mercurial/bundle2.py

bundle2: clarify stream parameter design in the documentation

Stream level parameter have very restricted use case. Clarify why we chosen a
textual format and point that applicative data goes in applicative parts.

# bundle2.py - generic container format to transmit arbitrary data.
#
# Copyright 2013 Facebook, Inc.
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
"""Handling of the new bundle2 format

The goal of bundle2 is to act as an atomically packet to transmit a set of
payloads in an application agnostic way. It consist in a sequence of "parts"
that will be handed to and processed by the application layer.


General format architecture
===========================

The format is architectured as follow

 - magic string
 - stream level parameters
 - payload parts (any number)
 - end of stream marker.

The current implementation accept some stream level option but no part.

Details on the Binary format
============================

All numbers are unsigned and big endian.

stream level parameters
------------------------

Binary format is as follow

:params size: (16 bits integer)

  The total number of Bytes used by the parameters

:params value: arbitrary number of Bytes

  A blob of `params size` containing the serialized version of all stream level
  parameters.

  The blob contains a space separated list of parameters.

  Parameter value are not supported yet.

  Special character in param name are not supported yet.

  Stream parameters use a simple textual format for two main reasons:

  - Stream level parameters should remains simple and we want to discourage any
    crazy usage.
  - Textual data allow easy human inspection of a the bundle2 header in case of
    troubles.

  Any Applicative level options MUST go into a bundle2 part instead.


Payload part
------------------------

Binary format is as follow

:header size: (16 bits inter)

  The total number of Bytes used by the part headers. When the header is empty
  (size = 0) this is interpreted as the end of stream marker.

  Currently forced to 0 in the current state of the implementation
"""

import util
import struct

import changegroup
from i18n import _

_pack = struct.pack
_unpack = struct.unpack

_magicstring = 'HG20'

_fstreamparamsize = '>H'

class bundle20(object):
    """represent an outgoing bundle2 container

    Use the `addparam` method to add stream level parameter. Then call
    `getchunks` to retrieve all the binary chunks of datathat compose the
    bundle2 container.

    This object does not support payload part yet."""

    def __init__(self):
        self._params = []
        self._parts = []

    def addparam(self, name, value=None):
        """add a stream level parameter"""
        self._params.append((name, value))

    def getchunks(self):
        yield _magicstring
        param = self._paramchunk()
        yield _pack(_fstreamparamsize, len(param))
        if param:
            yield param

        # no support for parts
        # to be obviously fixed soon.
        assert not self._parts
        yield '\0\0'

    def _paramchunk(self):
        """return a encoded version of all stream parameters"""
        blocks = []
        for key, value in self._params:
            # XXX no support for value yet
            assert value is None
            # XXX no escaping yet
            blocks.append(key)
        return ' '.join(blocks)

class unbundle20(object):
    """interpret a bundle2 stream

    (this will eventually yield parts)"""

    def __init__(self, fp):
        self._fp = fp
        header = self._readexact(4)
        magic, version = header[0:2], header[2:4]
        if magic != 'HG':
            raise util.Abort(_('not a Mercurial bundle'))
        if version != '20':
            raise util.Abort(_('unknown bundle version %s') % version)

    def _unpack(self, format):
        """unpack this struct format from the stream"""
        data = self._readexact(struct.calcsize(format))
        return _unpack(format, data)

    def _readexact(self, size):
        """read exactly <size> bytes from the stream"""
        return changegroup.readexactly(self._fp, size)

    @util.propertycache
    def params(self):
        """dictionnary of stream level parameters"""
        params = {}
        paramssize = self._unpack(_fstreamparamsize)[0]
        if paramssize:
            for p in self._readexact(paramssize).split(' '):
                params[p] = None
        return params

    def __iter__(self):
        """yield all parts contained in the stream"""
        # make sure param have been loaded
        self.params
        part = self._readpart()
        while part is not None:
            yield part
            part = self._readpart()

    def _readpart(self):
        """return None when an end of stream markers is reach"""
        headersize = self._readexact(2)
        assert headersize == '\0\0'
        return None