Implement the content/renderer and content/browser part of the Device Motion API.

content/renderer/shared_memory_seqlock_reader.cc

+// Copyright (c) 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.
+
+#include "shared_memory_seqlock_reader.h"
+
+namespace internal {
+
+SharedMemorySeqLockReaderBase::SharedMemorySeqLockReaderBase() { }
+
+SharedMemorySeqLockReaderBase::~SharedMemorySeqLockReaderBase() { }
+
+void*
+SharedMemorySeqLockReaderBase::InitializeSharedMemory(
+    base::SharedMemoryHandle shared_memory_handle, size_t buffer_size) {
+  renderer_shared_memory_handle_ = shared_memory_handle;
+  if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
+    return 0;
+  renderer_shared_memory_.reset(new base::SharedMemory(
+      renderer_shared_memory_handle_, true));
+
+  return (renderer_shared_memory_->Map(buffer_size))
+      ? renderer_shared_memory_->memory()
+      : 0;
+}
+
+bool SharedMemorySeqLockReaderBase::FetchFromBuffer(
+    content::OneWriterSeqLock* seqlock, void* final, void* temp, void* from,
+    size_t size) {
+
+  if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
+    return false;
+
+  // Only try to read this many times before failing to avoid waiting here
+  // very long in case of contention with the writer.
+  int contention_count = -1;
+  base::subtle::Atomic32 version;
+  do {
+    version = seqlock->ReadBegin();
+    memcpy(temp, from, size);
+    ++contention_count;
+    if (contention_count == kMaximumContentionCount)
+      break;
+  } while (seqlock->ReadRetry(version));
+
+  if (contention_count >= kMaximumContentionCount) {
+    // We failed to successfully read, presumably because the hardware
+    // thread was taking unusually long. Don't copy the data to the output
+    // buffer, and simply leave what was there before.
+    return false;
+  }
+
+  // New data was read successfully, copy it into the output buffer.
+  memcpy(final, temp, size);
+  return true;
+}
+
+}  // namespace internal