retry.py

from __future__ import absolute_import
import time
import logging
from collections import namedtuple
from itertools import takewhile
import email
import re

from ..exceptions import (
    ConnectTimeoutError,
    MaxRetryError,
    ProtocolError,
    ReadTimeoutError,
    ResponseError,
    InvalidHeader,
)
from ..packages import six


log = logging.getLogger(__name__)


# Data structure for representing the metadata of requests that result in a retry.
RequestHistory = namedtuple('RequestHistory', ["method", "url", "error",
                                               "status", "redirect_location"])


class Retry(object):
    """ Retry configuration.

    Each retry attempt will create a new Retry object with updated values, so
    they can be safely reused.

    Retries can be defined as a default for a pool::

        retries = Retry(connect=5, read=2, redirect=5)
        http = PoolManager(retries=retries)
        response = http.request('GET', 'http://example.com/')

    Or per-request (which overrides the default for the pool)::

        response = http.request('GET', 'http://example.com/', retries=Retry(10))

    Retries can be disabled by passing ``False``::

        response = http.request('GET', 'http://example.com/', retries=False)

    Errors will be wrapped in :class:`~urllib3.exceptions.MaxRetryError` unless
    retries are disabled, in which case the causing exception will be raised.

    :param int total:
        Total number of retries to allow. Takes precedence over other counts.

        Set to ``None`` to remove this constraint and fall back on other
        counts. It's a good idea to set this to some sensibly-high value to
        account for unexpected edge cases and avoid infinite retry loops.

        Set to ``0`` to fail on the first retry.

        Set to ``False`` to disable and imply ``raise_on_redirect=False``.

    :param int connect:
        How many connection-related errors to retry on.

        These are errors raised before the request is sent to the remote server,
        which we assume has not triggered the server to process the request.

        Set to ``0`` to fail on the first retry of this type.

    :param int read:
        How many times to retry on read errors.

        These errors are raised after the request was sent to the server, so the
        request may have side-effects.

        Set to ``0`` to fail on the first retry of this type.

    :param int redirect:
        How many redirects to perform. Limit this to avoid infinite redirect
        loops.

        A redirect is a HTTP response with a status code 301, 302, 303, 307 or
        308.

        Set to ``0`` to fail on the first retry of this type.

        Set to ``False`` to disable and imply ``raise_on_redirect=False``.

    :param int status:
        How many times to retry on bad status codes.

        These are retries made on responses, where status code matches
        ``status_forcelist``.

        Set to ``0`` to fail on the first retry of this type.

    :param iterable method_whitelist:
        Set of uppercased HTTP method verbs that we should retry on.

        By default, we only retry on methods which are considered to be
        idempotent (multiple requests with the same parameters end with the
        same state). See :attr:`Retry.DEFAULT_METHOD_WHITELIST`.

        Set to a ``False`` value to retry on any verb.

    :param iterable status_forcelist:
        A set of integer HTTP status codes that we should force a retry on.
        A retry is initiated if the request method is in ``method_whitelist``
        and the response status code is in ``status_forcelist``.

        By default, this is disabled with ``None``.

    :param float backoff_factor:
        A backoff factor to apply between attempts after the second try
        (most errors are resolved immediately by a second try without a
        delay). urllib3 will sleep for::

            {backoff factor} * (2 ** ({number of total retries} - 1))

        seconds. If the backoff_factor is 0.1, then :func:`.sleep` will sleep
        for [0.0s, 0.2s, 0.4s, ...] between retries. It will never be longer
        than :attr:`Retry.BACKOFF_MAX`.

        By default, backoff is disabled (set to 0).

    :param bool raise_on_redirect: Whether, if the number of redirects is
        exhausted, to raise a MaxRetryError, or to return a response with a
        response code in the 3xx range.

    :param bool raise_on_status: Similar meaning to ``raise_on_redirect``:
        whether we should raise an exception, or return a response,
        if status falls in ``status_forcelist`` range and retries have
        been exhausted.

    :param tuple history: The history of the request encountered during
        each call to :meth:`~Retry.increment`. The list is in the order
        the requests occurred. Each list item is of class :class:`RequestHistory`.

    :param bool respect_retry_after_header:
        Whether to respect Retry-After header on status codes defined as
        :attr:`Retry.RETRY_AFTER_STATUS_CODES` or not.

