wait();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(cd_watcher));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b_watcher));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(c));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(d));
+}
+
+TEST_F(WatcherTest, CancelSelfInNotificationCallback) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  static const char kTestMessageToA[] = "hey a";
+
+  MojoHandle w;
+  WatchHelper helper;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  static uintptr_t readable_a_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, MojoHandle w, MojoHandle a,
+         MojoResult result, MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+
+        // There should be no problem cancelling this watch from its own
+        // notification invocation.
+        EXPECT_EQ(MOJO_RESULT_OK, MojoCancelWatch(w, readable_a_context));
+        EXPECT_EQ(kTestMessageToA, ReadMessage(a));
+
+        // Arming should fail because there are no longer any registered
+        // watches on the watcher.
+        EXPECT_EQ(MOJO_RESULT_NOT_FOUND,
+                  MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+        // And closing |a| should be fine (and should not invoke this
+        // notification with MOJO_RESULT_CANCELLED) for the same reason.
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+
+        event->Signal();
+      },
+      &event, w, a));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  WriteMessage(b, kTestMessageToA);
+  event.Wait();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+}
+
+TEST_F(WatcherTest, CloseWatcherInNotificationCallback) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  static const char kTestMessageToA1[] = "hey a";
+  static const char kTestMessageToA2[] = "hey a again";
+
+  MojoHandle w;
+  WatchHelper helper;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  uintptr_t readable_a_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, MojoHandle w, MojoHandle a, MojoHandle b,
+         MojoResult result, MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        EXPECT_EQ(kTestMessageToA1, ReadMessage(a));
+        EXPECT_EQ(MOJO_RESULT_OK,
+                  MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+        // There should be no problem closing this watcher from its own
+        // notification callback.
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+
+        // And these should not trigger more notifications, because |w| has been
+        // closed already.
+        WriteMessage(b, kTestMessageToA2);
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+
+        event->Signal();
+      },
+      &event, w, a, b));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  WriteMessage(b, kTestMessageToA1);
+  event.Wait();
+}
+
+TEST_F(WatcherTest, CloseWatcherAfterImplicitCancel) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  static const char kTestMessageToA[] = "hey a";
+
+  MojoHandle w;
+  WatchHelper helper;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  uintptr_t readable_a_context = helper.CreateContext(base::Bind(
+      [](base::WaitableEvent* event, MojoHandle w, MojoHandle a,
+         MojoResult result, MojoHandleSignalsState state) {
+        EXPECT_EQ(MOJO_RESULT_OK, result);
+        EXPECT_EQ(kTestMessageToA, ReadMessage(a));
+        EXPECT_EQ(MOJO_RESULT_OK,
+                  MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+        // This will cue up a notification for |MOJO_RESULT_CANCELLED|...
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+
+        // ...but it should never fire because we close the watcher here.
+        EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+
+        event->Signal();
+      },
+      &event, w, a));
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  WriteMessage(b, kTestMessageToA);
+  event.Wait();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+}
+
+TEST_F(WatcherTest, OtherThreadCancelDuringNotification) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  static const char kTestMessageToA[] = "hey a";
+
+  MojoHandle w;
+  WatchHelper helper;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  base::WaitableEvent wait_for_notification(
+      base::WaitableEvent::ResetPolicy::MANUAL,
+      base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  base::WaitableEvent wait_for_cancellation(
+      base::WaitableEvent::ResetPolicy::MANUAL,
+      base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  static bool callback_done = false;
+  uintptr_t readable_a_context = helper.CreateContextWithCancel(
+      base::Bind(
+          [](base::WaitableEvent* wait_for_notification, MojoHandle w,
+             MojoHandle a, MojoResult result, MojoHandleSignalsState state) {
+            EXPECT_EQ(MOJO_RESULT_OK, result);
+            EXPECT_EQ(kTestMessageToA, ReadMessage(a));
+
+            wait_for_notification->Signal();
+
+            // Give the other thread sufficient time to race with the completion
+            // of this callback. There should be no race, since the cancellation
+            // notification must be mutually exclusive to this notification.
