[core worker] Python core worker task execution (#5783)

Executes tasks via the event loop in the C++ core worker. Also properly handles signals (including KeyboardInterrupt), so ctrl-C in a python interactive shell works now (if connecting to an existing cluster).

python/ray/workers/default_worker.py

@@ -3,7 +3,6 @@ from __future__ import division
 from __future__ import print_function
 
 import argparse
-import traceback
 
 import ray
 import ray.actor
@@ -86,30 +85,5 @@ if __name__ == "__main__":
     node = ray.node.Node(
         ray_params, head=False, shutdown_at_exit=False, connect_only=True)
     ray.worker._global_node = node
-
     ray.worker.connect(node, mode=ray.WORKER_MODE)
-
-    error_explanation = """
-  This error is unexpected and should not have happened. Somehow a worker
-  crashed in an unanticipated way causing the main_loop to throw an exception,
-  which is being caught in "python/ray/workers/default_worker.py".
-  """
-
-    try:
-        # This call to main_loop should never return if things are working.
-        # Most exceptions that are thrown (e.g., inside the execution of a
-        # task) should be caught and handled inside of the call to
-        # main_loop. If an exception is thrown here, then that means that
-        # there is some error that we didn't anticipate.
-        ray.worker.global_worker.main_loop()
-    except Exception:
-        traceback_str = traceback.format_exc() + error_explanation
-        ray.utils.push_error_to_driver(
-            ray.worker.global_worker,
-            "worker_crash",
-            traceback_str,
-            job_id=None)
-        # TODO(rkn): Note that if the worker was in the middle of executing
-        # a task, then any worker or driver that is blocking in a get call
-        # and waiting for the output of that task will hang. We need to
-        # address this.
+    ray.worker.global_worker.main_loop()