view mercurial/wireprotoserver.py @ 47965:f9e6f2bb721d

rhg: Don’t compare ambiguous files one byte at a time

Even though the use of `BufReader` reduces the number of syscalls to read
the file from disk, `.bytes()` yields a separate `Result` for every byte.
Creating those results and dispatching on them is most likely costly.

Instead, this commit opts for simplicity by reading the entire file into memory
and comparing a single pair of byte strings. Note that memory already needs to
contain the entire previous contents of the file, as read from the filelog.
So with an extremely large file this doubles memory use but does not make it
grow by orders of magnitude.

At first I wrote code that still avoids reading the entire file into memory
and compares one buffer at a time with `BufReader`. Find this code below for
posterity. However its correctness is subtle. I ended up preferring the
simplicity of the obviously-correct single comparison.

```rust
let mut reader = BufReader::new(fobj);
let mut expected = &contents_in_p1[..];
loop {
    let buf = reader.fill_buf().when_reading_file(&fs_path)?;
    if buf.is_empty() {
        // Found EOF
        return Ok(expected.is_empty());
    } else if let Some(rest) = expected.drop_prefix(buf) {
        // What we read so far matches the expected content, continue reading
        let buf_len = buf.len();
        reader.consume(buf_len);
        expected = rest
    } else {
        // Found different content
        return Ok(false);
    }
}
```

Differential Revision: https://phab.mercurial-scm.org/D11412

# Copyright 21 May 2005 - (c) 2005 Jake Edge <[e-mail]>
# Copyright 2005-2007 Olivia Mackall <[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.

from __future__ import absolute_import

import contextlib
import struct
import threading

from .i18n import _
from . import (
    encoding,
    error,
    pycompat,
    util,
    wireprototypes,
    wireprotov1server,
    wireprotov2server,
)
from .interfaces import util as interfaceutil
from .utils import (
    cborutil,
    compression,
    stringutil,
)

stringio = util.stringio

urlerr = util.urlerr
urlreq = util.urlreq

HTTP_OK = 200

HGTYPE = b'application/mercurial-0.1'
HGTYPE2 = b'application/mercurial-0.2'
HGERRTYPE = b'application/hg-error'

SSHV1 = wireprototypes.SSHV1
SSHV2 = wireprototypes.SSHV2


def decodevaluefromheaders(req, headerprefix):
    """Decode a long value from multiple HTTP request headers.

    Returns the value as a bytes, not a str.
    """
    chunks = []
    i = 1
    while True:
        v = req.headers.get(b'%s-%d' % (headerprefix, i))
        if v is None:
            break
        chunks.append(pycompat.bytesurl(v))
        i += 1

    return b''.join(chunks)


@interfaceutil.implementer(wireprototypes.baseprotocolhandler)
class httpv1protocolhandler(object):
    def __init__(self, req, ui, checkperm):
        self._req = req
        self._ui = ui
        self._checkperm = checkperm
        self._protocaps = None

    @property
    def name(self):
        return b'http-v1'

    def getargs(self, args):
        knownargs = self._args()
        data = {}
        keys = args.split()
        for k in keys:
            if k == b'*':
                star = {}
                for key in knownargs.keys():
                    if key != b'cmd' and key not in keys:
                        star[key] = knownargs[key][0]
                data[b'*'] = star
            else:
                data[k] = knownargs[k][0]
        return [data[k] for k in keys]

    def _args(self):
        args = self._req.qsparams.asdictoflists()
        postlen = int(self._req.headers.get(b'X-HgArgs-Post', 0))
        if postlen:
            args.update(
                urlreq.parseqs(
                    self._req.bodyfh.read(postlen), keep_blank_values=True
                )
            )
            return args

        argvalue = decodevaluefromheaders(self._req, b'X-HgArg')
        args.update(urlreq.parseqs(argvalue, keep_blank_values=True))
        return args

    def getprotocaps(self):
        if self._protocaps is None:
            value = decodevaluefromheaders(self._req, b'X-HgProto')
            self._protocaps = set(value.split(b' '))
        return self._protocaps

    def getpayload(self):
        # Existing clients *always* send Content-Length.
        length = int(self._req.headers[b'Content-Length'])

