~bzr-pqm/bzr/bzr.dev

# Copyright (C) 2006 Canonical Ltd
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
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# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

"""Smart-server protocol, client and server.

This code is fairly complex, so it has been split up into a package of modules,
rather than being a single large module.  Refer to the individual module
docstrings for details.

Overview
========

The smart protocol provides a way to send a requests and corresponding
responses to communicate with a remote bzr process.

Layering
========

Medium
------

At the bottom level there is either a socket, pipes, or an HTTP
request/response.  We call this layer the *medium*.  It is responsible for
carrying bytes between a client and server.  For sockets, we have the
idea that you have multiple requests and get a read error because the other side
did shutdown.  For pipes we have read pipe which will have a zero read which
marks end-of-file.  For HTTP server environment there is no end-of-stream
because each request coming into the server is independent.

So we need a wrapper around pipes and sockets to seperate out requests from
substrate and this will give us a single model which is consistent for HTTP,
sockets and pipes.

Protocol
--------

On top of the medium is the *protocol*.  This is the layer that deserialises
bytes into the structured data that requests and responses consist of.

Version one of the protocol (for requests and responses) is described by::

  REQUEST := MESSAGE_V1
  RESPONSE := MESSAGE_V1
  MESSAGE_V1 := ARGS BODY

  ARGS := ARG [MORE_ARGS] NEWLINE
  MORE_ARGS := SEP ARG [MORE_ARGS]
  SEP := 0x01

  BODY := LENGTH NEWLINE BODY_BYTES TRAILER
  LENGTH := decimal integer
  TRAILER := "done" NEWLINE

That is, a tuple of arguments separated by Ctrl-A and terminated with a newline,
followed by length prefixed body with a constant trailer.  Note that although
arguments are not 8-bit safe (they cannot include 0x01 or 0x0a bytes without
breaking the protocol encoding), the body is.

Version two of the request protocol is::

  REQUEST_V2 := "bzr request 2" NEWLINE MESSAGE_V1

Version two of the response protocol is::

  RESPONSE_V2 := "bzr request 2" NEWLINE MESSAGE_V1

Future versions should follow this structure, like version two does::

  FUTURE_MESSAGE := VERSION_STRING NEWLINE REST_OF_MESSAGE

This is that clients and servers can read bytes up to the first newline byte to
determine what version a message is.

For compatibility will all versions (past and future) of bzr clients, servers
that receive a request in an unknown protocol version should respond with a
single-line error terminated with 0x0a (NEWLINE), rather than structured
response prefixed with a version string.

Request/Response processing
---------------------------

On top of the protocol is the logic for processing requests (on the server) or
responses (on the client).

Server-side
-----------

Sketch::

 MEDIUM  (factory for protocol, reads bytes & pushes to protocol,
          uses protocol to detect end-of-request, sends written
          bytes to client) e.g. socket, pipe, HTTP request handler.
  ^
  | bytes.
  v

 PROTOCOL(serialization, deserialization)  accepts bytes for one
          request, decodes according to internal state, pushes
          structured data to handler.  accepts structured data from
          handler and encodes and writes to the medium.  factory for
          handler.
  ^
  | structured data
  v

 HANDLER  (domain logic) accepts structured data, operates state
          machine until the request can be satisfied,
          sends structured data to the protocol.

Request handlers are registered in `bzrlib.smart.request`.


Client-side
-----------

Sketch::

 CLIENT   domain logic, accepts domain requests, generated structured
          data, reads structured data from responses and turns into
          domain data.  Sends structured data to the protocol.
          Operates state machines until the request can be delivered
          (e.g. reading from a bundle generated in bzrlib to deliver a
          complete request).

