mitogen/ansible_mitogen/services.py

# Copyright 2017, David Wilson
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"""
Classes in this file define Mitogen 'services' that run (initially) within the
connection multiplexer process that is forked off the top-level controller
process.

Once a worker process connects to a multiplexer process
(Connection._connect()), it communicates with these services to establish new
connections, grant access to files by children, and register for notification
when a child has completed a job.
"""

from __future__ import absolute_import
import logging
import os
import os.path
import pprint
import sys
import threading
import zlib

import mitogen
import mitogen.service
import ansible_mitogen.target


LOG = logging.getLogger(__name__)


class Error(Exception):
    pass


class ContextService(mitogen.service.Service):
    """
    Used by workers to fetch the single Context instance corresponding to a
    connection configuration, creating the matching connection if it does not
    exist.

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

    This concentrates connections in the top-level process, which may become a
    bottleneck. The bottleneck can be removed using per-CPU connection
    processes and arranging for the worker to select one according to a hash of
    the connection parameters (sharding).
    """
    handle = 500
    max_message_size = 1000
    max_interpreters = int(os.getenv('MITOGEN_MAX_INTERPRETERS', '20'))

    def __init__(self, *args, **kwargs):
        super(ContextService, self).__init__(*args, **kwargs)
        self._lock = threading.Lock()
        #: Records the :meth:`get` result dict for successful calls, returned
        #: for identical subsequent calls. Keyed by :meth:`key_from_kwargs`.
        self._response_by_key = {}
        #: List of :class:`mitogen.core.Message` waiting for the result dict
        #: for a particular connection config. Keyed as sbove.
        self._waiters_by_key = {}
        #: Mapping of :class:`mitogen.core.Context` -> reference count. Each
        #: call to :meth:`get` increases this by one. Calls to :meth:`put`
        #: decrease it by one.
        self._refs_by_context = {}
        #: List of contexts in creation order by via= parameter. When
        #: :attr:`max_interpreters` is reached, the most recently used context
        #: is destroyed to make room for any additional context.
        self._update_lru_by_via = {}
        #: :meth:`key_from_kwargs` result by Context.
        self._key_by_context = {}

    @mitogen.service.expose(mitogen.service.AllowParents())
    @mitogen.service.arg_spec({
        'context': mitogen.core.Context
    })
    def put(self, context):
        """
        Return a reference, making it eligable for recycling once its reference
        count reaches zero.
        """
        LOG.debug('%r.put(%r)', self, context)
        assert self._refs_by_context[context] > 0
        self._refs_by_context[context] -= 1

    def key_from_kwargs(self, **kwargs):
        """
        Generate a deduplication key from the request. The default
        implementation returns a string based on a stable representation of the
        input dictionary generated by :py:func:`pprint.pformat`.
        """
        return pprint.pformat(kwargs)

    def _produce_response(self, key, response):
        """
        Reply to every waiting request matching a configuration key with a
        response dictionary, deleting the list of waiters when done.

        :param str key:
            Result of :meth:`key_from_kwargs`
        :param dict response:
            Response dictionary
        :returns:
            Number of waiters that were replied to.
        """
        self._lock.acquire()
        try:
            waiters = self._waiters_by_key.pop(key)
            count = len(waiters)
            for msg in waiters:
                msg.reply(response)
        finally:
            self._lock.release()
        return count

    def _update_lru(self, new_context, **kwargs):
        """
        Update the LRU ("MRU"?) list associated with the connection described
        by `kwargs`, destroying the most recently created context if the list
        is full. Finally add `new_context` to the list.
        """
        via = kwargs.get('via')
        if via is None:
            # We don't have a limit on the number of directly connections.
            return

        lru = self._update_lru_by_via.setdefault(via, [])
        if len(lru) < self.max_interpreters:
            lru.append(new_context)
            return

        for context in reversed(lru):
            if self._refs_by_context[context] == 0:
                break
        else:
            LOG.warning('via=%r reached maximum number of interpreters, '
                        'but they are all marked as in-use.', via)
            return

        LOG.info('%r._discard_one(): shutting down %r', self, context)
        context.shutdown()

        key = self._key_by_context[context]

        self._lock.acquire()
        try:
            del self._response_by_key[key]
            del self._refs_by_context[context]
            del self._key_by_context[context]
            lru.remove(context)
            lru.append(new_context)
        finally:
            self._lock.release()

    def _connect(self, key, method_name, **kwargs):
        """
        Actual connect implementation. Arranges for the Mitogen connection to
        be created and enqueues an asynchronous call to start the forked task
        parent in the remote context.