        # If httppostargs is used, we need to read Content-Length
        # minus the amount that was consumed by args.
        length -= int(self._req.headers.get(b'X-HgArgs-Post', 0))
        return util.filechunkiter(self._req.bodyfh, limit=length)

    @contextlib.contextmanager
    def mayberedirectstdio(self):
        oldout = self._ui.fout
        olderr = self._ui.ferr

        out = util.stringio()

        try:
            self._ui.fout = out
            self._ui.ferr = out
            yield out
        finally:
            self._ui.fout = oldout
            self._ui.ferr = olderr

    def client(self):
        return b'remote:%s:%s:%s' % (
            self._req.urlscheme,
            urlreq.quote(self._req.remotehost or b''),
            urlreq.quote(self._req.remoteuser or b''),
        )

    def addcapabilities(self, repo, caps):
        caps.append(b'batch')

        caps.append(
            b'httpheader=%d' % repo.ui.configint(b'server', b'maxhttpheaderlen')
        )
        if repo.ui.configbool(b'experimental', b'httppostargs'):
            caps.append(b'httppostargs')

        # FUTURE advertise 0.2rx once support is implemented
        # FUTURE advertise minrx and mintx after consulting config option
        caps.append(b'httpmediatype=0.1rx,0.1tx,0.2tx')

        compengines = wireprototypes.supportedcompengines(
            repo.ui, compression.SERVERROLE
        )
        if compengines:
            comptypes = b','.join(
                urlreq.quote(e.wireprotosupport().name) for e in compengines
            )
            caps.append(b'compression=%s' % comptypes)

        return caps

    def checkperm(self, perm):
        return self._checkperm(perm)


# This method exists mostly so that extensions like remotefilelog can
# disable a kludgey legacy method only over http. As of early 2018,
# there are no other known users, so with any luck we can discard this
# hook if remotefilelog becomes a first-party extension.
def iscmd(cmd):
    return cmd in wireprotov1server.commands


def handlewsgirequest(rctx, req, res, checkperm):
    """Possibly process a wire protocol request.

    If the current request is a wire protocol request, the request is
    processed by this function.

    ``req`` is a ``parsedrequest`` instance.
    ``res`` is a ``wsgiresponse`` instance.

    Returns a bool indicating if the request was serviced. If set, the caller
    should stop processing the request, as a response has already been issued.
    """
    # Avoid cycle involving hg module.
    from .hgweb import common as hgwebcommon

    repo = rctx.repo

    # HTTP version 1 wire protocol requests are denoted by a "cmd" query
    # string parameter. If it isn't present, this isn't a wire protocol
    # request.
    if b'cmd' not in req.qsparams:
        return False

    cmd = req.qsparams[b'cmd']

    # The "cmd" request parameter is used by both the wire protocol and hgweb.
    # While not all wire protocol commands are available for all transports,
    # if we see a "cmd" value that resembles a known wire protocol command, we
    # route it to a protocol handler. This is better than routing possible
    # wire protocol requests to hgweb because it prevents hgweb from using
    # known wire protocol commands and it is less confusing for machine
    # clients.
    if not iscmd(cmd):
        return False

    # The "cmd" query string argument is only valid on the root path of the
    # repo. e.g. ``/?cmd=foo``, ``/repo?cmd=foo``. URL paths within the repo
    # like ``/blah?cmd=foo`` are not allowed. So don't recognize the request
    # in this case. We send an HTTP 404 for backwards compatibility reasons.
    if req.dispatchpath:
        res.status = hgwebcommon.statusmessage(404)
        res.headers[b'Content-Type'] = HGTYPE
        # TODO This is not a good response to issue for this request. This
        # is mostly for BC for now.
        res.setbodybytes(b'0\n%s\n' % b'Not Found')
        return True

    proto = httpv1protocolhandler(
        req, repo.ui, lambda perm: checkperm(rctx, req, perm)
    )

