Force events to be non blocking if main thread is unresponsive.

device/base/synchronization/one_writer_seqlock.h

 // Copyright 2013 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.
 
 #ifndef DEVICE_BASE_SYNCHRONIZATION_ONE_WRITER_SEQLOCK_H_
 #define DEVICE_BASE_SYNCHRONIZATION_ONE_WRITER_SEQLOCK_H_
 
 #include "base/atomicops.h"
 #include "base/macros.h"
 #include "base/threading/platform_thread.h"
 
 namespace device {
 
 // This SeqLock handles only *one* writer and multiple readers. It may be
 // suitable for low-contention with relatively infrequent writes, and many
 // readers. See:
 //   http://en.wikipedia.org/wiki/Seqlock
 //   http://www.concurrencykit.org/doc/ck_sequence.html
 // This implementation is based on ck_sequence.h from http://concurrencykit.org.
 //
-// Currently this type of lock is used in two implementations (gamepad and
-// device motion, in particular see e.g. shared_memory_seqlock_buffer.h).
+// Currently this type of lock is used in at least two implementations (gamepad
+// and device motion, in particular see e.g. shared_memory_seqlock_buffer.h).
 // It may make sense to generalize this lock to multiple writers.
 //
 // You must be very careful not to operate on potentially inconsistent read
 // buffers. If the read must be retry'd, the data in the read buffer could
 // contain any random garbage. e.g., contained pointers might be
 // garbage, or indices could be out of range. Probably the only suitable thing
 // to do during the read loop is to make a copy of the data, and operate on it
 // only after the read was found to be consistent.
 class OneWriterSeqLock {
  public:
   OneWriterSeqLock();
   base::subtle::Atomic32 ReadBegin() const;
+  void ReadOrFail(bool* can_read, base::subtle::Atomic32* version) const;

[Ken Rockot(use gerrit already), 2016/12/15 19:20:37]

nitty nit: Maybe TryRead is a better name. There isn't really a "fail" action
that takes place so the name might be misleading.

[tdresser, 2016/12/15 19:27:22]

Done.

   bool ReadRetry(base::subtle::Atomic32 version) const;
   void WriteBegin();
   void WriteEnd();
 
  private:
   base::subtle::Atomic32 sequence_;
   DISALLOW_COPY_AND_ASSIGN(OneWriterSeqLock);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_BASE_SYNCHRONIZATION_ONE_WRITER_SEQLOCK_H_