          Possibly this should just be RemoteBzrDir, RemoteTransport,
          ...
  ^
  | structured data
  v

PROTOCOL  (serialization, deserialization)  accepts structured data for one
          request, encodes and writes to the medium.  Reads bytes from the
          medium, decodes and allows the client to read structured data.
  ^
  | bytes.
  v

 MEDIUM  (accepts bytes from the protocol & delivers to the remote server.
          Allows the potocol to read bytes e.g. socket, pipe, HTTP request.

The domain logic is in `bzrlib.remote`: `RemoteBzrDir`, `RemoteBranch`, and so
on.

There is also an plain file-level transport that calls remote methods to
manipulate files on the server in `bzrlib.transport.remote`.

Paths
=====

Paths are passed across the network.  The client needs to see a namespace that
includes any repository that might need to be referenced, and the client needs
to know about a root directory beyond which it cannot ascend.

Servers run over ssh will typically want to be able to access any path the user
can access.  Public servers on the other hand (which might be over http, ssh
or tcp) will typically want to restrict access to only a particular directory
and its children, so will want to do a software virtual root at that level.
In other words they'll want to rewrite incoming paths to be under that level
(and prevent escaping using ../ tricks.)

URLs that include ~ should probably be passed across to the server verbatim
and the server can expand them.  This will proably not be meaningful when
limited to a directory?
"""

# TODO: _translate_error should be on the client, not the transport because
#     error coding is wire protocol specific.

# TODO: A plain integer from query_version is too simple; should give some
# capabilities too?

# TODO: Server should probably catch exceptions within itself and send them
# back across the network.  (But shouldn't catch KeyboardInterrupt etc)
# Also needs to somehow report protocol errors like bad requests.  Need to
# consider how we'll handle error reporting, e.g. if we get halfway through a
# bulk transfer and then something goes wrong.

# TODO: Make each request and response self-validatable, e.g. with checksums.
#
# TODO: get/put objects could be changed to gradually read back the data as it
# comes across the network
#
# TODO: What should the server do if it hits an error and has to terminate?
#
# TODO: is it useful to allow multiple chunks in the bulk data?
#
# TODO: If we get an exception during transmission of bulk data we can't just
# emit the exception because it won't be seen.
#   John proposes:  I think it would be worthwhile to have a header on each
#   chunk, that indicates it is another chunk. Then you can send an 'error'
#   chunk as long as you finish the previous chunk.
#
# TODO: Clone method on Transport; should work up towards parent directory;
# unclear how this should be stored or communicated to the server... maybe
# just pass it on all relevant requests?
#
# TODO: Better name than clone() for changing between directories.  How about
# open_dir or change_dir or chdir?
#
# TODO: Is it really good to have the notion of current directory within the
# connection?  Perhaps all Transports should factor out a common connection
# from the thing that has the directory context?
#
# TODO: The server that manages a connection should be quite small and retain
# minimum state because each of the requests are supposed to be stateless.
# Then we can write another implementation that maps to http.
#
# TODO: What to do when a client connection is garbage collected?  Maybe just
# abruptly drop the connection?
#
# TODO: Server in some cases will need to restrict access to files outside of
# a particular root directory.  LocalTransport doesn't do anything to stop you
# ascending above the base directory, so we need to prevent paths
# containing '..' in either the server or transport layers.  (Also need to
# consider what happens if someone creates a symlink pointing outside the 
# directory tree...)
#
# TODO: Server should rebase absolute paths coming across the network to put
# them under the virtual root, if one is in use.  LocalTransport currently
# doesn't do that; if you give it an absolute path it just uses it.
# 
# XXX: Arguments can't contain newlines or ascii; possibly we should e.g.
# urlescape them instead.  Indeed possibly this should just literally be
# http-over-ssh.
#
# TODO: Probably want some way for server commands to gradually produce body
# data rather than passing it as a string; they could perhaps pass an
# iterator-like callback that will gradually yield data; it probably needs a
# close() method that will always be closed to do any necessary cleanup.


# Promote some attributes from submodules into this namespace
from bzrlib.smart.request import SmartServerRequestHandler