Make sure user seeding does not affect actor ID generation. (#506)

* Make sure user seeding does not affect actor ID generation.

* Fix linting.

* Add test.

python/ray/utils.py

@@ -3,11 +3,37 @@ from __future__ import division
 from __future__ import print_function
 
 import binascii
+import numpy as np
 import sys
 
 import ray.local_scheduler
 
 
+def random_string():
+  """Generate a random string to use as an ID.
+
+  Note that users may seed numpy, which could cause this function to generate
+  duplicate IDs. Therefore, we need to seed numpy ourselves, but we can't
+  interfere with the state of the user's random number generator, so we extract
+  the state of the random number generator and reset it after we are done.
+
+  TODO(rkn): If we want to later guarantee that these are generated in a
+  deterministic manner, then we will need to make some changes here.
+
+  Returns:
+    A random byte string of length 20.
+  """
+  # Get the state of the numpy random number generator.
+  numpy_state = np.random.get_state()
+  # Try to use true randomness.
+  np.random.seed(None)
+  # Generate the random ID.
+  random_id = np.random.bytes(20)
+  # Reset the state of the numpy random number generator.
+  np.random.set_state(numpy_state)
+  return random_id
+
+
 def decode(byte_str):
   """Make this unicode in Python 3, otherwise leave it as bytes."""
   if sys.version_info >= (3, 0):