    # The permissions checker should be the only thing that can raise an
    # ErrorResponse. It is kind of a layer violation to catch an hgweb
    # exception here. So consider refactoring into a exception type that
    # is associated with the wire protocol.
    try:
        _callhttp(repo, req, res, proto, cmd)
    except hgwebcommon.ErrorResponse as e:
        for k, v in e.headers:
            res.headers[k] = v
        res.status = hgwebcommon.statusmessage(
            e.code, stringutil.forcebytestr(e)
        )
        # TODO This response body assumes the failed command was
        # "unbundle." That assumption is not always valid.
        res.setbodybytes(b'0\n%s\n' % stringutil.forcebytestr(e))

    return True


def _availableapis(repo):
    apis = set()

    # Registered APIs are made available via config options of the name of
    # the protocol.
    for k, v in API_HANDLERS.items():
        section, option = v[b'config']
        if repo.ui.configbool(section, option):
            apis.add(k)

    return apis


def handlewsgiapirequest(rctx, req, res, checkperm):
    """Handle requests to /api/*."""
    assert req.dispatchparts[0] == b'api'

    repo = rctx.repo

    # This whole URL space is experimental for now. But we want to
    # reserve the URL space. So, 404 all URLs if the feature isn't enabled.
    if not repo.ui.configbool(b'experimental', b'web.apiserver'):
        res.status = b'404 Not Found'
        res.headers[b'Content-Type'] = b'text/plain'
        res.setbodybytes(_(b'Experimental API server endpoint not enabled'))
        return

    # The URL space is /api/<protocol>/*. The structure of URLs under varies
    # by <protocol>.

    availableapis = _availableapis(repo)

    # Requests to /api/ list available APIs.
    if req.dispatchparts == [b'api']:
        res.status = b'200 OK'
        res.headers[b'Content-Type'] = b'text/plain'
        lines = [
            _(
                b'APIs can be accessed at /api/<name>, where <name> can be '
                b'one of the following:\n'
            )
        ]
        if availableapis:
            lines.extend(sorted(availableapis))
        else:
            lines.append(_(b'(no available APIs)\n'))
        res.setbodybytes(b'\n'.join(lines))
        return

    proto = req.dispatchparts[1]

    if proto not in API_HANDLERS:
        res.status = b'404 Not Found'
        res.headers[b'Content-Type'] = b'text/plain'
        res.setbodybytes(
            _(b'Unknown API: %s\nKnown APIs: %s')
            % (proto, b', '.join(sorted(availableapis)))
        )
        return

    if proto not in availableapis:
        res.status = b'404 Not Found'
        res.headers[b'Content-Type'] = b'text/plain'
        res.setbodybytes(_(b'API %s not enabled\n') % proto)
        return

    API_HANDLERS[proto][b'handler'](
        rctx, req, res, checkperm, req.dispatchparts[2:]
    )


# Maps API name to metadata so custom API can be registered.
# Keys are:
#
# config
#    Config option that controls whether service is enabled.
# handler
#    Callable receiving (rctx, req, res, checkperm, urlparts) that is called
#    when a request to this API is received.
# apidescriptor
#    Callable receiving (req, repo) that is called to obtain an API
#    descriptor for this service. The response must be serializable to CBOR.
API_HANDLERS = {
    wireprotov2server.HTTP_WIREPROTO_V2: {
        b'config': (b'experimental', b'web.api.http-v2'),
        b'handler': wireprotov2server.handlehttpv2request,
        b'apidescriptor': wireprotov2server.httpv2apidescriptor,
    },
}


def _httpresponsetype(ui, proto, prefer_uncompressed):
    """Determine the appropriate response type and compression settings.

