manyi/venv/Lib/site-packages/tornado/http1connection.py

#
# Copyright 2014 Facebook
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations
# under the License.

"""Client and server implementations of HTTP/1.x.

.. versionadded:: 4.0
"""

from __future__ import absolute_import, division, print_function

import re
import warnings

from tornado.concurrent import (Future, future_add_done_callback,
                                future_set_result_unless_cancelled)
from tornado.escape import native_str, utf8
from tornado import gen
from tornado import httputil
from tornado import iostream
from tornado.log import gen_log, app_log
from tornado import stack_context
from tornado.util import GzipDecompressor, PY3


class _QuietException(Exception):
    def __init__(self):
        pass


class _ExceptionLoggingContext(object):
    """Used with the ``with`` statement when calling delegate methods to
    log any exceptions with the given logger.  Any exceptions caught are
    converted to _QuietException
    """
    def __init__(self, logger):
        self.logger = logger

    def __enter__(self):
        pass

    def __exit__(self, typ, value, tb):
        if value is not None:
            self.logger.error("Uncaught exception", exc_info=(typ, value, tb))
            raise _QuietException


class HTTP1ConnectionParameters(object):
    """Parameters for `.HTTP1Connection` and `.HTTP1ServerConnection`.
    """
    def __init__(self, no_keep_alive=False, chunk_size=None,
                 max_header_size=None, header_timeout=None, max_body_size=None,
                 body_timeout=None, decompress=False):
        """
        :arg bool no_keep_alive: If true, always close the connection after
            one request.
        :arg int chunk_size: how much data to read into memory at once
        :arg int max_header_size:  maximum amount of data for HTTP headers
        :arg float header_timeout: how long to wait for all headers (seconds)
        :arg int max_body_size: maximum amount of data for body
        :arg float body_timeout: how long to wait while reading body (seconds)
        :arg bool decompress: if true, decode incoming
            ``Content-Encoding: gzip``
        """
        self.no_keep_alive = no_keep_alive
        self.chunk_size = chunk_size or 65536
        self.max_header_size = max_header_size or 65536
        self.header_timeout = header_timeout
        self.max_body_size = max_body_size
        self.body_timeout = body_timeout
        self.decompress = decompress


class HTTP1Connection(httputil.HTTPConnection):
    """Implements the HTTP/1.x protocol.

    This class can be on its own for clients, or via `HTTP1ServerConnection`
    for servers.
    """
    def __init__(self, stream, is_client, params=None, context=None):
        """
        :arg stream: an `.IOStream`
        :arg bool is_client: client or server
        :arg params: a `.HTTP1ConnectionParameters` instance or ``None``
        :arg context: an opaque application-defined object that can be accessed
            as ``connection.context``.
        """
        self.is_client = is_client
        self.stream = stream
        if params is None:
            params = HTTP1ConnectionParameters()
        self.params = params
        self.context = context
        self.no_keep_alive = params.no_keep_alive
        # The body limits can be altered by the delegate, so save them
        # here instead of just referencing self.params later.
        self._max_body_size = (self.params.max_body_size or
                               self.stream.max_buffer_size)
        self._body_timeout = self.params.body_timeout
        # _write_finished is set to True when finish() has been called,
        # i.e. there will be no more data sent.  Data may still be in the
        # stream's write buffer.
        self._write_finished = False
        # True when we have read the entire incoming body.
        self._read_finished = False
        # _finish_future resolves when all data has been written and flushed
        # to the IOStream.
        self._finish_future = Future()
        # If true, the connection should be closed after this request
        # (after the response has been written in the server side,
        # and after it has been read in the client)
        self._disconnect_on_finish = False
        self._clear_callbacks()
        # Save the start lines after we read or write them; they
        # affect later processing (e.g. 304 responses and HEAD methods
        # have content-length but no bodies)
        self._request_start_line = None
        self._response_start_line = None
        self._request_headers = None
        # True if we are writing output with chunked encoding.
        self._chunking_output = None
        # While reading a body with a content-length, this is the
        # amount left to read.
        self._expected_content_remaining = None
        # A Future for our outgoing writes, returned by IOStream.write.
        self._pending_write = None

    def read_response(self, delegate):
        """Read a single HTTP response.

        Typical client-mode usage is to write a request using `write_headers`,
        `write`, and `finish`, and then call ``read_response``.

