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/threadpool.h

@@ -0,0 +1,103 @@
+/*
+ * 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_THREADPOOL_H_
+#define COMMON_THREADPOOL_H_
+
+#include <atomic>
+#include <condition_variable>
+#include <functional>
+#include <memory>
+#include <queue>
+#include <thread>
+#include <vector>
+
+#include "absl/synchronization/mutex.h"
+
+namespace cdc_ft {
+
+class Task {
+ public:
+  using IsCancelledPredicate = std::function<bool()>;
+
+  virtual ~Task() = default;
+
+  // Method that's doing all the work.
+  // Called on a background thread.
+  virtual void ThreadRun(IsCancelledPredicate is_cancelled) = 0;
+};
+
+// Manages a pool of worker threads and schedules tasks to run on the threads.
+class Threadpool {
+ public:
+  // Creates a new thread pool with |num_threads| worker threads.
+  explicit Threadpool(size_t num_threads);
+  ~Threadpool();
+
+  // Waits for all queued tasks to finish.
+  void Wait() ABSL_LOCKS_EXCLUDED(task_queue_mutex_);
+
+  // Stops the worker threads. Cancels currently active tasks.
+  void Shutdown() ABSL_LOCKS_EXCLUDED(task_queue_mutex_);
+
+  // Queues a task for execution in a worker thread.
+  void QueueTask(std::unique_ptr<Task> task)
+      ABSL_LOCKS_EXCLUDED(task_queue_mutex_);
+
+  // If available, returns the next completed task.
+  // For a single worker thread (|num_threads| == 1), tasks are completed in
+  // FIFO order. This is no longer the case for multiple threads
+  // (|num_threads| > 1). Tasks that got queued later might complete first.
+  std::unique_ptr<Task> TryGetCompletedTask()
+      ABSL_LOCKS_EXCLUDED(completed_tasks_mutex_);
+
+  // Returns the next completed task, possibly blocking until it is available.
+  // For a single worker thread (|num_threads| == 1), tasks are completed in
+  // FIFO order. This is no longer the case for multiple threads
+  // (|num_threads| > 1). Tasks that got queued later might complete first.
+  std::unique_ptr<Task> GetCompletedTask()
+      ABSL_LOCKS_EXCLUDED(completed_tasks_mutex_);
+
+  // Returns the total number of worker threads in the pool.
+  size_t NumThreads() const { return workers_.size(); }
+
+  // Returns the number of tasks that are either queued or in progress.
+  size_t NumQueuedTasks() const ABSL_LOCKS_EXCLUDED(task_queue_mutex_) {
+    absl::ReaderMutexLock lock(&task_queue_mutex_);
+    return outstanding_task_count_;
+  }
+
+ private:
+  // Background thread worker method. Picks tasks and runs them.
+  void ThreadWorkerMain()
+      ABSL_LOCKS_EXCLUDED(task_queue_mutex_, completed_tasks_mutex_);
+
+  mutable absl::Mutex task_queue_mutex_;
+  std::queue<std::unique_ptr<Task>> task_queue_
+      ABSL_GUARDED_BY(task_queue_mutex_);
+  size_t outstanding_task_count_ ABSL_GUARDED_BY(task_queue_mutex_) = 0;
+  std::atomic_bool shutdown_ ABSL_GUARDED_BY(task_queue_mutex_);
+
+  absl::Mutex completed_tasks_mutex_;
+  std::queue<std::unique_ptr<Task>> completed_tasks_
+      ABSL_GUARDED_BY(completed_tasks_mutex_);
+
+  std::vector<std::thread> workers_;
+};
+
+}  // namespace cdc_ft
+
+#endif  // COMMON_THREADPOOL_H_