Fix sky_server.py on Mac and make sky_server executable if it's not already

sky/tools/skypy/skyserver.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import socket
 import subprocess
 import logging
 import os.path
+import stat
 import platform
 
 SKYPY_PATH = os.path.dirname(os.path.abspath(__file__))
 SKY_TOOLS_PATH = os.path.dirname(SKYPY_PATH)
 SKYGO_PATH = os.path.join(SKY_TOOLS_PATH, 'skygo')
 SKY_ROOT = os.path.dirname(SKY_TOOLS_PATH)
 SRC_ROOT = os.path.dirname(SKY_ROOT)
 
 class SkyServer(object):
     def __init__(self, port, configuration, root, package_root):
         self.port = port
         self.configuration = configuration
         self.root = root
         self.package_root = package_root
         self.server = None
 
     @staticmethod
     def _port_in_use(port):
         sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         return sock.connect_ex(('localhost', port)) == 0
 
     @staticmethod
     def sky_server_path():
 
         if platform.system() == 'Linux':
             platform_dir = 'linux64'
-        elif platform.system() == 'Mac':
+        elif platform.system() == 'Darwin':
             platform_dir = 'mac'
         else:
-            assert False, 'No sky_server binary for this platform?'
+            assert False, 'No sky_server binary for this platform: ' + platform.system()
 
         return os.path.join(SKYGO_PATH, platform_dir, 'sky_server')
 
     def start(self):
         if self._port_in_use(self.port):
             logging.warn(
                 'Port %s already in use, assuming custom sky_server started.' %
                 self.port)
             return
 
         server_path = self.sky_server_path()
+        st = os.stat(self.sky_server_path())
+        if not (stat.S_IXUSR & st[stat.ST_MODE]):
+            logging.warn('Changing the permissions of %s to be executable.', self.sky_server_path())
+            os.chmod(self.sky_server_path(), st[stat.ST_MODE] | stat.S_IEXEC)
         server_command = [
             server_path,
             '-t', self.configuration,
             self.root,
             str(self.port),
             self.package_root,
         ]
         self.server = subprocess.Popen(server_command)
         return self.server.pid
 
     def stop(self):
         if self.server:
             self.server.terminate()
 
     def __enter__(self):
         self.start()
 
     def __exit__(self, exc_type, exc_value, traceback):
         self.stop()
 
     def path_as_url(self, path):
         return self.url_for_path(self.port, self.root, path)
 
     @staticmethod
     def url_for_path(port, root, path):
         relative_path = os.path.relpath(path, root)
         return 'http://localhost:%s/%s' % (port, relative_path)