meow_base/core/base_handler.py

"""
This file contains the base MEOW handler defintion. This should be inherited 
from for all handler instances.

Author(s): David Marchant
"""


from threading import Event, Thread
from typing import Any, Tuple, Dict, Union
from time import sleep

from meow_base.core.vars import VALID_CHANNELS, \
    VALID_HANDLER_NAME_CHARS, get_drt_imp_msg
from meow_base.core.meow import valid_event
from meow_base.functionality.validation import check_implementation, \
    valid_string, valid_natural
from meow_base.functionality.naming import generate_handler_id

class BaseHandler:
    # An identifier for a handler within the runner. Can be manually set in 
    # the constructor, or autogenerated if no name provided.
    name:str
    # A channel for sending messages to the runner event queue. Note that this 
    # will be overridden by a MeowRunner, if a handler instance is passed to 
    # it, and so does not need to be initialised within the handler itself, 
    # unless the handler is running independently of a runner.
    to_runner_event: VALID_CHANNELS
    # A channel for sending messages to the runner job queue. Note that this 
    # will be overridden by a MeowRunner, if a handler instance is passed to 
    # it, and so does not need to be initialised within the handler itself, 
    # unless the handler is running independently of a runner.
    to_runner_job: VALID_CHANNELS    
    # Directory where queued jobs are initially written to. Note that this 
    # will be overridden by a MeowRunner, if a handler instance is passed to 
    # it, and so does not need to be initialised within the handler itself.
    job_queue_dir:str
    # A count, for how long a handler will wait if told that there are no 
    # events in the runner, before polling again. Default is 5 seconds.
    pause_time: int
    def __init__(self, name:str='', pause_time:int=5)->None:
        """BaseHandler Constructor. This will check that any class inheriting 
        from it implements its validation functions."""
        check_implementation(type(self).handle, BaseHandler)
        check_implementation(type(self).valid_handle_criteria, BaseHandler)
        if not name:
            name = generate_handler_id()
        self._is_valid_name(name)
        self.name = name
        self._is_valid_pause_time(pause_time)
        self.pause_time = pause_time

    def __new__(cls, *args, **kwargs):
        """A check that this base class is not instantiated itself, only 
        inherited from"""
        if cls is BaseHandler:
            msg = get_drt_imp_msg(BaseHandler)
            raise TypeError(msg)
        return object.__new__(cls)

    def _is_valid_name(self, name:str)->None:
        """Validation check for 'name' variable from main constructor. Is 
        automatically called during initialisation. This does not need to be 
        overridden by child classes."""
        valid_string(name, VALID_HANDLER_NAME_CHARS)

    def _is_valid_pause_time(self, pause_time:int)->None:
        """Validation check for 'pause_time' variable from main constructor. Is 
        automatically called during initialisation. This does not need to be 
        overridden by child classes."""
        valid_natural(pause_time, hint="BaseHandler.pause_time")

    def prompt_runner_for_event(self)->Union[Dict[str,Any],Any]:
        self.to_runner_event.send(1)

        if self.to_runner_event.poll(self.pause_time):
            return self.to_runner_event.recv()
        return None

    def send_job_to_runner(self, job_id:str)->None:
        self.to_runner_job.send(job_id)

    def start(self)->None:
        """Function to start the handler as an ongoing thread, as defined by 
        the main_loop function. Together, these will execute any code in a 
        implemented handlers handle function sequentially, but concurrently to 
        any other handlers running or other runner operations. This is intended 
        as a naive mmultiprocessing implementation, and any more in depth 
        parallelisation of execution must be implemented by a user by 
        overriding this function, and the stop function."""
        self._stop_event = Event()        
        self._handle_thread = Thread(
            target=self.main_loop, 
            args=(self._stop_event,),
            daemon=True,
            name="handler_thread"
        )
        self._handle_thread.start()

    def stop(self)->None:
        """Function to stop the handler as an ongoing thread. May be overidden 
        by any child class. This function should also be overriden if the start
        function has been."""

        self._stop_event.set()
        self._handle_thread.join()
        
    def main_loop(self, stop_event)->None:
        """Function defining an ongoing thread, as started by the start 
        function and stoped by the stop function. """

        while not stop_event.is_set():
            reply = self.prompt_runner_for_event()

            # If we have recieved 'None' then we have already timed out so skip 
            # this loop and start again
            if reply is None:
                continue

            try:
                valid_event(reply)
            except Exception as e:
                # Were not given an event, so sleep before trying again
                sleep(self.pause_time)


            try:
                self.handle(reply)
            except Exception as e:
                # TODO some error reporting here
                pass

    def valid_handle_criteria(self, event:Dict[str,Any])->Tuple[bool,str]:
        """Function to determine given an event defintion, if this handler can 
        process it or not. Must be implemented by any child process."""
        pass

    def handle(self, event:Dict[str,Any])->None:
        """Function to handle a given event. Must be implemented by any child 
        process. Note that once any handling has occured, the 
        send_job_to_runner function should be called to inform the runner of 
        any resultant jobs."""
        pass