Allow clients to subscribe to notifications about sealed objects. (#40)

* Create libplasma_client.a in Makefile.

* Implement plasma subscribe.

* Fixes and tests.

* Buffer notifications in the Plasma store when the socket send buffer is full.

* Fix formatting.

* Turn off -Werror in Makefile.

* Fixes.

* Fix formatting.

lib/python/plasma.py

@@ -1,17 +1,19 @@
 import os
 import socket
 import ctypes
+import time
 
 Addr = ctypes.c_ubyte * 4
 
-ID = ctypes.c_ubyte * 20
+PLASMA_ID_SIZE = 20
+ID = ctypes.c_ubyte * PLASMA_ID_SIZE
 
 class PlasmaID(ctypes.Structure):
   _fields_ = [("plasma_id", ID)]
 
 def make_plasma_id(string):
-  if len(string) != 20:
-    raise Exception("PlasmaIDs must be 20 characters long")
+  if len(string) != PLASMA_ID_SIZE:
+    raise Exception("PlasmaIDs must be {} characters long".format(PLASMA_ID_SIZE))
   object_id = map(ord, string)
   return PlasmaID(plasma_id=ID(*object_id))
 
@@ -46,6 +48,7 @@ class PlasmaClient(object):
     self.client.plasma_contains.restype = None
     self.client.plasma_seal.restype = None
     self.client.plasma_delete.restype = None
+    self.client.plasma_subscribe.restype = ctypes.c_int
 
     self.buffer_from_memory = ctypes.pythonapi.PyBuffer_FromMemory
     self.buffer_from_memory.argtypes = [ctypes.c_void_p, ctypes.c_int64]
@@ -161,3 +164,25 @@ class PlasmaClient(object):
     if self.manager_conn == -1:
       raise Exception("Not connected to the plasma manager socket")
     self.client.plasma_transfer(self.manager_conn, addr, port, make_plasma_id(object_id))
+
+  def subscribe(self):
+    """Subscribe to notifications about sealed objects."""
+    fd = self.client.plasma_subscribe(self.store_conn)
+    self.notification_sock = socket.fromfd(fd, socket.AF_UNIX, socket.SOCK_STREAM)
+    # Make the socket non-blocking.
+    self.notification_sock.setblocking(0)
+
+  def get_next_notification(self):
+    """Get the next notification from the notification socket."""
+    if not self.notification_sock:
+      raise Exception("To get notifications, first call subscribe.")
+    # Loop until we've read PLASMA_ID_SIZE bytes from the socket.
+    while True:
+      try:
+        message_data = self.notification_sock.recv(PLASMA_ID_SIZE)
+      except socket.error:
+        time.sleep(0.001)
+      else:
+        assert len(message_data) == PLASMA_ID_SIZE
+        break
+    return message_data