[Android] Fix DeviceUtils.__str__ when no serial is provided.

This CL also changes two instances of DeviceUtils.__init__(None) s.t.
an explicit serial number is now provided.

BUG=398127

Review URL: https://codereview.chromium.org/420273006

git-svn-id: http://src.chromium.org/svn/trunk/src/build@286124 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

android/pylib/device/device_errors.py

@@ -37,3 +37,10 @@ class DeviceUnreachableError(BaseError):
   """Exception for device unreachable failures."""
   pass
 
+
+class NoDevicesError(BaseError):
+  """Exception for having no devices attached."""
+
+  def __init__(self):
+    super(NoDevicesError, self).__init__('No devices attached.')
+

android/pylib/device/device_utils.py

@@ -790,7 +790,12 @@ class DeviceUtils(object):
 
   def __str__(self):
     """Returns the device serial."""
-    return self.old_interface.GetDevice()
+    s = self.old_interface.GetDevice()
+    if not s:
+      s = self.old_interface.Adb().GetSerialNumber()
+      if s == 'unknown':
+        raise device_errors.NoDevicesError()
+    return s
 
   @staticmethod
   def parallel(devices=None, async=False):

android/pylib/device/device_utils_test.py

@@ -1484,6 +1484,17 @@ class DeviceUtilsStrTest(DeviceUtilsOldImplTest):
     with self.assertNoAdbCalls():
       self.assertEqual('0123456789abcdef', str(self.device))
 
+  def testStr_noSerial(self):
+    self.device = device_utils.DeviceUtils(None)
+    with self.assertCalls('adb  get-serialno', '0123456789abcdef'):
+      self.assertEqual('0123456789abcdef', str(self.device))
+
+  def testStr_noSerial_noDevices(self):
+    self.device = device_utils.DeviceUtils(None)
+    with self.assertCalls('adb  get-serialno', 'unknown'), (
+         self.assertRaises(device_errors.NoDevicesError)):
+      str(self.device)
+
 
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.DEBUG)