[cdc_rsync] Enable local syncing (#75)

Adds support for local syncs of files and folders on the same Windows
machine, e.g. cdc_rsync C:\source C:\dest. The two main changes are

- Skip the check whether the port is available remotely with PortManager.
- Do not deploy cdc_rsync_server.
- Run cdc_rsync_server directly, not through an SSH tunnel.

The current implementation is not optimal as it starts
cdc_rsync_server as a separate process and communicates to it via a
TCP port.

common/path.cc

@@ -291,8 +291,8 @@ std::string GetDrivePrefix(const std::string& path) {
 
   if (path[0] != '\\') {
     size_t pos = path.find(":");
-    if (pos == std::string::npos) {
-      // E.g. "\path\to\file" or "path\to\file".
+    if (pos != 1) {
+      // E.g. "\path\to\file", "path\to\file" or "user@host:file".
       return std::string();
     }
 

common/path_test.cc

@@ -302,6 +302,7 @@ TEST_F(PathTest, GetDrivePrefix) {
   EXPECT_EQ(path::GetDrivePrefix("C:\\"), "C:");
   EXPECT_EQ(path::GetDrivePrefix("C:\\dir"), "C:");
   EXPECT_EQ(path::GetDrivePrefix("C:\\dir\\file"), "C:");
+  EXPECT_EQ(path::GetDrivePrefix("host:C:\\dir\\file"), "");
 }
 #endif
 

common/port_manager.h

@@ -40,8 +40,8 @@ class PortManager {
   // synchronize port reservation. The range of possible ports managed by this
   // instance is [|first_port|, |last_port|]. |process_factory| is a valid
   // pointer to a ProcessFactory instance to run processes locally.
-  // |remote_util| is a valid pointer to a RemoteUtil instance to run processes
-  // remotely.
+  // |remote_util| is the RemoteUtil instance to run processes remotely. If it
+  //   is nullptr, no remote ports are reserved.
   PortManager(std::string unique_name, int first_port, int last_port,
               ProcessFactory* process_factory, RemoteUtil* remote_util,
               SystemClock* system_clock = DefaultSystemClock::GetInstance(),

common/port_manager_win.cc

@@ -131,11 +131,13 @@ absl::StatusOr<int> PortManager::ReservePort(int remote_timeout_sec) {
 
   // Find available port on remote instance.
   std::unordered_set<int> remote_ports = local_ports;
-  ASSIGN_OR_RETURN(remote_ports,
-                   FindAvailableRemotePorts(first_port_, last_port_, "0.0.0.0",
-                                            process_factory_, remote_util_,
-                                            remote_timeout_sec, steady_clock_),
-                   "Failed to find available ports on instance");
+  if (remote_util_ != nullptr) {
+    ASSIGN_OR_RETURN(remote_ports,
+                     FindAvailableRemotePorts(
+                         first_port_, last_port_, "0.0.0.0", process_factory_,
+                         remote_util_, remote_timeout_sec, steady_clock_),
+                     "Failed to find available ports on instance");
+  }
 
   // Fetch shared memory.
   void* mem;