Change mojo::Watcher to survive closing a handle and cancelling.

Currently, if a handle is closed while a Watcher is watching it, its
callback will notify it that the watch has been cancelled. However, if
the Watcher is explicitly cancelled after the handle is closed but
before the callback is called, it will attempt to call MojoCancelWatch()
on the closed handle and DCHECK that it succeeds. This changes the
DCHECK to also accept MOJO_ERROR_INVALID_ARGUMENT.

Review URL: https://codereview.chromium.org/1795193005

Cr-Commit-Position: refs/heads/master@{#380936}

2 files changed, 23 insertions, 1 deletions

diff --git a/mojo/public/cpp/system/tests/watcher_unittest.cc b/mojo/public/cpp/system/tests/watcher_unittest.cc
index ea78a56..f72f495 100644
--- a/mojo/public/cpp/system/tests/watcher_unittest.cc
+++ b/mojo/public/cpp/system/tests/watcher_unittest.cc
@@ -182,5 +182,25 @@ TEST_F(WatcherTest, NotifyOnMessageLoopDestruction) {
   b_watcher.Cancel();
 }
 
+TEST_F(WatcherTest, CloseAndCancel) {
+  ScopedMessagePipeHandle a, b;
+  CreateMessagePipe(nullptr, &a, &b);
+
+  Watcher b_watcher;
+  EXPECT_EQ(MOJO_RESULT_OK,
+            b_watcher.Start(b.get(), MOJO_HANDLE_SIGNAL_READABLE,
+                            OnReady([](MojoResult result) { FAIL(); })));
+  EXPECT_TRUE(b_watcher.IsWatching());
+
+  // This should trigger the watcher above...
+  b.reset();
+  // ...but the watcher is cancelled first.
+  b_watcher.Cancel();
+
+  EXPECT_FALSE(b_watcher.IsWatching());
+
+  base::RunLoop().RunUntilIdle();
+}
+
 }  // namespace
 }  // namespace mojo
diff --git a/mojo/public/cpp/system/watcher.cc b/mojo/public/cpp/system/watcher.cc
index ad965ff..5723533 100644
--- a/mojo/public/cpp/system/watcher.cc
+++ b/mojo/public/cpp/system/watcher.cc
@@ -96,7 +96,9 @@ void Watcher::Cancel() {
   MojoResult result =
       MojoCancelWatch(handle_.value(), reinterpret_cast<uintptr_t>(this));
   message_loop_observer_.reset();
-  DCHECK_EQ(result, MOJO_RESULT_OK);
+  // |result| may be MOJO_RESULT_INVALID_ARGUMENT if |handle_| has closed, but
+  // OnHandleReady has not yet been called.
+  DCHECK(result == MOJO_RESULT_INVALID_ARGUMENT || result == MOJO_RESULT_OK);
   handle_.set_value(kInvalidHandleValue);
   callback_.Reset();
 }