        :arg delegate: a `.HTTPMessageDelegate`

        Returns a `.Future` that resolves to None after the full response has
        been read.
        """
        if self.params.decompress:
            delegate = _GzipMessageDelegate(delegate, self.params.chunk_size)
        return self._read_message(delegate)

    @gen.coroutine
    def _read_message(self, delegate):
        need_delegate_close = False
        try:
            header_future = self.stream.read_until_regex(
                b"\r?\n\r?\n",
                max_bytes=self.params.max_header_size)
            if self.params.header_timeout is None:
                header_data = yield header_future
            else:
                try:
                    header_data = yield gen.with_timeout(
                        self.stream.io_loop.time() + self.params.header_timeout,
                        header_future,
                        quiet_exceptions=iostream.StreamClosedError)
                except gen.TimeoutError:
                    self.close()
                    raise gen.Return(False)
            start_line, headers = self._parse_headers(header_data)
            if self.is_client:
                start_line = httputil.parse_response_start_line(start_line)
                self._response_start_line = start_line
            else:
                start_line = httputil.parse_request_start_line(start_line)
                self._request_start_line = start_line
                self._request_headers = headers

            self._disconnect_on_finish = not self._can_keep_alive(
                start_line, headers)
            need_delegate_close = True
            with _ExceptionLoggingContext(app_log):
                header_future = delegate.headers_received(start_line, headers)
                if header_future is not None:
                    yield header_future
            if self.stream is None:
                # We've been detached.
                need_delegate_close = False
                raise gen.Return(False)
            skip_body = False
            if self.is_client:
                if (self._request_start_line is not None and
                        self._request_start_line.method == 'HEAD'):
                    skip_body = True
                code = start_line.code
                if code == 304:
                    # 304 responses may include the content-length header
                    # but do not actually have a body.
                    # http://tools.ietf.org/html/rfc7230#section-3.3
                    skip_body = True
                if code >= 100 and code < 200:
                    # 1xx responses should never indicate the presence of
                    # a body.
                    if ('Content-Length' in headers or
                            'Transfer-Encoding' in headers):
                        raise httputil.HTTPInputError(
                            "Response code %d cannot have body" % code)
                    # TODO: client delegates will get headers_received twice
                    # in the case of a 100-continue.  Document or change?
                    yield self._read_message(delegate)
            else:
                if (headers.get("Expect") == "100-continue" and
                        not self._write_finished):
                    self.stream.write(b"HTTP/1.1 100 (Continue)\r\n\r\n")
            if not skip_body:
                body_future = self._read_body(
                    start_line.code if self.is_client else 0, headers, delegate)
                if body_future is not None:
                    if self._body_timeout is None:
                        yield body_future
                    else:
                        try:
                            yield gen.with_timeout(
                                self.stream.io_loop.time() + self._body_timeout,
                                body_future,
                                quiet_exceptions=iostream.StreamClosedError)
                        except gen.TimeoutError:
                            gen_log.info("Timeout reading body from %s",
                                         self.context)
                            self.stream.close()
                            raise gen.Return(False)
            self._read_finished = True
            if not self._write_finished or self.is_client:
                need_delegate_close = False
                with _ExceptionLoggingContext(app_log):
                    delegate.finish()
            # If we're waiting for the application to produce an asynchronous
            # response, and we're not detached, register a close callback
            # on the stream (we didn't need one while we were reading)
            if (not self._finish_future.done() and
                    self.stream is not None and
                    not self.stream.closed()):
                self.stream.set_close_callback(self._on_connection_close)
                yield self._finish_future
            if self.is_client and self._disconnect_on_finish:
                self.close()
            if self.stream is None:
                raise gen.Return(False)
        except httputil.HTTPInputError as e:
            gen_log.info("Malformed HTTP message from %s: %s",
                         self.context, e)
            if not self.is_client:
                yield self.stream.write(b'HTTP/1.1 400 Bad Request\r\n\r\n')
            self.close()
            raise gen.Return(False)
        finally:
            if need_delegate_close:
                with _ExceptionLoggingContext(app_log):
                    delegate.on_connection_close()
            header_future = None
            self._clear_callbacks()
        raise gen.Return(True)

    def _clear_callbacks(self):
        """Clears the callback attributes.

