# Copyright 2017, David Wilson
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from __future__ import absolute_import
import mitogen.service


class ContextService(mitogen.service.Service):
    """
    Used by worker processes connecting back into the top-level process to
    fetch the single Context instance corresponding to the supplied connection
    configuration, creating a matching connection if it does not exist.

    For connection methods and their parameters, refer to:
        https://mitogen.readthedocs.io/en/latest/api.html#context-factories

    This concentrates all SSH connections in the top-level process, which may
    become a bottleneck. There are multiple ways to fix that: 
        * creating one .local() child context per CPU and sharding connections
          between them, using the master process to route messages, or
        * as above, but having each child create a unique UNIX listener and
          having workers connect in directly.

    :param dict dct:
        Parameters passed to mitogen.master.Router.[method](). One key,
        "method", is popped from the dictionary and used to look up the method.

    :returns mitogen.master.Context:
        Corresponding Context instance.
    """
    handle = 500
    max_message_size = 1000

    def __init__(self, router):
        super(ContextService, self).__init__(router)
        self._context_by_key = {}

    def validate_args(self, args):
        return isinstance(args, dict)

    def dispatch(self, dct, msg):
        key = repr(sorted(dct.items()))
        if key not in self._context_by_key:
            method = getattr(self.router, dct.pop('method'))
            self._context_by_key[key] = method(**dct)
        return self._context_by_key[key]