Fix IsADirectoryError in _rm_cluster() when /etc/ceph/ceph.conf exists as a directory instead of a file.

The Error:

During cluster cleanup, _rm_cluster() fails with:

  Traceback (most recent call last):
    File "/home/ubuntu/cephtest/cephadm", line 8634, in <module>
      main()
    File "/home/ubuntu/cephtest/cephadm", line 8622, in main
      r = ctx.func(ctx)
    File "/home/ubuntu/cephtest/cephadm", line 6538, in command_rm_cluster
      with open(files[0]) as f:
  IsADirectoryError: [Errno 21] Is a directory: '/etc/ceph/ceph.conf'

This occurs when attempting to remove a cluster where /etc/ceph/ceph.conf is a directory instead of the expected file.

Root Cause:

Container services (iSCSI, NFS, NVMe-oF) create bind mounts like:

  mounts[os.path.join(data_dir, 'config')] = '/etc/ceph/ceph.conf:z'

Docker/Podman has a quirk: when bind mounting to a destination that doesn't exist, the container runtime creates the destination as a DIRECTORY, not a file.

This occurs in test environments when:

1. Test framework creates /etc/ceph/ directory
2. Container service starts before bootstrap writes /etc/ceph/ceph.conf
3. Container runtime creates /etc/ceph/ceph.conf as a directory for the bind mount
4. Later, _rm_cluster() tries to open('/etc/ceph/ceph.conf') assuming it's a file, causing IsADirectoryError

Scenarios where /etc/ceph/ceph.conf may not exist as a file:

• Bootstrap with custom --output-config location
• Bootstrap failure/interruption before config write
• Race condition: container service starts before bootstrap completes
• Manual deletion of config file
• Container-only deployment without host-level config

The Fix:

Use early returns and guard clauses to handle different states of /etc/ceph/ceph.conf:

1. If ceph.conf doesn't exist: return early (nothing to clean up)
2. If ceph.conf is an empty directory: remove it and return (leftover bind mount point from container)
3. If ceph.conf exists but is not a file: return early (unexpected state)
4. If ceph.conf is a file: validate fsid and remove all config files

Fixes: https://tracker.ceph.com/issues/75275

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