Fix excessive buffering of worker stdout/stderr. (#4094)

* Start workers with 'python -u' to prevent buffering of prints.

* Set sys.stdout and sys.stderr.

* Add comment.

python/ray/worker.py

@@ -1877,6 +1877,12 @@ def connect(info,
             # be redirected.
             os.dup2(log_stdout_file.fileno(), sys.stdout.fileno())
             os.dup2(log_stderr_file.fileno(), sys.stderr.fileno())
+            # We also manually set sys.stdout and sys.stderr because that seems
+            # to have an affect on the output buffering. Without doing this,
+            # stdout and stderr are heavily buffered resulting in seemingly
+            # lost logging statements.
+            sys.stdout = log_stdout_file
+            sys.stderr = log_stderr_file
             # This should always be the first message to appear in the worker's
             # stdout and stderr log files. The string "Ray worker pid:" is
             # parsed in the log monitor process.