Passing file descriptors to Xvfb to retrieve allocated DISPLAY

README.md

Problem

Passing a file descriptor to the command used in create_subprocess_exec would result in Xvfb complaining it could not write to the file descriptor.

Explanation

File descriptors are closed by default when passed to a subprocess, plus file descriptors do not inherit by default.

Solution

I have in this example shown how to use a pipe to pass in a file descriptor which we then read from to get the result. I set file descriptors to not be automatically closed and I set the pipe to allow inheritance.

docker build -t xfd .
docker run -it --rm xfd

Example result. DISPLAY 0 was allocated to one of the executions, DISPLAY 1 to the other.

$ docker run -it --rm xfd
_XSERVTransSocketUNIXCreateListener: ...SocketCreateListener() failed
_XSERVTransMakeAllCOTSServerListeners: server already running
DISPLAY: 0
Await xvfb
DISPLAY: 1
Await xvfb

Dockerfile

FROM ubuntu:20.10
RUN apt-get -y update
RUN apt-get install -y python3 xvfb
ADD main.py main.py
CMD python3 main.py & python3 main.py

main.py

import os
import asyncio

async def create_xvfb():

    dpipe = os.pipe()
    os.set_inheritable(dpipe[1], True)

    xvfb_cmd = [
        "Xvfb",
        "-displayfd",
        str(dpipe[1]),
    ]

    xvfb = await asyncio.create_subprocess_exec(*xvfb_cmd, close_fds=False)

    while True:
        s = os.read(dpipe[0], 1).decode("utf-8")
        if s == "\n":
            break
        print(f"DISPLAY: {s}")

    print("Await xvfb")
    print(await xvfb.wait())
    os.close(dpipe[1])
    os.close(dpipe[0])

def main():

    loop = asyncio.get_event_loop()
    loop.run_until_complete(create_xvfb())
    loop.close()


if __name__ == "__main__":
    main()