        This allows the request handler to be garbage collected more
        quickly in CPython by breaking up reference cycles.
        """
        self._write_callback = None
        self._write_future = None
        self._close_callback = None
        if self.stream is not None:
            self.stream.set_close_callback(None)

    def set_close_callback(self, callback):
        """Sets a callback that will be run when the connection is closed.

        Note that this callback is slightly different from
        `.HTTPMessageDelegate.on_connection_close`: The
        `.HTTPMessageDelegate` method is called when the connection is
        closed while recieving a message. This callback is used when
        there is not an active delegate (for example, on the server
        side this callback is used if the client closes the connection
        after sending its request but before receiving all the
        response.
        """
        self._close_callback = stack_context.wrap(callback)

    def _on_connection_close(self):
        # Note that this callback is only registered on the IOStream
        # when we have finished reading the request and are waiting for
        # the application to produce its response.
        if self._close_callback is not None:
            callback = self._close_callback
            self._close_callback = None
            callback()
        if not self._finish_future.done():
            future_set_result_unless_cancelled(self._finish_future, None)
        self._clear_callbacks()

    def close(self):
        if self.stream is not None:
            self.stream.close()
        self._clear_callbacks()
        if not self._finish_future.done():
            future_set_result_unless_cancelled(self._finish_future, None)

    def detach(self):
        """Take control of the underlying stream.

        Returns the underlying `.IOStream` object and stops all further
        HTTP processing.  May only be called during
        `.HTTPMessageDelegate.headers_received`.  Intended for implementing
        protocols like websockets that tunnel over an HTTP handshake.
        """
        self._clear_callbacks()
        stream = self.stream
        self.stream = None
        if not self._finish_future.done():
            future_set_result_unless_cancelled(self._finish_future, None)
        return stream

    def set_body_timeout(self, timeout):
        """Sets the body timeout for a single request.

        Overrides the value from `.HTTP1ConnectionParameters`.
        """
        self._body_timeout = timeout

    def set_max_body_size(self, max_body_size):
        """Sets the body size limit for a single request.

        Overrides the value from `.HTTP1ConnectionParameters`.
        """
        self._max_body_size = max_body_size

    def write_headers(self, start_line, headers, chunk=None, callback=None):
        """Implements `.HTTPConnection.write_headers`."""
        lines = []
        if self.is_client:
            self._request_start_line = start_line
            lines.append(utf8('%s %s HTTP/1.1' % (start_line[0], start_line[1])))
            # Client requests with a non-empty body must have either a
            # Content-Length or a Transfer-Encoding.
            self._chunking_output = (
                start_line.method in ('POST', 'PUT', 'PATCH') and
                'Content-Length' not in headers and
                'Transfer-Encoding' not in headers)
        else:
            self._response_start_line = start_line
            lines.append(utf8('HTTP/1.1 %d %s' % (start_line[1], start_line[2])))
            self._chunking_output = (
                # TODO: should this use
                # self._request_start_line.version or
                # start_line.version?
                self._request_start_line.version == 'HTTP/1.1' and
                # 1xx, 204 and 304 responses have no body (not even a zero-length
                # body), and so should not have either Content-Length or
                # Transfer-Encoding headers.
                start_line.code not in (204, 304) and
                (start_line.code < 100 or start_line.code >= 200) and
                # No need to chunk the output if a Content-Length is specified.
                'Content-Length' not in headers and
                # Applications are discouraged from touching Transfer-Encoding,
                # but if they do, leave it alone.
                'Transfer-Encoding' not in headers)
            # If connection to a 1.1 client will be closed, inform client
            if (self._request_start_line.version == 'HTTP/1.1' and self._disconnect_on_finish):
                headers['Connection'] = 'close'
            # If a 1.0 client asked for keep-alive, add the header.
            if (self._request_start_line.version == 'HTTP/1.0' and
                    self._request_headers.get('Connection', '').lower() == 'keep-alive'):
                headers['Connection'] = 'Keep-Alive'
        if self._chunking_output:
            headers['Transfer-Encoding'] = 'chunked'
        if (not self.is_client and
            (self._request_start_line.method == 'HEAD' or
             start_line.code == 304)):
            self._expected_content_remaining = 0
        elif 'Content-Length' in headers:
            self._expected_content_remaining = int(headers['Content-Length'])
        else:
            self._expected_content_remaining = None
        # TODO: headers are supposed to be of type str, but we still have some
        # cases that let bytes slip through. Remove these native_str calls when those
        # are fixed.
        header_lines = (native_str(n) + ": " + native_str(v) for n, v in headers.get_all())
        if PY3:
            lines.extend(l.encode('latin1') for l in header_lines)
        else:
            lines.extend(header_lines)
        for line in lines:
            if b'\n' in line:
                raise ValueError('Newline in header: ' + repr(line))
        future = None
        if self.stream.closed():
            future = self._write_future = Future()
            future.set_exception(iostream.StreamClosedError())
            future.exception()
        else:
            if callback is not None:
                warnings.warn("callback argument is deprecated, use returned Future instead",
                              DeprecationWarning)
                self._write_callback = stack_context.wrap(callback)
            else:
                future = self._write_future = Future()
            data = b"\r\n".join(lines) + b"\r\n\r\n"
            if chunk:
                data += self._format_chunk(chunk)
            self._pending_write = self.stream.write(data)
            future_add_done_callback(self._pending_write, self._on_write_complete)
        return future

