Remove usage of fcntl (#347)

fcntl is only available on linux systems. Importing sagemaker on Windows fails cause of this. This change removes usage of fcntl.

_stream_out exists because jupyter swallows stderr and stdout from a child process. We have to capture it from the main process and pipe to stdout here. This change redirects the child process's stderr to stdout so we only need to read from stdout which is already non-blocking.

Author: icywang86rui

src/sagemaker/local/image.py

@@ -26,7 +26,6 @@
 import sys
 import tarfile
 import tempfile
-from fcntl import fcntl, F_GETFL, F_SETFL
 from six.moves.urllib.parse import urlparse
 from threading import Thread
 
@@ -105,7 +104,7 @@ def train(self, input_data_config, hyperparameters):
         compose_command = self._compose()
 
         _ecr_login_if_needed(self.sagemaker_session.boto_session, self.image)
-        process = subprocess.Popen(compose_command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+        process = subprocess.Popen(compose_command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
 
         try:
             _stream_output(process)
@@ -555,34 +554,20 @@ def __init__(self, host_dir, container_dir=None, channel=None):
 def _stream_output(process):
     """Stream the output of a process to stdout
 
-    This function takes an existing process that will be polled for output. Both stdout and
-    stderr will be polled and both will be sent to sys.stdout.
+    This function takes an existing process that will be polled for output. Only stdout
+    will be polled and sent to sys.stdout.
 
     Args:
         process(subprocess.Popen): a process that has been started with
-            stdout=PIPE and stderr=PIPE
+            stdout=PIPE and stderr=STDOUT
 
     Returns (int): process exit code
     """
     exit_code = None
 
-    # Get the current flags for the  stderr file descriptor
-    # And add the NONBLOCK flag to allow us to read even if there is no data.
-    # Since usually stderr will be empty unless there is an error.
-    flags = fcntl(process.stderr, F_GETFL)  # get current process.stderr flags
-    fcntl(process.stderr, F_SETFL, flags | os.O_NONBLOCK)
-
     while exit_code is None:
         stdout = process.stdout.readline().decode("utf-8")
         sys.stdout.write(stdout)
-        try:
-            stderr = process.stderr.readline().decode("utf-8")
-            sys.stdout.write(stderr)
-        except IOError:
-            # If there is nothing to read on stderr we will get an IOError
-            # this is fine.
-            pass
-
         exit_code = process.poll()
 
     if exit_code != 0: