Make Telemetry more robust during ephemeral port handshake.

On Windows, if the browser is still writing the DevToolsActivePort
file when Telemetry attempts to open it, Python will raise an IOError.
Make the Telemetry-side code more robust when reading the file
containing the port.

BUG=372560

Review URL: https://codereview.chromium.org/317163008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275620 0039d316-1c4b-4281-b951-d872f2087c98

tools/telemetry/telemetry/core/backends/chrome/desktop_browser_backend.py

@@ -142,10 +142,21 @@ class DesktopBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
       if not os.path.isfile(port_file):
         # File isn't ready yet. Return false. Will retry.
         return False
-      with open(port_file) as f:
-        port_string = f.read()
-        self._port = int(port_string)
-        logging.info('Discovered ephemeral port %s' % self._port)
+      # Attempt to avoid reading the file until it's populated.
+      got_port = False
+      try:
+        if os.stat(port_file).st_size > 0:
+          with open(port_file) as f:
+            port_string = f.read()
+            self._port = int(port_string)
+            logging.info('Discovered ephemeral port %s' % self._port)
+            got_port = True
+      except Exception:
+        # Both stat and open can throw exceptions.
+        pass
+      if not got_port:
+        # File isn't ready yet. Return false. Will retry.
+        return False
     return super(DesktopBrowserBackend, self).HasBrowserFinishedLaunching()
 
   def GetBrowserStartupArgs(self):