    def _format_chunk(self, chunk):
        if self._expected_content_remaining is not None:
            self._expected_content_remaining -= len(chunk)
            if self._expected_content_remaining < 0:
                # Close the stream now to stop further framing errors.
                self.stream.close()
                raise httputil.HTTPOutputError(
                    "Tried to write more data than Content-Length")
        if self._chunking_output and chunk:
            # Don't write out empty chunks because that means END-OF-STREAM
            # with chunked encoding
            return utf8("%x" % len(chunk)) + b"\r\n" + chunk + b"\r\n"
        else:
            return chunk

    def write(self, chunk, callback=None):
        """Implements `.HTTPConnection.write`.

        For backwards compatibility it is allowed but deprecated to
        skip `write_headers` and instead call `write()` with a
        pre-encoded header block.
        """
        future = None
        if self.stream.closed():
            future = self._write_future = Future()
            self._write_future.set_exception(iostream.StreamClosedError())
            self._write_future.exception()
        else:
            if callback is not None:
                warnings.warn("callback argument is deprecated, use returned Future instead",
                              DeprecationWarning)
                self._write_callback = stack_context.wrap(callback)
            else:
                future = self._write_future = Future()
            self._pending_write = self.stream.write(self._format_chunk(chunk))
            self._pending_write.add_done_callback(self._on_write_complete)
        return future

    def finish(self):
        """Implements `.HTTPConnection.finish`."""
        if (self._expected_content_remaining is not None and
                self._expected_content_remaining != 0 and
                not self.stream.closed()):
            self.stream.close()
            raise httputil.HTTPOutputError(
                "Tried to write %d bytes less than Content-Length" %
                self._expected_content_remaining)
        if self._chunking_output:
            if not self.stream.closed():
                self._pending_write = self.stream.write(b"0\r\n\r\n")
                self._pending_write.add_done_callback(self._on_write_complete)
        self._write_finished = True
        # If the app finished the request while we're still reading,
        # divert any remaining data away from the delegate and
        # close the connection when we're done sending our response.
        # Closing the connection is the only way to avoid reading the
        # whole input body.
        if not self._read_finished:
            self._disconnect_on_finish = True
        # No more data is coming, so instruct TCP to send any remaining
        # data immediately instead of waiting for a full packet or ack.
        self.stream.set_nodelay(True)
        if self._pending_write is None:
            self._finish_request(None)
        else:
            future_add_done_callback(self._pending_write, self._finish_request)

    def _on_write_complete(self, future):
        exc = future.exception()
        if exc is not None and not isinstance(exc, iostream.StreamClosedError):
            future.result()
        if self._write_callback is not None:
            callback = self._write_callback
            self._write_callback = None
            self.stream.io_loop.add_callback(callback)
        if self._write_future is not None:
            future = self._write_future
            self._write_future = None
            future_set_result_unless_cancelled(future, None)

