Getting coverage results in Too many open files error in some cases

[afsafzal]

When build_traces is building many traces, it stops working after a while due to this error:

failed to build trace 475: 512eae84
Traceback (most recent call last):
  File "experiments/build_traces.py", line 69, in trace
    trace = sandbox.run_and_trace(mission.commands)
  File "/home/afsoon/Houston-forked/houston/ardu/sandbox.py", line 345, in run_and_trace
    coverage = self.__get_coverage(directory=directory)
  File "/home/afsoon/Houston-forked/houston/ardu/sandbox.py", line 362, in __get_coverage
    coverage = self._bugzoo.coverage.extract(self.container)
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/bugzoo/mgr/coverage.py", line 325, in extract
    mgr_ctr.copy_from(container, fn_temp_ctr, fn_temp_host)
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/bugzoo/mgr/container.py", line 478, in copy_from
    subprocess.check_output(cmd, shell=True)
  File "/usr/lib/python3.5/subprocess.py", line 626, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.5/subprocess.py", line 693, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.5/subprocess.py", line 947, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.5/subprocess.py", line 1454, in _execute_child
    errpipe_read, errpipe_write = os.pipe()
OSError: [Errno 24] Too many open files
generating trace for mission 478: 5fa8c670
>>> Skipping invalid cmd #176^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@
last_wp: [2, 5] len: 10
failed to build trace 477: 2d3183ea
Traceback (most recent call last):
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/urllib3/connectionpool.py", line 600, in urlopen
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/urllib3/connectionpool.py", line 354, in _make_request
  File "/usr/lib/python3.5/http/client.py", line 1106, in request
  File "/usr/lib/python3.5/http/client.py", line 1151, in _send_request
  File "/usr/lib/python3.5/http/client.py", line 1102, in endheaders
  File "/usr/lib/python3.5/http/client.py", line 934, in _send_output
  File "/usr/lib/python3.5/http/client.py", line 877, in send
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/docker/transport/unixconn.py", line 44, in connect
  File "/usr/lib/python3.5/socket.py", line 134, in __init__
OSError: [Errno 24] Too many open files

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/requests/adapters.py", line 445, in send
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/urllib3/connectionpool.py", line 638, in urlopen
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/urllib3/util/retry.py", line 367, in increment
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/urllib3/packages/six.py", line 685, in reraise
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/urllib3/connectionpool.py", line 600, in urlopen
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/urllib3/connectionpool.py", line 354, in _make_request
  File "/usr/lib/python3.5/http/client.py", line 1106, in request
  File "/usr/lib/python3.5/http/client.py", line 1151, in _send_request
  File "/usr/lib/python3.5/http/client.py", line 1102, in endheaders
  File "/usr/lib/python3.5/http/client.py", line 934, in _send_output
  File "/usr/lib/python3.5/http/client.py", line 877, in send
  File "/home/afsoon/python3.5-venv/lib/python3.5/site-packages/docker/transport/unixconn.py", line 44, in connect
  File "/usr/lib/python3.5/socket.py", line 134, in __init__
urllib3.exceptions.ProtocolError: ('Connection aborted.', OSError(24, 'Too many open files'))

[afsafzal]

This is a known BugZoo issue. Relevant issue in flask: psf/requests#239