`json_formatter` in config causes `--show-config` option to crash

[vkarak]

Here is a simple reproducer:

def foo(record, extras, ignore_keys):
    return {'foo': 1}


site_configuration = {
   'logging': [
        {
            'handlers_perflog': [
                {
                    'type': 'httpjson',
                    'url': 'https://elastic-endpoint',
                    'level': 'info',
                    'debug': True,
                    'json_formatter': foo
                }
            ]
        }
    ]
}

Running reframe -C myconf.py --show-config or anything that includes the handlers_perflog yields the following error:

Traceback (most recent call last):
  File "/Users/karakasv/Repositories/reframe/./bin/reframe", line 22, in <module>
    cli.main()
  File "/Users/karakasv/Repositories/reframe/reframe/core/logging.py", line 969, in _fn
    return fn(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^
  File "/Users/karakasv/Repositories/reframe/reframe/frontend/cli.py", line 823, in main
    printer.info(str(rt.site_config))
                 ^^^^^^^^^^^^^^^^^^^
  File "/Users/karakasv/Repositories/reframe/reframe/core/config.py", line 228, in __str__
    return json.dumps(self._pick_config(), indent=2)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
          ^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 202, in encode
    chunks = list(chunks)
             ^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 432, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 406, in _iterencode_dict
    yield from chunks
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 326, in _iterencode_list
    yield from chunks
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 406, in _iterencode_dict
    yield from chunks
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 326, in _iterencode_list
    yield from chunks
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 406, in _iterencode_dict
    yield from chunks
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 439, in _iterencode
    o = _default(o)
        ^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 180, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type function is not JSON serializable

If you instead try reframe -C myconf.py --show-config=systems, it works as expected:

[
  {
    "name": "generic",
    "descr": "Generic example system",
    "hostnames": [
      ".*"
    ],
    "partitions": [
      {
        "name": "default",
        "scheduler": "local",
        "launcher": "local",
        "environs": [
          "builtin"
        ]
      }
    ]
  }
]