    def _can_keep_alive(self, start_line, headers):
        if self.params.no_keep_alive:
            return False
        connection_header = headers.get("Connection")
        if connection_header is not None:
            connection_header = connection_header.lower()
        if start_line.version == "HTTP/1.1":
            return connection_header != "close"
        elif ("Content-Length" in headers or
              headers.get("Transfer-Encoding", "").lower() == "chunked" or
              getattr(start_line, 'method', None) in ("HEAD", "GET")):
            # start_line may be a request or response start line; only
            # the former has a method attribute.
            return connection_header == "keep-alive"
        return False

    def _finish_request(self, future):
        self._clear_callbacks()
        if not self.is_client and self._disconnect_on_finish:
            self.close()
            return
        # Turn Nagle's algorithm back on, leaving the stream in its
        # default state for the next request.
        self.stream.set_nodelay(False)
        if not self._finish_future.done():
            future_set_result_unless_cancelled(self._finish_future, None)

    def _parse_headers(self, data):
        # The lstrip removes newlines that some implementations sometimes
        # insert between messages of a reused connection.  Per RFC 7230,
        # we SHOULD ignore at least one empty line before the request.
        # http://tools.ietf.org/html/rfc7230#section-3.5
        data = native_str(data.decode('latin1')).lstrip("\r\n")
        # RFC 7230 section allows for both CRLF and bare LF.
        eol = data.find("\n")
        start_line = data[:eol].rstrip("\r")
        headers = httputil.HTTPHeaders.parse(data[eol:])
        return start_line, headers

    def _read_body(self, code, headers, delegate):
        if "Content-Length" in headers:
            if "Transfer-Encoding" in headers:
                # Response cannot contain both Content-Length and
                # Transfer-Encoding headers.
                # http://tools.ietf.org/html/rfc7230#section-3.3.3
                raise httputil.HTTPInputError(
                    "Response with both Transfer-Encoding and Content-Length")
            if "," in headers["Content-Length"]:
                # Proxies sometimes cause Content-Length headers to get
                # duplicated.  If all the values are identical then we can
                # use them but if they differ it's an error.
                pieces = re.split(r',\s*', headers["Content-Length"])
                if any(i != pieces[0] for i in pieces):
                    raise httputil.HTTPInputError(
                        "Multiple unequal Content-Lengths: %r" %
                        headers["Content-Length"])
                headers["Content-Length"] = pieces[0]

            try:
                content_length = int(headers["Content-Length"])
            except ValueError:
                # Handles non-integer Content-Length value.
                raise httputil.HTTPInputError(
                    "Only integer Content-Length is allowed: %s" % headers["Content-Length"])

            if content_length > self._max_body_size:
                raise httputil.HTTPInputError("Content-Length too long")
        else:
            content_length = None

        if code == 204:
            # This response code is not allowed to have a non-empty body,
            # and has an implicit length of zero instead of read-until-close.
            # http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.3
            if ("Transfer-Encoding" in headers or
                    content_length not in (None, 0)):
                raise httputil.HTTPInputError(
                    "Response with code %d should not have body" % code)
            content_length = 0

        if content_length is not None:
            return self._read_fixed_body(content_length, delegate)
        if headers.get("Transfer-Encoding", "").lower() == "chunked":
            return self._read_chunked_body(delegate)
        if self.is_client:
            return self._read_body_until_close(delegate)
        return None

    @gen.coroutine
    def _read_fixed_body(self, content_length, delegate):
        while content_length > 0:
            body = yield self.stream.read_bytes(
                min(self.params.chunk_size, content_length), partial=True)
            content_length -= len(body)
            if not self._write_finished or self.is_client:
                with _ExceptionLoggingContext(app_log):
                    ret = delegate.data_received(body)
                    if ret is not None:
                        yield ret

    @gen.coroutine
    def _read_chunked_body(self, delegate):
        # TODO: "chunk extensions" http://tools.ietf.org/html/rfc2616#section-3.6.1
        total_size = 0
        while True:
            chunk_len = yield self.stream.read_until(b"\r\n", max_bytes=64)
            chunk_len = int(chunk_len.strip(), 16)
            if chunk_len == 0:
                crlf = yield self.stream.read_bytes(2)
                if crlf != b'\r\n':
                    raise httputil.HTTPInputError("improperly terminated chunked request")
                return
            total_size += chunk_len
            if total_size > self._max_body_size:
                raise httputil.HTTPInputError("chunked body too large")
            bytes_to_read = chunk_len
            while bytes_to_read:
                chunk = yield self.stream.read_bytes(
                    min(bytes_to_read, self.params.chunk_size), partial=True)
                bytes_to_read -= len(chunk)
                if not self._write_finished or self.is_client:
                    with _ExceptionLoggingContext(app_log):
                        ret = delegate.data_received(chunk)
                        if ret is not None:
                            yield ret
            # chunk ends with \r\n
            crlf = yield self.stream.read_bytes(2)
            assert crlf == b"\r\n"

