annotate mercurial/wireprotoframing.py @ 37051:40206e227412

wireproto: define and implement protocol for issuing requests

The existing HTTP and SSH wire protocols suffer from a host of flaws
and shortcomings. I've been wanting to rewrite the protocol for a while
now. Supporting partial clone - which will require new wire protocol
commands and capabilities - and other advanced server functionality
will be much easier if we start from a clean slate and don't have
to be constrained by limitations of the existing wire protocol.

This commit starts to introduce a new data exchange format for
use over the wire protocol.

The new protocol is built on top of "frames," which are atomic
units of metadata + data. Frames will make it easier to implement
proxies and other mechanisms that want to inspect data without
having to maintain state. The existing frame metadata is very
minimal and it will evolve heavily. (We will eventually support
things like concurrent requests, out-of-order responses,
compression, side-channels for status updates, etc. Some of
these will require additions to the frame header.)

Another benefit of frames is that all reads are of a fixed size.
A reader works by consuming a frame header, extracting the payload
length, then reading that many bytes. No lookahead, buffering, or
memory reallocations are needed.

The new protocol attempts to be transport agnostic. I want all that's
required to use the new protocol to be a pair of unidirectional,
half-duplex pipes. (Yes, we will eventually make use of full-duplex
pipes, but that's for another commit.) Notably, when the SSH
transport switches to this new protocol, stderr will be unused.
This is by design: the lack of stderr on HTTP harms protocol
behavior there. By shoehorning everything into a pair of pipes,
we can have more consistent behavior across transports.

We currently only define the client side parts of the new protocol,
specifically the bits for requesting that a command run. This keeps
the new code and feature small and somewhat easy to review.

We add support to `hg debugwireproto` for writing frames into
HTTP request bodies. Our tests that issue commands to the new
HTTP endpoint have been updated to transmit frames. The server
bits haven't been touched to consume the frames yet. This will
occur in the next commit...

Astute readers may notice that the command name is transmitted in
both the HTTP request URL and the command request frame. This is
partially a kludge from me initially implementing the frame-based
protocol for SSH first. But it is also a feature: I intend to
eventually support issuing multiple commands per HTTP request. This
will allow us to replace the abomination that is the "batch" wire
protocol command with a protocol-level mechanism for performing
multi-dispatch. Because I want the frame-based protocol to be
as similar as possible across transports, I'd rather we (redundantly)
include the command name in the frame than differ behavior between
transports that have out-of-band routing information (like HTTP)
readily available.

Differential Revision: https://phab.mercurial-scm.org/D2851

# wireprotoframing.py - unified framing protocol for wire protocol
#
# Copyright 2018 Gregory Szorc <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

# This file contains functionality to support the unified frame-based wire
# protocol. For details about the protocol, see
# `hg help internals.wireprotocol`.

from __future__ import absolute_import

import struct

from . import (
    util,
)

FRAME_HEADER_SIZE = 4
DEFAULT_MAX_FRAME_SIZE = 32768

FRAME_TYPE_COMMAND_NAME = 0x01
FRAME_TYPE_COMMAND_ARGUMENT = 0x02
FRAME_TYPE_COMMAND_DATA = 0x03

FRAME_TYPES = {
    b'command-name': FRAME_TYPE_COMMAND_NAME,
    b'command-argument': FRAME_TYPE_COMMAND_ARGUMENT,
    b'command-data': FRAME_TYPE_COMMAND_DATA,
}

FLAG_COMMAND_NAME_EOS = 0x01
FLAG_COMMAND_NAME_HAVE_ARGS = 0x02
FLAG_COMMAND_NAME_HAVE_DATA = 0x04

FLAGS_COMMAND = {
    b'eos': FLAG_COMMAND_NAME_EOS,
    b'have-args': FLAG_COMMAND_NAME_HAVE_ARGS,
    b'have-data': FLAG_COMMAND_NAME_HAVE_DATA,
}

