[cdc_rsync] Move sockets to common (#95)

There are no real changes, just moving files around. Sockets will be
used in the future to find available ports in cdc_stream. Therefore,
they need to be in common.

common/fake_socket.h

@@ -0,0 +1,57 @@
+/*
+ * 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_FAKE_SOCKET_H_
+#define COMMON_FAKE_SOCKET_H_
+
+#include <condition_variable>
+#include <mutex>
+
+#include "absl/status/status.h"
+#include "common/socket.h"
+
+namespace cdc_ft {
+
+// Fake socket that receives the same data it sends.
+class FakeSocket : public Socket {
+ public:
+  FakeSocket();
+  ~FakeSocket();
+
+  // Socket:
+  absl::Status Send(const void* buffer, size_t size) override;  // thread-safe
+  absl::Status Receive(void* buffer, size_t size, bool allow_partial_read,
+                       size_t* bytes_received) override;  // thread-safe
+
+  void ShutdownSendingEnd();
+
+  // If set to true, blocks on Send() until it is set to false again.
+  void SuspendSending(bool suspended);
+
+ private:
+  std::mutex data_mutex_;
+  std::condition_variable data_cv_;
+  std::string data_;
+  bool shutdown_ = false;
+
+  bool sending_suspended_ = false;
+  std::mutex suspend_mutex_;
+  std::condition_variable suspend_cv_;
+};
+
+}  // namespace cdc_ft
+
+#endif  // CDC_RSYNC_BASE_FAKE_SOCKET_H_