add append option to LoggingPlugin.set_log_path and return old path

[RolfBippus]

What's the problem this feature will solve?

set_log_path() currently overwrites existing log-files.
I would like to add fixtures that produce e.g. a logfile per module (scope = 'module') or for selected tests (scope = 'function').
Using set_log_path() is great for this, however the old path (if it existed) cannot be restored when upon fixture teardown. This does not allow the fixture to be e.g. combined with the --log-file commandline option for tests not using the fixture.

Describe the solution you'd like

Thus I propose to return the old path and to add an option to append to the file instead of overwriting it.

Alternative Solutions

I have tried different things, however using the set_log_path() seems the most simple way to achieve this.

Additional context

Here are the proposed changes to set_log_path:

    def set_log_path(self, fname: str, append=False) -> None:
        """Set the filename parameter for Logging.FileHandler().

        Creates parent directory if it does not exist.

        .. warning::
            This is an experimental API.
        """
        fpath = Path(fname)

        if not fpath.is_absolute():
            fpath = self._config.rootpath / fpath

        if not fpath.parent.exists():
            fpath.parent.mkdir(exist_ok=True, parents=True)

        stream = fpath.open(mode="w" if not append else "a", encoding="UTF-8")
        if sys.version_info >= (3, 7):
            old_stream = self.log_file_handler.setStream(stream)
        else:
            old_stream = self.log_file_handler.stream
            self.log_file_handler.acquire()
            try:
                self.log_file_handler.flush()
                self.log_file_handler.stream = stream
            finally:
                self.log_file_handler.release()
        old_path = None
        if old_stream:
            old_path = os.path.abspath(old_stream.name)
            old_stream.close()
        return old_path

Example of application for a fixture generating one log file per test:

It allows to write a specific logfile for selected tests and at the same time still allows the usage of --log-file from the commandline to work on all other tests. (attention, I added a pytestconfig.option.logdir option)

@pytest.fixture(scope='function')
def logfile_per_test(request, pytestconfig):
    """
    A fixture that creates a logfile per test/ function call.
    If the function is parametrized, one logfile per call(!) is created.
    """
    logging_plugin=request.config.pluginmanager.get_plugin("logging-plugin")
    filename=os.path.join(pytestconfig.option.logdir,
                          request.module.__name__ + '__' + request.node.name.replace('[','__').replace(']', '__') +".log")
    oldfile = logging_plugin.set_log_path(filename, append=False) # this overwrites the logfile!
    # write some intitial info to the logfile
    log = logging.getLogger(request.module.__name__)
    log.info(f"========= {request.module.__name__}.{request.node.name} =======================")
    
    yield
    
    # write some closing info to the logfile
    log.info(f"========= finished {request.module.__name__}.{request.node.name} ==============")
    
    # restore the old logfile, in case there had been one, and append to it
    if oldfile:
        logging_plugin.set_log_path(oldfile, append=True) # this further appends to the existing logfile!