    @gen.coroutine
    def _read_body_until_close(self, delegate):
        body = yield self.stream.read_until_close()
        if not self._write_finished or self.is_client:
            with _ExceptionLoggingContext(app_log):
                delegate.data_received(body)


class _GzipMessageDelegate(httputil.HTTPMessageDelegate):
    """Wraps an `HTTPMessageDelegate` to decode ``Content-Encoding: gzip``.
    """
    def __init__(self, delegate, chunk_size):
        self._delegate = delegate
        self._chunk_size = chunk_size
        self._decompressor = None

    def headers_received(self, start_line, headers):
        if headers.get("Content-Encoding") == "gzip":
            self._decompressor = GzipDecompressor()
            # Downstream delegates will only see uncompressed data,
            # so rename the content-encoding header.
            # (but note that curl_httpclient doesn't do this).
            headers.add("X-Consumed-Content-Encoding",
                        headers["Content-Encoding"])
            del headers["Content-Encoding"]
        return self._delegate.headers_received(start_line, headers)

    @gen.coroutine
    def data_received(self, chunk):
        if self._decompressor:
            compressed_data = chunk
            while compressed_data:
                decompressed = self._decompressor.decompress(
                    compressed_data, self._chunk_size)
                if decompressed:
                    ret = self._delegate.data_received(decompressed)
                    if ret is not None:
                        yield ret
                compressed_data = self._decompressor.unconsumed_tail
        else:
            ret = self._delegate.data_received(chunk)
            if ret is not None:
                yield ret

    def finish(self):
        if self._decompressor is not None:
            tail = self._decompressor.flush()
            if tail:
                # I believe the tail will always be empty (i.e.
                # decompress will return all it can).  The purpose
                # of the flush call is to detect errors such
                # as truncated input.  But in case it ever returns
                # anything, treat it as an extra chunk
                self._delegate.data_received(tail)
        return self._delegate.finish()

    def on_connection_close(self):
        return self._delegate.on_connection_close()


class HTTP1ServerConnection(object):
    """An HTTP/1.x server."""
    def __init__(self, stream, params=None, context=None):
        """
        :arg stream: an `.IOStream`
        :arg params: a `.HTTP1ConnectionParameters` or None
        :arg context: an opaque application-defined object that is accessible
            as ``connection.context``
        """
        self.stream = stream
        if params is None:
            params = HTTP1ConnectionParameters()
        self.params = params
        self.context = context
        self._serving_future = None

    @gen.coroutine
    def close(self):
        """Closes the connection.

        Returns a `.Future` that resolves after the serving loop has exited.
        """
        self.stream.close()
        # Block until the serving loop is done, but ignore any exceptions
        # (start_serving is already responsible for logging them).
        try:
            yield self._serving_future
        except Exception:
            pass

    def start_serving(self, delegate):
        """Starts serving requests on this connection.

        :arg delegate: a `.HTTPServerConnectionDelegate`
        """
        assert isinstance(delegate, httputil.HTTPServerConnectionDelegate)
        self._serving_future = self._server_request_loop(delegate)
        # Register the future on the IOLoop so its errors get logged.
        self.stream.io_loop.add_future(self._serving_future,
                                       lambda f: f.result())

    @gen.coroutine
    def _server_request_loop(self, delegate):
        try:
            while True:
                conn = HTTP1Connection(self.stream, False,
                                       self.params, self.context)
                request_delegate = delegate.start_request(self, conn)
                try:
                    ret = yield conn.read_response(request_delegate)
                except (iostream.StreamClosedError,
                        iostream.UnsatisfiableReadError):
                    return
                except _QuietException:
                    # This exception was already logged.
                    conn.close()
                    return
                except Exception:
                    gen_log.error("Uncaught exception", exc_info=True)
                    conn.close()
                    return
                if not ret:
                    return
                yield gen.moment
        finally:
            delegate.on_close(self)