        :param key:
            Deduplication key representing the connection configuration.
        :param method_name:
            :class:`mitogen.parent.Router` method implementing the connection
            type.
        :param kwargs:
            Keyword arguments passed to the router method.
        :returns:
            Dict like::

                {
                    'context': mitogen.core.Context or None,
                    'home_dir': str or None,
                    'msg': str or None
                }

            Where either `msg` is an error message and the remaining fields are
            :data:`None`, or `msg` is :data:`None` and the remaining fields are
            set.
        """
        method = getattr(self.router, method_name, None)
        if method is None:
            raise Error('no such Router method: %s' % (method_name,))

        try:
            context = method(**kwargs)
        except mitogen.core.StreamError as e:
            return {
                'context': None,
                'home_dir': None,
                'msg': str(e),
            }

        if kwargs.get('via'):
            self._update_lru(context, method_name=method_name, **kwargs)
        home_dir = context.call(os.path.expanduser, '~')

        # We don't need to wait for the result of this. Ideally we'd check its
        # return value somewhere, but logs will catch a failure anyway.
        context.call_async(ansible_mitogen.target.start_fork_parent)
        self._key_by_context[context] = key
        self._refs_by_context[context] = 0
        return {
            'context': context,
            'home_dir': home_dir,
            'msg': None,
        }

    @mitogen.service.expose(mitogen.service.AllowParents())
    @mitogen.service.arg_spec({
        'method_name': str
    })
    def get(self, msg, **kwargs):
        """
        Return a Context referring to an established connection with the given
        configuration, establishing a new connection as necessary.

        :param str method_name:
            The :class:`mitogen.parent.Router` connection method to use.
        :param dict kwargs:
            Keyword arguments passed to `mitogen.master.Router.[method_name]()`.

        :returns tuple:
            Tuple of `(context, home_dir)`, where:
                * `context` is the mitogen.master.Context referring to the
                  target context.
                * `home_dir` is a cached copy of the remote directory.
        """
        key = self.key_from_kwargs(**kwargs)
        self._lock.acquire()
        try:
            response = self._response_by_key.get(key)
            if response is not None:
                self._refs_by_context[response['context']] += 1
                return response

            waiters = self._waiters_by_key.get(key)
            if waiters is not None:
                waiters.append(msg)
                return self.NO_REPLY

            self._waiters_by_key[key] = [msg]
        finally:
            self._lock.release()

        # I'm the first thread to wait, so I will create the connection.
        try:
            response = self._connect(key, **kwargs)
            count = self._produce_response(key, response)
            if response['msg'] is None:
                # Only record the response for non-error results.
                self._response_by_key[key] = response
                # Set the reference count to the number of waiters.
                self._refs_by_context[response['context']] += count
        except mitogen.core.CallError:
            e = sys.exc_info()[1]
            self._produce_response(key, e)
        except Exception:
            e = sys.exc_info()[1]
            self._produce_response(key, mitogen.core.CallError(e))

        return self.NO_REPLY


class FileService(mitogen.service.Service):
    """
    Primitive latency-inducing file server for old-style incantations of the
    module runner. This is to be replaced later with a scheme that forwards
    files known to be missing without the target having to ask for them,
    avoiding a corresponding roundtrip per file.