    :param iterable remove_headers_on_redirect:
        Sequence of headers to remove from the request when a response
        indicating a redirect is returned before firing off the redirected
        request.
    """

    DEFAULT_METHOD_WHITELIST = frozenset([
        'HEAD', 'GET', 'PUT', 'DELETE', 'OPTIONS', 'TRACE'])

    RETRY_AFTER_STATUS_CODES = frozenset([413, 429, 503])

    DEFAULT_REDIRECT_HEADERS_BLACKLIST = frozenset(['Authorization'])

    #: Maximum backoff time.
    BACKOFF_MAX = 120

    def __init__(self, total=10, connect=None, read=None, redirect=None, status=None,
                 method_whitelist=DEFAULT_METHOD_WHITELIST, status_forcelist=None,
                 backoff_factor=0, raise_on_redirect=True, raise_on_status=True,
                 history=None, respect_retry_after_header=True,
                 remove_headers_on_redirect=DEFAULT_REDIRECT_HEADERS_BLACKLIST):

        self.total = total
        self.connect = connect
        self.read = read
        self.status = status

        if redirect is False or total is False:
            redirect = 0
            raise_on_redirect = False

        self.redirect = redirect
        self.status_forcelist = status_forcelist or set()
        self.method_whitelist = method_whitelist
        self.backoff_factor = backoff_factor
        self.raise_on_redirect = raise_on_redirect
        self.raise_on_status = raise_on_status
        self.history = history or tuple()
        self.respect_retry_after_header = respect_retry_after_header
        self.remove_headers_on_redirect = remove_headers_on_redirect

    def new(self, **kw):
        params = dict(
            total=self.total,
            connect=self.connect, read=self.read, redirect=self.redirect, status=self.status,
            method_whitelist=self.method_whitelist,
            status_forcelist=self.status_forcelist,
            backoff_factor=self.backoff_factor,
            raise_on_redirect=self.raise_on_redirect,
            raise_on_status=self.raise_on_status,
            history=self.history,
            remove_headers_on_redirect=self.remove_headers_on_redirect
        )
        params.update(kw)
        return type(self)(**params)

    @classmethod
    def from_int(cls, retries, redirect=True, default=None):
        """ Backwards-compatibility for the old retries format."""
        if retries is None:
            retries = default if default is not None else cls.DEFAULT

        if isinstance(retries, Retry):
            return retries

        redirect = bool(redirect) and None
        new_retries = cls(retries, redirect=redirect)
        log.debug("Converted retries value: %r -> %r", retries, new_retries)
        return new_retries

    def get_backoff_time(self):
        """ Formula for computing the current backoff

        :rtype: float
        """
        # We want to consider only the last consecutive errors sequence (Ignore redirects).
        consecutive_errors_len = len(list(takewhile(lambda x: x.redirect_location is None,
                                                    reversed(self.history))))
        if consecutive_errors_len <= 1:
            return 0

        backoff_value = self.backoff_factor * (2 ** (consecutive_errors_len - 1))
        return min(self.BACKOFF_MAX, backoff_value)

    def parse_retry_after(self, retry_after):
        # Whitespace: https://tools.ietf.org/html/rfc7230#section-3.2.4
        if re.match(r"^\s*[0-9]+\s*$", retry_after):
            seconds = int(retry_after)
        else:
            retry_date_tuple = email.utils.parsedate(retry_after)
            if retry_date_tuple is None:
                raise InvalidHeader("Invalid Retry-After header: %s" % retry_after)
            retry_date = time.mktime(retry_date_tuple)
            seconds = retry_date - time.time()

        if seconds < 0:
            seconds = 0

        return seconds

    def get_retry_after(self, response):
        """ Get the value of Retry-After in seconds. """

        retry_after = response.getheader("Retry-After")

        if retry_after is None:
            return None

        return self.parse_retry_after(retry_after)

    def sleep_for_retry(self, response=None):
        retry_after = self.get_retry_after(response)
        if retry_after:
            time.sleep(retry_after)
            return True

        return False

    def _sleep_backoff(self):
        backoff = self.get_backoff_time()
        if backoff <= 0:
            return
        time.sleep(backoff)

    def sleep(self, response=None):
        """ Sleep between retry attempts.

