webrtc: Fix missing port forwarding on CrOS

telemetry/telemetry/internal/forwarders/cros_forwarder.py

Index: telemetry/telemetry/internal/forwarders/cros_forwarder.py
diff --git a/telemetry/telemetry/internal/forwarders/cros_forwarder.py b/telemetry/telemetry/internal/forwarders/cros_forwarder.py
index 4316954559177e6428cc5584d4359be3ea1fd13a..e59167bbd628c46356d26cd0b8bf7ee669750613 100644
--- a/telemetry/telemetry/internal/forwarders/cros_forwarder.py
+++ b/telemetry/telemetry/internal/forwarders/cros_forwarder.py
@@ -3,7 +3,9 @@
 # found in the LICENSE file.
 
 import logging
+import re
 import subprocess
+import tempfile
 
 from telemetry.internal import forwarders
 from telemetry.internal.forwarders import do_nothing_forwarder
@@ -30,16 +32,36 @@ class CrOsSshForwarder(forwarders.Forwarder):
     super(CrOsSshForwarder, self).__init__(port_pair)
     self._cri = cri
     self._proc = None
+    self._remote_port = None
     forwarding_args = self._ForwardingArgs(
         use_remote_port_forwarding, self.host_ip, port_pair)
+    err_file = tempfile.NamedTemporaryFile()
     self._proc = subprocess.Popen(
-        self._cri.FormSSHCommandLine(['sleep', '999999999'], forwarding_args),
+        self._cri.FormSSHCommandLine(['-NT'], forwarding_args,
+                                     port_forward=use_remote_port_forwarding),
         stdout=subprocess.PIPE,
-        stderr=subprocess.PIPE,
+        stderr=err_file,
         stdin=subprocess.PIPE,
         shell=False)
+    def _get_remote_port(err_file):
+      # When we specify the remote port '0' in ssh remote port forwarding,
+      # the remote ssh server should return the port it binds to in stderr.
+      # e.g. 'Allocated port 42360 for remote forward to localhost:12345',
+      # the port 42360 is the port created remotely and the traffic to the
+      # port will be relayed to localhost port 12345.
+      line = err_file.readline()
+      tokens = re.search(r'port (\d+) for remote forward to', line)
+      if tokens:
+        self._remote_port = int(tokens.group(1))
+      return tokens
+
+    if use_remote_port_forwarding and port_pair.remote_port == 0:
+      with open(err_file.name, 'r') as err_file_reader:
+        py_utils.WaitFor(lambda: _get_remote_port(err_file_reader), 60)
+
     py_utils.WaitFor(
         lambda: self._cri.IsHTTPServerRunningOnPort(self.host_port), 60)
+    err_file.close()
     logging.debug('Server started on %s:%d', self.host_ip, self.host_port)
 
   # pylint: disable=unused-argument
@@ -55,6 +77,9 @@ class CrOsSshForwarder(forwarders.Forwarder):
 
   @property
   def host_port(self):
+    # Return remote port if it is resolved remotely.
+    if self._remote_port:
+      return self._remote_port
     return self._port_pair.remote_port
 
   def Close(self):