Releasing the former Stadia file transfer tools

The tools allow efficient and fast synchronization of large directory
trees from a Windows workstation to a Linux target machine.

cdc_rsync* support efficient copy of files by using content-defined
chunking (CDC) to identify chunks within files that can be reused.

asset_stream_manager + cdc_fuse_fs support efficient streaming of a
local directory to a remote virtual file system based on FUSE. It also
employs CDC to identify and reuse unchanged data chunks.

common/semaphore.h

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2022 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef COMMON_SEMAPHORE_H_
+#define COMMON_SEMAPHORE_H_
+
+#include <condition_variable>
+#include <mutex>
+
+namespace cdc_ft {
+
+// Standard semaphore. Can be replaced by std::counting_semaphore in C++20.
+// Not fair! Wait() does not unlock in FIFO order.
+class Semaphore {
+ public:
+  // Initialize the semaphore with |initial_count|. The count indicates how
+  // often Wait() can be called without blocking. There is no max count.
+  explicit Semaphore(int initial_count);
+  ~Semaphore();
+
+  // If the count is larger than zero, decreases the count and returns
+  // immediately. If the count is zero, blocks until some other thread calls
+  // Signal() and decreases the count.
+  // This method is thread-safe.
+  void Wait();
+
+  // Increases the count.
+  // This method is thread-safe.
+  void Signal();
+
+ private:
+  uint32_t count_;
+  std::mutex mutex_;
+  std::condition_variable condition_;
+};
+
+}  // namespace cdc_ft
+
+#endif  // COMMON_SEMAPHORE_H_