"""
A basic "plugin loader" implementation which searches for default packaged and
user-supplied parser modules and verifies them so they can be executed by
logparse.interface. The requirements for parser modules and classes are
specified in the docstring of the Parser class.

Classes in this module:
    Parser          Base class that every parser should inherit
    ParserLoader    Class used internally by logparse.interface to load parsers
"""

import importlib
from os.path import dirname
from pkgutil import iter_modules
import inspect
from pathlib import Path
from typing import get_type_hints

import logging
logger = logging.getLogger(__name__)


PARSER_DIR = "/usr/share/logparse/user-parsers"
PARSER_PKG = "logparse.parsers"


class Parser():
    """
    This is the base class that every parser should inherit. Parsers should
    each exist in their own module and contain a Parser class whose name is the
    same as the parser (e.g. `example.py` contains class `Example(Parser)`).
    Each parser module must contain exactly one Parser class definition, and
    this class cannot be a redefinition of the base Parser class (i.e. this
    class). This class must provide the parse_log() method which returns a
    logparse.formatting.Section object.
    """

    def __init__(self, name=None, path=None, info=None, deprecated=False):
        """
        The following variables can be set to display information about the
        parser. The object `self.logger` can be used as for outputting messages
        to whatever sink is set up in logparse.interface (no setup required in
        the parser module itself).
        """
        self.name = str(name) if name else None
        self.path = Path(path) if path else None
        self.info = dict(info) if info else None
        self.logger = logging.getLogger(__name__)
        self.deprecated = deprecated

    def load(self):
        """
        A generic loading method to import a parser, only used for debugging
        """
        logger.debug("Loading parser {0} from {1}".format(self.name, str(self.path) if self.path != None else "defaults"))
        return importlib.import_module(self.name)

    def parse_log(self, **args) -> None:
        """
        Every parser should provide the parse_log method which is executed at
        runtime to analyse logs. Verification checks should prevent the below
        exception from ever being raised.
        """
        raise NotImplementedError("Failed to find an entry point for parser")



class ParserLoader:
    """
    This class searches for parsers in the main logparse package
    (logparser.parsers) and optionally in another external package (default
    /usr/share/logparse).
    """

    def __init__(self, pkg=PARSER_PKG, path=PARSER_DIR):
        """
        The pkg and path attributes shouldn't need to be set on object
        creation, the default values should work fine. They are hard-coded here
        for security so that a module can't force-load a module from another
        package/location, e.g. from the internet.
        """

        self.pkg = pkg
        self.path = path
        self.parsers = []

    def search(self, pattern):
        """
        Basic wrapper for the two search functions below.
        """

        default_parser = self._search_default(pattern)
        if default_parser != None:
            self.parsers.append(default_parser)
            return default_parser
        else:
            user_parser = self._search_user(pattern)
            if user_parser != None:
                self.parsers.append(user_parser)
                return user_parser
            else:
                logger.warning("Couldn't find a matching parser module for search term {0}".format(pattern))
                return None

    def _search_user(self, pattern):
        """
        Search for a parser name `pattern` in the user-managed parser directory
        """

        logger.debug("Searching for {0} in {1}".format(pattern, self.path))
        try:
            spec = importlib.machinery.PathFinder.find_spec(pattern, path=[self.path])
            parser_module = spec.loader.load_module(spec.name)
            return self._validate_module(parser_module)
        except Exception as e:
            logger.debug("Couldn't find parser {0} in {1}: {2}".format(pattern, self.path, str(e)))
            return None

    def _search_default(self, pattern):
        """
        Search for a parser name `pattern` in the default parser package
        """
        
        # TODO use importlib.resources.is_resources() once there is a backport to Python 3.6 or below
        logger.debug("Searching for {0} in default parsers".format(pattern))
        try:
            parser_module = importlib.import_module(self.pkg + "." + pattern)
            return self._validate_module(parser_module)
        except Exception as e:
            return None 

    def _validate_module(self, parser_module):
        """
        Some basic security tests for candidate modules:
            1. Must contain exactly one Parser object
            3. This class cannot be a redefinition of the base Parser class
            4. Must provide the parse_log() method
            5. Must not return None
            6. Must not match an already-loaded class
        """

        logger.debug("Checking validity of module {0} at {1}".format(parser_module.__name__, parser_module.__file__))
        available_parsers = []
        clsmembers = inspect.getmembers(parser_module, inspect.isclass)

        # Check individual classes
        for (_, c) in clsmembers:
            if not (issubclass(c, Parser) & (c is not Parser)):
                continue
            if c in self.parsers:
                logger.warning("Parser class {0} has already been loaded from another source, ignoring it".format(c.__class__.__name__, c.__file__))
            if not inspect.isroutine(c.parse_log):
                logger.warning("Parser class {0} in {1} does not contain a parse_log() method".format(c.__class__.__name__, c.__file__))
                continue
            if None in get_type_hints(c):
                logger.warning("Parser class {0} in {1} contains a null-returning parse_log() method".format(c.__class__.__name__, c.__file__))
                continue
            logger.debug("Found parser {0}.{1}".format(c.__module__, c.__class__.__name__))
            available_parsers.append(c())

        # Check module structure
        if len(available_parsers) == 1:
            logger.debug("Parser module {0} at {1} passed validity checks".format(parser_module.__name__, parser_module.__file__))
            return available_parsers[0]
        elif len(available_parsers) == 0:
            logger.warning("No valid classes in {0} at {1}".format(parser_module.__name__, parser_module.__file__))
            return None
        elif len(available_parsers) > 1:
            logger.warning("Found multiple valid parser classes in {0} at {1} - ignoring this module".format(parser_module.__name__, parser_module.__file__))
            return None

    def load_pkg(self):
        """
        Clear the list of currently loaded packages and load all valid and
        non-deprecated parser classes from self.pkg using importlib.
        """

        available_parsers = [name for _, name, _ in iter_modules([dirname(importlib.import_module(self.pkg).__file__)])]
        for parser_name in available_parsers:
            parser_module = importlib.import_module("logparse.parsers." + parser_name)
            parser_class = self._validate_module(parser_module)
            if parser_class == None:
                continue
            parser_obj = parser_class
            if parser_obj.deprecated:
                logger.debug("Ignoring parser {0} because it is deprecated".format(parser_class.__class__.__name__))
                continue
            self.parsers.append(parser_class)
        return self.parsers

    def ignore(self, pattern):
        """
        Remove a parser from the list of currently loaded parsers
        """

        for parser in self.parsers:
            if parser.__module__ == pattern:
                self.parsers.remove(parser)
                logger.debug("Ignoring parser {0}".format(parser.__name__))
        return self.parsers