    Returns a tuple of (mediatype, compengine, engineopts).
    """
    # Determine the response media type and compression engine based
    # on the request parameters.

    if b'0.2' in proto.getprotocaps():
        # All clients are expected to support uncompressed data.
        if prefer_uncompressed:
            return HGTYPE2, compression._noopengine(), {}

        # Now find an agreed upon compression format.
        compformats = wireprotov1server.clientcompressionsupport(proto)
        for engine in wireprototypes.supportedcompengines(
            ui, compression.SERVERROLE
        ):
            if engine.wireprotosupport().name in compformats:
                opts = {}
                level = ui.configint(b'server', b'%slevel' % engine.name())
                if level is not None:
                    opts[b'level'] = level

                return HGTYPE2, engine, opts

        # No mutually supported compression format. Fall back to the
        # legacy protocol.

    # Don't allow untrusted settings because disabling compression or
    # setting a very high compression level could lead to flooding
    # the server's network or CPU.
    opts = {b'level': ui.configint(b'server', b'zliblevel')}
    return HGTYPE, util.compengines[b'zlib'], opts


def processcapabilitieshandshake(repo, req, res, proto):
    """Called during a ?cmd=capabilities request.

    If the client is advertising support for a newer protocol, we send
    a CBOR response with information about available services. If no
    advertised services are available, we don't handle the request.
    """
    # Fall back to old behavior unless the API server is enabled.
    if not repo.ui.configbool(b'experimental', b'web.apiserver'):
        return False

    clientapis = decodevaluefromheaders(req, b'X-HgUpgrade')
    protocaps = decodevaluefromheaders(req, b'X-HgProto')
    if not clientapis or not protocaps:
        return False

    # We currently only support CBOR responses.
    protocaps = set(protocaps.split(b' '))
    if b'cbor' not in protocaps:
        return False

    descriptors = {}

    for api in sorted(set(clientapis.split()) & _availableapis(repo)):
        handler = API_HANDLERS[api]

        descriptorfn = handler.get(b'apidescriptor')
        if not descriptorfn:
            continue

        descriptors[api] = descriptorfn(req, repo)

    v1caps = wireprotov1server.dispatch(repo, proto, b'capabilities')
    assert isinstance(v1caps, wireprototypes.bytesresponse)

    m = {
        # TODO allow this to be configurable.
        b'apibase': b'api/',
        b'apis': descriptors,
        b'v1capabilities': v1caps.data,
    }

    res.status = b'200 OK'
    res.headers[b'Content-Type'] = b'application/mercurial-cbor'
    res.setbodybytes(b''.join(cborutil.streamencode(m)))

    return True


def _callhttp(repo, req, res, proto, cmd):
    # Avoid cycle involving hg module.
    from .hgweb import common as hgwebcommon

    def genversion2(gen, engine, engineopts):
        # application/mercurial-0.2 always sends a payload header
        # identifying the compression engine.
        name = engine.wireprotosupport().name
        assert 0 < len(name) < 256
        yield struct.pack(b'B', len(name))
        yield name

        for chunk in gen:
            yield chunk

    def setresponse(code, contenttype, bodybytes=None, bodygen=None):
        if code == HTTP_OK:
            res.status = b'200 Script output follows'
        else:
            res.status = hgwebcommon.statusmessage(code)

        res.headers[b'Content-Type'] = contenttype

        if bodybytes is not None:
            res.setbodybytes(bodybytes)
        if bodygen is not None:
            res.setbodygen(bodygen)

    if not wireprotov1server.commands.commandavailable(cmd, proto):
        setresponse(
            HTTP_OK,
            HGERRTYPE,
            _(
                b'requested wire protocol command is not available over '
                b'HTTP'
            ),
        )
        return

    proto.checkperm(wireprotov1server.commands[cmd].permission)

    # Possibly handle a modern client wanting to switch protocols.
    if cmd == b'capabilities' and processcapabilitieshandshake(
        repo, req, res, proto
    ):

        return

    rsp = wireprotov1server.dispatch(repo, proto, cmd)

    if isinstance(rsp, bytes):
        setresponse(HTTP_OK, HGTYPE, bodybytes=rsp)
    elif isinstance(rsp, wireprototypes.bytesresponse):
        setresponse(HTTP_OK, HGTYPE, bodybytes=rsp.data)
    elif isinstance(rsp, wireprototypes.streamreslegacy):
        setresponse(HTTP_OK, HGTYPE, bodygen=rsp.gen)
    elif isinstance(rsp, wireprototypes.streamres):
        gen = rsp.gen

