ffmpeg.Reader._get_cam_inputname causes "ResourceWarning: unclosed file"

Description

In my tests I keep seeing ResourceWarning: unclosed file <_io.BufferedWriter name=3>

With PYTHONTRACEMALLOC=1, here’s what I get:

...\lib\site-packages\imageio\plugins\ffmpeg.py:303: ResourceWarning: unclosed file <_io.BufferedWriter name=3>
  self._filename = self._get_cam_inputname(index)
Object allocated at (most recent call last):
  File "...\lib\subprocess.py", lineno 935
    self.stdin = io.open(p2cwrite, 'wb', bufsize)
...\lib\site-packages\imageio\plugins\ffmpeg.py:303: ResourceWarning: unclosed file <_io.BufferedReader name=4>
  self._filename = self._get_cam_inputname(index)
Object allocated at (most recent call last):
  File "...\lib\subprocess.py", lineno 941
    self.stdout = io.open(c2pread, 'rb', bufsize)
...\lib\site-packages\imageio\plugins\ffmpeg.py:303: ResourceWarning: unclosed file <_io.BufferedReader name=5>
  self._filename = self._get_cam_inputname(index)
Object allocated at (most recent call last):
  File "...\lib\subprocess.py", lineno 946
    self.stderr = io.open(errread, 'rb', bufsize)

How to reproduce

On a Windows 10 laptop with built-in webcam (<video0>), using Python 3.9.7, imageio 2.11.0, imageio-ffmpeg 0.4.5, run the following test:

from imageio import get_reader
import unittest

class ReaderTests(unittest.TestCase):
    def test_get_reader(self):
        reader = get_reader(uri='<video0>')

Cause

Looking at the source of _get_cam_inputname, and looking at the trace above, I suspect the issue is caused by the stdin, stdout, and stderr streams not being closed properly after use in subprocess.Popen.

Proposed solution

Use subprocess.Popen as a context manager, to ensure streams are always properly closed, as suggested e.g. here.

Replace this (source):

...
proc = sp.Popen(
    cmd, stdin=sp.PIPE, stdout=sp.PIPE, stderr=sp.PIPE, shell=True
)
proc.stdout.readline()
proc.terminate()
infos = proc.stderr.read().decode("utf-8", errors="ignore")
...

by this:

...
with sp.Popen(
         cmd, stdin=sp.PIPE, stdout=sp.PIPE, stderr=sp.PIPE,
         shell=True) as proc:
    proc.stdout.readline()
    proc.terminate()
    infos = proc.stderr.read().decode("utf-8", errors="ignore")
...

[dennisvang]

@FirefoxMetzger Sorry it took so long, I had some other, more pressing, concerns.

I tried to address the issue both here and in imageio-ffmpeg, see above.

[dennisvang]

@FirefoxMetzger It looks like you are right. I’ll give it a try.