+            base::PlatformThread::Sleep(base::TimeDelta::FromSeconds(1));
+
+            callback_done = true;
+          },
+          &wait_for_notification, w, a),
+      base::Bind(
+          [](base::WaitableEvent* wait_for_cancellation) {
+            EXPECT_TRUE(callback_done);
+            wait_for_cancellation->Signal();
+          },
+          &wait_for_cancellation));
+
+  ThreadedRunner runner(base::Bind(
+      [](base::WaitableEvent* wait_for_notification,
+         base::WaitableEvent* wait_for_cancellation, MojoHandle w,
+         uintptr_t readable_a_context) {
+        wait_for_notification->Wait();
+
+        // Cancel the watch while the notification is still running.
+        EXPECT_EQ(MOJO_RESULT_OK, MojoCancelWatch(w, readable_a_context));
+
+        wait_for_cancellation->Wait();
+
+        EXPECT_TRUE(callback_done);
+      },
+      &wait_for_notification, &wait_for_cancellation, w, readable_a_context));
+  runner.Start();
+
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(w, nullptr, nullptr, nullptr, nullptr));
+
+  WriteMessage(b, kTestMessageToA);
+  runner.Join();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+}
+
+TEST_F(WatcherTest, WatchesCancelEachOtherFromNotifications) {
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  static const char kTestMessageToA[] = "hey a";
+  static const char kTestMessageToB[] = "hey b";
+
+  base::WaitableEvent wait_for_a_to_notify(
+      base::WaitableEvent::ResetPolicy::MANUAL,
+      base::WaitableEvent::InitialState::NOT_SIGNALED);
+  base::WaitableEvent wait_for_b_to_notify(
+      base::WaitableEvent::ResetPolicy::MANUAL,
+      base::WaitableEvent::InitialState::NOT_SIGNALED);
+  base::WaitableEvent wait_for_a_to_cancel(
+      base::WaitableEvent::ResetPolicy::MANUAL,
+      base::WaitableEvent::InitialState::NOT_SIGNALED);
+  base::WaitableEvent wait_for_b_to_cancel(
+      base::WaitableEvent::ResetPolicy::MANUAL,
+      base::WaitableEvent::InitialState::NOT_SIGNALED);
+
+  MojoHandle a_watcher;
+  MojoHandle b_watcher;
+  WatchHelper helper;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&a_watcher));
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&b_watcher));
+
+  // We set up two watchers, one on |a| and one on |b|. They cancel each other
+  // from within their respective watch notifications. This should be safe,
+  // i.e., it should not deadlock, in spite of the fact that we also guarantee
+  // mutually exclusive notification execution (including cancellations) on any
+  // given watch.
+  bool a_cancelled = false;
+  bool b_cancelled = false;
+  static uintptr_t readable_b_context;
+  uintptr_t readable_a_context = helper.CreateContextWithCancel(
+      base::Bind(
+          [](base::WaitableEvent* wait_for_a_to_notify,
+             base::WaitableEvent* wait_for_b_to_notify, MojoHandle b_watcher,
+             MojoHandle a, MojoResult result, MojoHandleSignalsState state) {
+            EXPECT_EQ(MOJO_RESULT_OK, result);
+            EXPECT_EQ(kTestMessageToA, ReadMessage(a));
+            wait_for_a_to_notify->Signal();
+            wait_for_b_to_notify->Wait();
+            EXPECT_EQ(MOJO_RESULT_OK,
+                      MojoCancelWatch(b_watcher, readable_b_context));
+            EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b_watcher));
+          },
+          &wait_for_a_to_notify, &wait_for_b_to_notify, b_watcher, a),
+      base::Bind(
+          [](base::WaitableEvent* wait_for_a_to_cancel,
+             base::WaitableEvent* wait_for_b_to_cancel, bool* a_cancelled) {
+            *a_cancelled = true;
+            wait_for_a_to_cancel->Signal();
+            wait_for_b_to_cancel->Wait();
+          },
+          &wait_for_a_to_cancel, &wait_for_b_to_cancel, &a_cancelled));
+
+  readable_b_context = helper.CreateContextWithCancel(
