stdin.encode does not properly encode the stdin on python3

[Scronkfinkle]

Link to issue: https://tracker.ceph.com/issues/51291

I was getting the following error when trying to use Ceph wih the docker container for 15.2.13:

RuntimeError: Failed to execute command: sudo /usr/bin/cephadm --image docker.io/ceph/ceph:v15 --no-container-init deploy --fsid 85361255-4989-4e27-bdb3-e017b9081911 --name mds.cephfs.athos4.nsevry --config-json -
2021-06-28T12:19:36.468436+0000 mgr.athos2 (mgr.5211678) 1623980 : cephadm [INF] Deploying daemon mds.cephfs.athos2.adupvw on athos2
2021-06-28T12:19:36.504448+0000 mgr.athos2 (mgr.5211678) 1623985 : cephadm [ERR] Traceback (most recent call last):
2021-06-28T12:19:36.504677+0000 mgr.athos2 (mgr.5211678) 1623986 : cephadm [ERR]   File "/lib/python3.6/site-packages/remoto/process.py", line 188, in check
2021-06-28T12:19:36.504883+0000 mgr.athos2 (mgr.5211678) 1623987 : cephadm [ERR]     response = result.receive(timeout)
2021-06-28T12:19:36.505087+0000 mgr.athos2 (mgr.5211678) 1623988 : cephadm [ERR]   File "/lib/python3.6/site-packages/execnet/gateway_base.py", line 749, in receive
2021-06-28T12:19:36.505290+0000 mgr.athos2 (mgr.5211678) 1623989 : cephadm [ERR]     raise self._getremoteerror() or EOFError()
2021-06-28T12:19:36.505525+0000 mgr.athos2 (mgr.5211678) 1623990 : cephadm [ERR] execnet.gateway_base.RemoteError: Traceback (most recent call last):
2021-06-28T12:19:36.505741+0000 mgr.athos2 (mgr.5211678) 1623991 : cephadm [ERR]   File "<string>", line 1088, in executetask
2021-06-28T12:19:36.505951+0000 mgr.athos2 (mgr.5211678) 1623992 : cephadm [ERR]   File "/lib/python3.6/site-packages/remoto/process.py", line 151, in _remote_check
2021-06-28T12:19:36.506161+0000 mgr.athos2 (mgr.5211678) 1623993 : cephadm [ERR]   File "/usr/lib/python3.6/subprocess.py", line 863, in communicate
2021-06-28T12:19:36.506370+0000 mgr.athos2 (mgr.5211678) 1623994 : cephadm [ERR]     stdout, stderr = self._communicate(input, endtime, timeout)
2021-06-28T12:19:36.506579+0000 mgr.athos2 (mgr.5211678) 1623995 : cephadm [ERR]   File "/usr/lib/python3.6/subprocess.py", line 1519, in _communicate
2021-06-28T12:19:36.506785+0000 mgr.athos2 (mgr.5211678) 1623996 : cephadm [ERR]     input_view = memoryview(self._input)
2021-06-28T12:19:36.506993+0000 mgr.athos2 (mgr.5211678) 1623997 : cephadm [ERR] TypeError: memoryview: a bytes-like object is required, not 'str'
2021-06-28T12:19:36.507202+0000 mgr.athos2 (mgr.5211678) 1623998 : cephadm [ERR] 
2021-06-28T12:19:36.507409+0000 mgr.athos2 (mgr.5211678) 1623999 : cephadm [ERR] 
2021-06-28T12:19:36.508516+0000 mgr.athos2 (mgr.5211678) 1624001 : cephadm [ERR] Failed to execute command: sudo /usr/bin/cephadm --image docker.io/ceph/ceph:v15 --no-container-init deploy --fsid 85361255-4989-4e27-bdb3-e017b9081911 --name mds.cephfs.athos2.adupvw --config-json -
Traceback (most recent call last):
  File "/lib/python3.6/site-packages/remoto/process.py", line 188, in check
    response = result.receive(timeout)
  File "/lib/python3.6/site-packages/execnet/gateway_base.py", line 749, in receive
    raise self._getremoteerror() or EOFError()
execnet.gateway_base.RemoteError: Traceback (most recent call last):
  File "<string>", line 1088, in executetask
  File "/lib/python3.6/site-packages/remoto/process.py", line 151, in _remote_check
  File "/usr/lib/python3.6/subprocess.py", line 863, in communicate
    stdout, stderr = self._communicate(input, endtime, timeout)
  File "/usr/lib/python3.6/subprocess.py", line 1519, in _communicate
    input_view = memoryview(self._input)
TypeError: memoryview: a bytes-like object is required, not 'str'

I dug into the source code and found this on line 151 of remoto/process.py

  if stdin:
        if not isinstance(stdin, bytes):
            stdin.encode('utf-8', errors='ignore')
        stdout_stream, stderr_stream = process.communicate(stdin)
    else:

In python3.6 (the version shipped with the ceph container) stdin.encode('utf-8', errors='ignore') does not mutate stdin, and instead only returns the modified variable. To verify this, I modified the code like so such that it sets stdin to the encoded value

  if stdin:
        if not isinstance(stdin, bytes):
            stdin = stdin.encode('utf-8', errors='ignore')
        stdout_stream, stderr_stream = process.communicate(stdin)
    else:

Ceph was able to reconfigure and run without issue after this

[Scronkfinkle]

Digging deeper, python2.7 doesn't appear to mutate the variable in place either. I'm unsure if this call ever followed expected behavior