forwarder2: Make the Forwarder instances operate on a single thread.

tools/android/forwarder2/pipe_notifier.cc

Index: tools/android/forwarder2/pipe_notifier.cc
diff --git a/tools/android/forwarder2/pipe_notifier.cc b/tools/android/forwarder2/pipe_notifier.cc
index 9110fffcea1812dd1df8a648a1556fb408af4478..94c13541bbe852791e791e3543fb2469217a3eaa 100644
--- a/tools/android/forwarder2/pipe_notifier.cc
+++ b/tools/android/forwarder2/pipe_notifier.cc
@@ -34,10 +34,21 @@ bool PipeNotifier::Notify() {
   errno = 0;
   int ret = HANDLE_EINTR(write(sender_fd_, "1", 1));
   if (ret < 0) {
-    LOG(WARNING) << "Error while notifying pipe. " << safe_strerror(errno);
+    PLOG(ERROR) << "write";
     return false;
   }
   return true;
 }
 
+void PipeNotifier::Reset() {
+  char buffer[1];
+  int ret = HANDLE_EINTR(read(receiver_fd_, buffer, sizeof(buffer)));
+  if (ret < 0) {
+    PLOG(ERROR) << "read";
+    return;
+  }
+  DCHECK_EQ(1, ret);
+  DCHECK_EQ('1', buffer[0]);
+}
+
 }  // namespace forwarder