+      base::Bind(
+          [](base::WaitableEvent* wait_for_a_to_notify,
+             base::WaitableEvent* wait_for_b_to_notify,
+             uintptr_t readable_a_context, MojoHandle a_watcher, MojoHandle b,
+             MojoResult result, MojoHandleSignalsState state) {
+            EXPECT_EQ(MOJO_RESULT_OK, result);
+            EXPECT_EQ(kTestMessageToB, ReadMessage(b));
+            wait_for_b_to_notify->Signal();
+            wait_for_a_to_notify->Wait();
+            EXPECT_EQ(MOJO_RESULT_OK,
+                      MojoCancelWatch(a_watcher, readable_a_context));
+            EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a_watcher));
+          },
+          &wait_for_a_to_notify, &wait_for_b_to_notify, readable_a_context,
+          a_watcher, b),
+      base::Bind(
+          [](base::WaitableEvent* wait_for_a_to_cancel,
+             base::WaitableEvent* wait_for_b_to_cancel, bool* b_cancelled) {
+            *b_cancelled = true;
+            wait_for_b_to_cancel->Signal();
+            wait_for_a_to_cancel->Wait();
+          },
+          &wait_for_a_to_cancel, &wait_for_b_to_cancel, &b_cancelled));
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(a_watcher, a, MOJO_HANDLE_SIGNAL_READABLE,
+                                      readable_a_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(a_watcher, nullptr, nullptr, nullptr, nullptr));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoWatch(b_watcher, b, MOJO_HANDLE_SIGNAL_READABLE,
+                                      readable_b_context));
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoArmWatcher(b_watcher, nullptr, nullptr, nullptr, nullptr));
+
+  ThreadedRunner runner(
+      base::Bind([](MojoHandle b) { WriteMessage(b, kTestMessageToA); }, b));
+  runner.Start();
+
+  WriteMessage(a, kTestMessageToB);
+
+  wait_for_a_to_cancel.Wait();
+  wait_for_b_to_cancel.Wait();
+  runner.Join();
+
+  EXPECT_TRUE(a_cancelled);
+  EXPECT_TRUE(b_cancelled);
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+}
+
+TEST_F(WatcherTest, AlwaysCancel) {
+  // Basic sanity check to ensure that all possible ways to cancel a watch
+  // result in a final MOJO_RESULT_CANCELLED notification.
+
+  MojoHandle a, b;
+  CreateMessagePipe(&a, &b);
+
+  MojoHandle w;
+  WatchHelper helper;
+  EXPECT_EQ(MOJO_RESULT_OK, helper.CreateWatcher(&w));
+
+  base::WaitableEvent event(base::WaitableEvent::ResetPolicy::MANUAL,
+                            base::WaitableEvent::InitialState::NOT_SIGNALED);
+  const base::Closure signal_event =
+      base::Bind(&base::WaitableEvent::Signal, base::Unretained(&event));
+
+  // Cancel via |MojoCancelWatch()|.
+  uintptr_t context = helper.CreateContextWithCancel(
+      WatchHelper::ContextCallback(), signal_event);
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, context));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoCancelWatch(w, context));
+  event.Wait();
+  event.Reset();
+
+  // Cancel by closing the watched handle.
+  context = helper.CreateContextWithCancel(WatchHelper::ContextCallback(),
+                                           signal_event);
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, a, MOJO_HANDLE_SIGNAL_READABLE, context));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(a));
+  event.Wait();
+  event.Reset();
+
+  // Cancel by closing the watcher handle.
+  context = helper.CreateContextWithCancel(WatchHelper::ContextCallback(),
+                                           signal_event);
+  EXPECT_EQ(MOJO_RESULT_OK,
+            MojoWatch(w, b, MOJO_HANDLE_SIGNAL_READABLE, context));
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(w));
+  event.Wait();
+
+  EXPECT_EQ(MOJO_RESULT_OK, MojoClose(b));
+}
+
+}  // namespace
+}  // namespace edk
+}  // namespace mojo