Fix handle_watcher when start is called twice on the same handle.

This carries over a fix by qsr@ for the mojo helper handle_watcher
landed in the mojo repo in https://codereview.chromium.org/781753004.

The fix allows the network service apptests not to crash when built
against the Chromium copy of mojo/common.

BUG=450356

Review URL: https://codereview.chromium.org/878513003

Cr-Commit-Position: refs/heads/master@{#313261}

2 files changed, 18 insertions, 0 deletions

diff --git a/mojo/common/handle_watcher.cc b/mojo/common/handle_watcher.cc
index 7819779..79cdcdb 100644
--- a/mojo/common/handle_watcher.cc
+++ b/mojo/common/handle_watcher.cc
@@ -459,6 +459,8 @@ void HandleWatcher::Start(const Handle& handle,
   DCHECK(handle.is_valid());
   DCHECK_NE(MOJO_HANDLE_SIGNAL_NONE, handle_signals);
 
+  // Need to clear the state before creating a new one.
+  state_.reset();
   if (MessagePumpMojo::IsCurrent()) {
     state_.reset(new SameThreadWatchingState(
         this, handle, handle_signals, deadline, callback));
diff --git a/mojo/common/handle_watcher_unittest.cc b/mojo/common/handle_watcher_unittest.cc
index 3323421..41d5ffb 100644
--- a/mojo/common/handle_watcher_unittest.cc
+++ b/mojo/common/handle_watcher_unittest.cc
@@ -280,6 +280,22 @@ TEST_P(HandleWatcherTest, Restart) {
   EXPECT_FALSE(callback_helper2.got_callback());
 }
 
+// Verifies Start() invoked a second time on the same handle works.
+TEST_P(HandleWatcherTest, RestartOnSameHandle) {
+  MessagePipe test_pipe;
+  CallbackHelper callback_helper;
+  ASSERT_TRUE(test_pipe.handle0.is_valid());
+
+  HandleWatcher watcher;
+  callback_helper.Start(&watcher, test_pipe.handle0.get());
+  RunUntilIdle();
+  EXPECT_FALSE(callback_helper.got_callback());
+
+  callback_helper.Start(&watcher, test_pipe.handle0.get());
+  RunUntilIdle();
+  EXPECT_FALSE(callback_helper.got_callback());
+}
+
 // Verifies deadline is honored.
 TEST_P(HandleWatcherTest, Deadline) {
   InstallTickClock();