    Paths must be explicitly added to the service by a trusted context before
    they will be served to an untrusted context.
    """
    handle = 501
    max_message_size = 1000
    unregistered_msg = 'Path is not registered with FileService.'

    def __init__(self, router):
        super(FileService, self).__init__(router)
        self._paths = {}

    @mitogen.service.expose(policy=mitogen.service.AllowParents())
    @mitogen.service.arg_spec({
        'path': basestring
    })
    def register(self, path):
        """
        Authorize a path for access by child contexts. Calling this repeatedly
        with the same path is harmless.

        :param str path:
            File path.
        """
        if path not in self._paths:
            LOG.debug('%r: registering %r', self, path)
            with open(path, 'rb') as fp:
                self._paths[path] = zlib.compress(fp.read())

    @mitogen.service.expose(policy=mitogen.service.AllowAny())
    @mitogen.service.arg_spec({
        'path': basestring
    })
    def fetch(self, path):
        """
        Fetch a file's data.

        :param str path:
            File path.

        :returns:
            The file data.

        :raises mitogen.core.CallError:
            The path was not registered.
        """
        if path not in self._paths:
            raise mitogen.core.CallError(self.unregistered_msg)

        LOG.debug('Serving %r', path)
        return self._paths[path]


class JobResultService(mitogen.service.Service):
    """
    Receive the result of a task from a child and forward it to interested
    listeners. If no listener exists, store the result until it is requested.

    Storing results in an intermediary service allows:

    * the lifetime of the worker to be decoupled from the lifetime of the job,
    * for new and unrelated workers to request the job result after the original
      worker that spawned it has exitted,
    * for synchronous and asynchronous jobs to be treated identically,
    * for latency-free polling and waiting on job results, and
    * for Ansible job IDs to be be used to refer to a job in preference to
      Mitogen-internal identifiers such as Sender and Context.

    Results are keyed by job ID.
    """
    handle = 502
    max_message_size = 1048576 * 64

    def __init__(self, router):
        super(JobResultService, self).__init__(router)
        self._lock = threading.Lock()
        self._result_by_job_id = {}
        self._sender_by_job_id = {}

    @mitogen.service.expose(mitogen.service.AllowParents())
    @mitogen.service.arg_spec({
        'job_id': str,
        'sender': mitogen.core.Sender,
    })
    def listen(self, job_id, sender):
        """
        Register to receive the result of a job when it becomes available.

        :param str job_id:
            Job ID to listen for.
        :param mitogen.core.Sender sender:
            Sender on which to deliver the job result.
        """
        LOG.debug('%r.listen(job_id=%r, sender=%r)', self, job_id, sender)
        with self._lock:
            if job_id in self._sender_by_job_id:
                raise Error('Listener already exists for job: %s' % (job_id,))
            self._sender_by_job_id[job_id] = sender

    @mitogen.service.expose(mitogen.service.AllowParents())
    @mitogen.service.arg_spec({
        'job_id': basestring,
    })
    def get(self, job_id):
        """
        Return a job's result if it is available, otherwise return immediately.
        The job result is forgotten once it has been returned by this method.

        :param str job_id:
            Job ID to return.
        :returns:
            Job result dictionary, or :data:`None`.
        """
        LOG.debug('%r.get(job_id=%r)', self, job_id)
        with self._lock:
            return self._result_by_job_id.pop(job_id, None)

    @mitogen.service.expose(mitogen.service.AllowAny())
    @mitogen.service.arg_spec({
        'job_id': basestring,
        'result': dict
    })
    def push(self, job_id, result):
        """
        Deliver a job's result from a child context, notifying any listener
        registred via :meth:`listen` of the result.

        :param str job_id:
            Job ID whose result is being pushed.
        :param dict result:
            Job result dictionary.
        """
        LOG.debug('%r.push(job_id=%r, result=%r)', self, job_id, result)
        with self._lock:
            if job_id in self._result_by_job_id:
                raise Error('Result already exists for job: %s' % (job_id,))
            sender = self._sender_by_job_id.pop(job_id, None)
            if sender:
                sender.send(result)
            else:
                self._result_by_job_id[job_id] = result