task: fix some bugs in 'dcos task exec' command. (#886)

* Only set tty into raw mode for `dcos task exec` when interactive.

Previously, the terminal for 'dcos task exec' was set into raw mode
anytime the '--tty' flags was set (even if '--interactive' was not set).
This would cause the CLI to hang when '--interactive' was not set since
control characters are not interpreted by the local tty once it is set
into raw mode (they are sent to STDIN instead). Since we don't actually
stream STDIN to the remote container without '--interactive' , its safe
to leave the tty in default mode and allow the user to '^C' out of it.

This commit addresses this issue.

* Updated attach input path of 'dcos task exec'.

Previously, we would error out no matter what type of exception was
thrown while trying to establish an ATTACH_CONTAINER_INPUT session with
an agent. This would cause us to sometimes drop the output of command we
launched because it happened to complete before we made the
ATTACH_CONTAINER_INPUT call (because the command actually didn't need to
accept input and just ran to completion). We should only error
out if we we are certain that the error occurred while the container was
still running.

This commit adds logic to update this error handling properly.

dcos/mesos.py

@@ -1103,9 +1103,8 @@ class TaskIO(object):
         oldtermios = termios.tcgetattr(fd)
 
         try:
-            tty.setraw(fd, when=termios.TCSANOW)
-
             if self.interactive:
+                tty.setraw(fd, when=termios.TCSANOW)
                 self._window_resize(signal.SIGWINCH, None)
                 signal.signal(signal.SIGWINCH, self._window_resize)
 
@@ -1315,19 +1314,28 @@ class TaskIO(object):
         # `_process_output_stream` function signals us that it's ready.
         self.attach_input_event.wait()
 
-        # Send an intial "Test" message to ensure that we can establish a
-        # connection with the agent at all. If we can't we will throw an
-        # exception and break out of this thread.
-        http.post(
-            self.agent_url,
-            data=_initial_input_streamer(),
-            **req_extra_args)
+        # Send an intial "Test" message to ensure that we are able to
+        # establish a connection with the agent. If we aren't we will throw
+        # an exception and break out of this thread. However, in cases where
+        # we receive a 500 response from the agent, we actually want to
+        # continue without throwing an exception. A 500 error indicates that
+        # we can't connect to the container because it has already finished
+        # running. In that case we continue running to allow the output queue
+        # to be flushed.
+        try:
+            http.post(
+                self.agent_url,
+                data=_initial_input_streamer(),
+                **req_extra_args)
+        except DCOSHTTPException as e:
+            if not e.response.status_code == 500:
+                raise e
 
         # If we succeeded with that connection, unblock process_output_stream()
         # from sending output data to the output thread.
         self.print_output_event.set()
 
-        # Begin streaming the the input.
+        # Begin streaming the input.
         http.post(
             self.agent_url,
             data=_input_streamer(),