Closing stdin on a Running Container

I am using docker version 2.1.0.

When starting a container the parameter stdin_open can be used to prepare the stdin socket for writing. I have had success doing this with code along these lines,

stdin_data = 'some data to send to stdin'

container = client.containers.run(image, detach=True, stdin_open=True)

socket = container.attach_socket(params={'stdin': 1, 'stream': 1})
os.write(socket.fileno(), stdin_data.encode())
socket.close()

exit_code = container.wait()

I have also found that this approach can easily hang indefinitely, if the underlying container is waiting for input. The reason seems to be be that the socket status is independent of the stdin_open parameter, so the code running in the container never receives an EOF-like sentinel on stdin.

Any suggestions on how to address this?

[mschwager]

Okay, I think I figured it out! The missing ingredient was a _sock.close call. So the complete example:

#!/usr/bin/env python

import docker

client = docker.APIClient()
container = client.create_container(
    'stdin-test',
    stdin_open=True,
)

sock = client.attach_socket(container, params={"stdin": 1, "stdout": 1, "stderr": 1, "stream": 1})
client.start(container)
sock._sock.send(b'sent data')
sock._sock.close()
sock.close()

status = client.wait(container)
status_code = status["StatusCode"]
stdout = client.logs(container, stderr=False).decode()
stderr = client.logs(container, stdout=False).decode()

client.remove_container(container)

print('Exit: {}'.format(status_code))
print('log stdout: {}'.format(stdout))
print('log stderr: {}'.format(stderr))

Running against the Docker image above, which simply runs the following script, produces the following output:

#!/usr/bin/env python

import sys

print("Running output.py")

data = sys.stdin.read()
print("stdout: {}".format(data), file=sys.stdout)
print("stderr: {}".format(data), file=sys.stderr)

$ python run.py 
Exit: 0
log stdout: Running output.py
stdout: b'sent data'
log stderr: stderr: b'sent data'

So we can see that the output.py script received stdin and successfully printed it!

I believe the original issue can be resolved with the following:

socket._sock.close()
socket.close()

The API ergonomics leave something to be desired here. Having the muck with sock._sock is not ideal, but this workaround appears to work as expected 👍

[mschwager]

Interestingly, it looks like there’s differences between Python 2 and Python 3.

If I remove detach=True the following code works as expected in Python 2:

print('Attaching...')
sock = client.attach_socket(container, params={"stdin": 1, "stream": 1})
print('Sending...')
sock.send(b'sent data')
print('Closing...')
sock.close()

$ python2 run.py 
Starting...
Attaching...
Sending...
Closing...
Waiting...
Exit: 0
stdout: Running output.py
stdout: sent data

stderr: stderr: sent data

$ python3 run.py 
Starting...
Attaching...
Sending...
Traceback (most recent call last):
  File "run.py", line 17, in <module>
    sock.send(b'sent data')
AttributeError: 'SocketIO' object has no attribute 'send'

This is expected due to the Python 2 and 3 differences. However, changing to the following code produces different results:

print('Attaching...')
sock = client.attach_socket(container, params={"stdin": 1, "stream": 1})
print('Sending...')
sock._sock.send(b'sent data')
print('Closing...')
sock.close()

$ python2 run.py 
Starting...
Attaching...
Sending...
Traceback (most recent call last):
  File "run.py", line 17, in <module>
    sock._sock.send(b'sent data')
AttributeError: '_socket.socket' object has no attribute '_sock'

$ python3 run.py 
Starting...
Attaching...
Sending...
Closing...
Waiting...
...
Hangs

So the question is: can we achieve the old Python 2 behavior from _socket.socket with Python 3’s SocketIO?