        This method will respect a server's ``Retry-After`` response header
        and sleep the duration of the time requested. If that is not present, it
        will use an exponential backoff. By default, the backoff factor is 0 and
        this method will return immediately.
        """

        if response:
            slept = self.sleep_for_retry(response)
            if slept:
                return

        self._sleep_backoff()

    def _is_connection_error(self, err):
        """ Errors when we're fairly sure that the server did not receive the
        request, so it should be safe to retry.
        """
        return isinstance(err, ConnectTimeoutError)

    def _is_read_error(self, err):
        """ Errors that occur after the request has been started, so we should
        assume that the server began processing it.
        """
        return isinstance(err, (ReadTimeoutError, ProtocolError))

    def _is_method_retryable(self, method):
        """ Checks if a given HTTP method should be retried upon, depending if
        it is included on the method whitelist.
        """
        if self.method_whitelist and method.upper() not in self.method_whitelist:
            return False

        return True

    def is_retry(self, method, status_code, has_retry_after=False):
        """ Is this method/status code retryable? (Based on whitelists and control
        variables such as the number of total retries to allow, whether to
        respect the Retry-After header, whether this header is present, and
        whether the returned status code is on the list of status codes to
        be retried upon on the presence of the aforementioned header)
        """
        if not self._is_method_retryable(method):
            return False

        if self.status_forcelist and status_code in self.status_forcelist:
            return True

        return (self.total and self.respect_retry_after_header and
                has_retry_after and (status_code in self.RETRY_AFTER_STATUS_CODES))

    def is_exhausted(self):
        """ Are we out of retries? """
        retry_counts = (self.total, self.connect, self.read, self.redirect, self.status)
        retry_counts = list(filter(None, retry_counts))
        if not retry_counts:
            return False

        return min(retry_counts) < 0

    def increment(self, method=None, url=None, response=None, error=None,
                  _pool=None, _stacktrace=None):
        """ Return a new Retry object with incremented retry counters.

        :param response: A response object, or None, if the server did not
            return a response.
        :type response: :class:`~urllib3.response.HTTPResponse`
        :param Exception error: An error encountered during the request, or
            None if the response was received successfully.

        :return: A new ``Retry`` object.
        """
        if self.total is False and error:
            # Disabled, indicate to re-raise the error.
            raise six.reraise(type(error), error, _stacktrace)

        total = self.total
        if total is not None:
            total -= 1

        connect = self.connect
        read = self.read
        redirect = self.redirect
        status_count = self.status
        cause = 'unknown'
        status = None
        redirect_location = None

        if error and self._is_connection_error(error):
            # Connect retry?
            if connect is False:
                raise six.reraise(type(error), error, _stacktrace)
            elif connect is not None:
                connect -= 1

        elif error and self._is_read_error(error):
            # Read retry?
            if read is False or not self._is_method_retryable(method):
                raise six.reraise(type(error), error, _stacktrace)
            elif read is not None:
                read -= 1

        elif response and response.get_redirect_location():
            # Redirect retry?
            if redirect is not None:
                redirect -= 1
            cause = 'too many redirects'
            redirect_location = response.get_redirect_location()
            status = response.status

        else:
            # Incrementing because of a server error like a 500 in
            # status_forcelist and a the given method is in the whitelist
            cause = ResponseError.GENERIC_ERROR
            if response and response.status:
                if status_count is not None:
                    status_count -= 1
                cause = ResponseError.SPECIFIC_ERROR.format(
                    status_code=response.status)
                status = response.status

        history = self.history + (RequestHistory(method, url, error, status, redirect_location),)

        new_retry = self.new(
            total=total,
            connect=connect, read=read, redirect=redirect, status=status_count,
            history=history)

        if new_retry.is_exhausted():
            raise MaxRetryError(_pool, url, error or ResponseError(cause))

        log.debug("Incremented Retry for (url='%s'): %r", url, new_retry)

        return new_retry

    def __repr__(self):
        return ('{cls.__name__}(total={self.total}, connect={self.connect}, '
                'read={self.read}, redirect={self.redirect}, status={self.status})').format(
                    cls=type(self), self=self)


# For backwards compatibility (equivalent to pre-v1.9):
Retry.DEFAULT = Retry(3)