[cdc_rsync] Use any available server port (#94)

Instead of calling netstat on the remote device to detect available
ports, simply call bind with port 0 to bind to any available port.
Since the port is not yet known when cdc_rsync_server.exe is called,
port forwarding needs to be started AFTER the server reports its port.

cdc_rsync_server/cdc_rsync_server.cc

@@ -288,7 +288,7 @@ bool CdcRsyncServer::CheckComponents(
   return true;
 }
 
-absl::Status CdcRsyncServer::Run(int port) {
+absl::Status CdcRsyncServer::Run() {
   absl::Status status = Socket::Initialize();
   if (!status.ok()) {
     return WrapStatus(status, "Failed to initialize sockets");
@@ -296,15 +296,15 @@ absl::Status CdcRsyncServer::Run(int port) {
   socket_finalizer_ = std::make_unique<SocketFinalizer>();
 
   socket_ = std::make_unique<ServerSocket>();
-  int new_port;
-  ASSIGN_OR_RETURN(new_port, socket_->StartListening(port),
-                   "Failed to start listening on port %i", port);
-  assert(port != 0);
-  assert(port == new_port);
+  int port;
+  ASSIGN_OR_RETURN(port, socket_->StartListening(0),
+                   "Failed to start listening for connections");
   LOG_INFO("cdc_rsync_server listening on port %i", port);
 
   // This is the marker for the client, so it knows it can connect.
-  printf("Server is listening\n");
+  // Print port first so the client can easily parse it when it sees "Server is
+  // listening" without dealing with half-transmitted data.
+  printf("Port %i: Server is listening\n", port);
   fflush(stdout);
 
   status = socket_->WaitForConnection();
@@ -607,7 +607,7 @@ absl::Status CdcRsyncServer::CreateMissingDirs() {
 template <typename T>
 absl::Status CdcRsyncServer::SendFileIndices(const char* file_type,
                                              const std::vector<T>& files) {
-  LOG_INFO("Sending indices of missing files to client");
+  LOG_INFO("Sending indices of %s files to client", file_type);
   constexpr char error_fmt[] = "Failed to send indices of %s files.";
 
   AddFileIndicesResponse response;

cdc_rsync_server/cdc_rsync_server.h

@@ -43,9 +43,10 @@ class CdcRsyncServer {
   // up-to-date by checking their sizes and timestamps.
   bool CheckComponents(const std::vector<GameletComponent>& components);
 
-  // Listens to |port|, accepts a connection from the client and runs the rsync
-  // procedure.
-  absl::Status Run(int port);
+  // Listens to any available port, accepts a connection from the client and
+  // runs the rsync procedure. Prints "Port <n>: Server is listening" to stdout,
+  // so the client can retrieve the selected port.
+  absl::Status Run();
 
   // Returns the verbosity sent from the client. 0 by default.
   int GetVerbosity() const { return verbosity_; }

cdc_rsync_server/main.cc

@@ -62,29 +62,22 @@ ServerExitCode GetExitCode(const absl::Status& status) {
 
 int main(int argc, const char** argv) {
   if (argc < 5) {
-    printf("cdc_rsync_server - Remote component of cdc_rsync. Version: %s\n\n",
-           BUILD_VERSION);
-    printf(
-        "Usage: cdc_rsync_server <port> <build_version> cdc_rsync_server "
-        "<size> <time> \n");
-    printf("       where <size> is the file size, <time> is the  modified\n");
-    printf(
-        "       timestamp (Unix epoch) and <build_version> is the build "
-        "version of the server.\n");
-    return cdc_ft::kServerExitCodeGenericStartup;
-  }
+    printf(R"("cdc_rsync_server - Remote component of cdc_rsync. Version: %s
+
+Usage: cdc_rsync_server <build_version> cdc_rsync_server <size> <modified_time>
+       <build_version>            build version embedded in the component
+       <size>                     file size of the component
+       <modified_time>            timestamp (Unix epoch) of the component)",
+           BUILD_VERSION);
 
-  int port = atoi(argv[1]);
-  if (port == 0) {
-    SendErrorMessage(absl::StrFormat("Invalid port '%s'", argv[1]).c_str());
     return cdc_ft::kServerExitCodeGenericStartup;
   }
 
   // The rest is expected to be sets of gamelet component info consisting of
-  // (filename, filesize, modified_time). This is used check whether the
+  // (version, filename, size, modified_time). This is used check whether the
   // components are up-to-date.
   std::vector<cdc_ft::GameletComponent> components =
-      cdc_ft::GameletComponent::FromCommandLineArgs(argc - 2, argv + 2);
+      cdc_ft::GameletComponent::FromCommandLineArgs(argc - 1, argv + 1);
 
   cdc_ft::Log::Initialize(
       std::make_unique<cdc_ft::ConsoleLog>(cdc_ft::LogLevel::kWarning));
@@ -94,7 +87,7 @@ int main(int argc, const char** argv) {
     return cdc_ft::kServerExitCodeOutOfDate;
   }
 
-  absl::Status status = server.Run(port);
+  absl::Status status = server.Run();
   if (status.ok()) {
     return 0;
   }