FLAG_COMMAND_ARGUMENT_CONTINUATION = 0x01
FLAG_COMMAND_ARGUMENT_EOA = 0x02

FLAGS_COMMAND_ARGUMENT = {
    b'continuation': FLAG_COMMAND_ARGUMENT_CONTINUATION,
    b'eoa': FLAG_COMMAND_ARGUMENT_EOA,
}

FLAG_COMMAND_DATA_CONTINUATION = 0x01
FLAG_COMMAND_DATA_EOS = 0x02

FLAGS_COMMAND_DATA = {
    b'continuation': FLAG_COMMAND_DATA_CONTINUATION,
    b'eos': FLAG_COMMAND_DATA_EOS,
}

# Maps frame types to their available flags.
FRAME_TYPE_FLAGS = {
    FRAME_TYPE_COMMAND_NAME: FLAGS_COMMAND,
    FRAME_TYPE_COMMAND_ARGUMENT: FLAGS_COMMAND_ARGUMENT,
    FRAME_TYPE_COMMAND_DATA: FLAGS_COMMAND_DATA,
}

ARGUMENT_FRAME_HEADER = struct.Struct(r'<HH')

def makeframe(frametype, frameflags, payload):
    """Assemble a frame into a byte array."""
    # TODO assert size of payload.
    frame = bytearray(FRAME_HEADER_SIZE + len(payload))

    l = struct.pack(r'<I', len(payload))
    frame[0:3] = l[0:3]
    frame[3] = (frametype << 4) | frameflags
    frame[4:] = payload

    return frame

def makeframefromhumanstring(s):
    """Given a string of the form: <type> <flags> <payload>, creates a frame.

    This can be used by user-facing applications and tests for creating
    frames easily without having to type out a bunch of constants.

    Frame type and flags can be specified by integer or named constant.
    Flags can be delimited by `|` to bitwise OR them together.
    """
    frametype, frameflags, payload = s.split(b' ', 2)

    if frametype in FRAME_TYPES:
        frametype = FRAME_TYPES[frametype]
    else:
        frametype = int(frametype)

    finalflags = 0
    validflags = FRAME_TYPE_FLAGS[frametype]
    for flag in frameflags.split(b'|'):
        if flag in validflags:
            finalflags |= validflags[flag]
        else:
            finalflags |= int(flag)

    payload = util.unescapestr(payload)

    return makeframe(frametype, finalflags, payload)

def createcommandframes(cmd, args, datafh=None):
    """Create frames necessary to transmit a request to run a command.

    This is a generator of bytearrays. Each item represents a frame
    ready to be sent over the wire to a peer.
    """
    flags = 0
    if args:
        flags |= FLAG_COMMAND_NAME_HAVE_ARGS
    if datafh:
        flags |= FLAG_COMMAND_NAME_HAVE_DATA

    if not flags:
        flags |= FLAG_COMMAND_NAME_EOS

    yield makeframe(FRAME_TYPE_COMMAND_NAME, flags, cmd)

    for i, k in enumerate(sorted(args)):
        v = args[k]
        last = i == len(args) - 1

        # TODO handle splitting of argument values across frames.
        payload = bytearray(ARGUMENT_FRAME_HEADER.size + len(k) + len(v))
        offset = 0
        ARGUMENT_FRAME_HEADER.pack_into(payload, offset, len(k), len(v))
        offset += ARGUMENT_FRAME_HEADER.size
        payload[offset:offset + len(k)] = k
        offset += len(k)
        payload[offset:offset + len(v)] = v

        flags = FLAG_COMMAND_ARGUMENT_EOA if last else 0
        yield makeframe(FRAME_TYPE_COMMAND_ARGUMENT, flags, payload)

    if datafh:
        while True:
            data = datafh.read(DEFAULT_MAX_FRAME_SIZE)

            done = False
            if len(data) == DEFAULT_MAX_FRAME_SIZE:
                flags = FLAG_COMMAND_DATA_CONTINUATION
            else:
                flags = FLAG_COMMAND_DATA_EOS
                assert datafh.read(1) == b''
                done = True

            yield makeframe(FRAME_TYPE_COMMAND_DATA, flags, data)

            if done:
                break