        # This code for compression should not be streamres specific. It
        # is here because we only compress streamres at the moment.
        mediatype, engine, engineopts = _httpresponsetype(
            repo.ui, proto, rsp.prefer_uncompressed
        )
        gen = engine.compressstream(gen, engineopts)

        if mediatype == HGTYPE2:
            gen = genversion2(gen, engine, engineopts)

        setresponse(HTTP_OK, mediatype, bodygen=gen)
    elif isinstance(rsp, wireprototypes.pushres):
        rsp = b'%d\n%s' % (rsp.res, rsp.output)
        setresponse(HTTP_OK, HGTYPE, bodybytes=rsp)
    elif isinstance(rsp, wireprototypes.pusherr):
        rsp = b'0\n%s\n' % rsp.res
        res.drain = True
        setresponse(HTTP_OK, HGTYPE, bodybytes=rsp)
    elif isinstance(rsp, wireprototypes.ooberror):
        setresponse(HTTP_OK, HGERRTYPE, bodybytes=rsp.message)
    else:
        raise error.ProgrammingError(b'hgweb.protocol internal failure', rsp)


def _sshv1respondbytes(fout, value):
    """Send a bytes response for protocol version 1."""
    fout.write(b'%d\n' % len(value))
    fout.write(value)
    fout.flush()


def _sshv1respondstream(fout, source):
    write = fout.write
    for chunk in source.gen:
        write(chunk)
    fout.flush()


def _sshv1respondooberror(fout, ferr, rsp):
    ferr.write(b'%s\n-\n' % rsp)
    ferr.flush()
    fout.write(b'\n')
    fout.flush()


@interfaceutil.implementer(wireprototypes.baseprotocolhandler)
class sshv1protocolhandler(object):
    """Handler for requests services via version 1 of SSH protocol."""

    def __init__(self, ui, fin, fout):
        self._ui = ui
        self._fin = fin
        self._fout = fout
        self._protocaps = set()

    @property
    def name(self):
        return wireprototypes.SSHV1

    def getargs(self, args):
        data = {}
        keys = args.split()
        for n in pycompat.xrange(len(keys)):
            argline = self._fin.readline()[:-1]
            arg, l = argline.split()
            if arg not in keys:
                raise error.Abort(_(b"unexpected parameter %r") % arg)
            if arg == b'*':
                star = {}
                for k in pycompat.xrange(int(l)):
                    argline = self._fin.readline()[:-1]
                    arg, l = argline.split()
                    val = self._fin.read(int(l))
                    star[arg] = val
                data[b'*'] = star
            else:
                val = self._fin.read(int(l))
                data[arg] = val
        return [data[k] for k in keys]

    def getprotocaps(self):
        return self._protocaps

    def getpayload(self):
        # We initially send an empty response. This tells the client it is
        # OK to start sending data. If a client sees any other response, it
        # interprets it as an error.
        _sshv1respondbytes(self._fout, b'')

        # The file is in the form:
        #
        # <chunk size>\n<chunk>
        # ...
        # 0\n
        count = int(self._fin.readline())
        while count:
            yield self._fin.read(count)
            count = int(self._fin.readline())

    @contextlib.contextmanager
    def mayberedirectstdio(self):
        yield None

    def client(self):
        client = encoding.environ.get(b'SSH_CLIENT', b'').split(b' ', 1)[0]
        return b'remote:ssh:' + client

    def addcapabilities(self, repo, caps):
        if self.name == wireprototypes.SSHV1:
            caps.append(b'protocaps')
        caps.append(b'batch')
        return caps

    def checkperm(self, perm):
        pass


class sshv2protocolhandler(sshv1protocolhandler):
    """Protocol handler for version 2 of the SSH protocol."""

    @property
    def name(self):
        return wireprototypes.SSHV2

    def addcapabilities(self, repo, caps):
        return caps


def _runsshserver(ui, repo, fin, fout, ev):
    # This function operates like a state machine of sorts. The following
    # states are defined:
    #
    # protov1-serving
    #    Server is in protocol version 1 serving mode. Commands arrive on
    #    new lines. These commands are processed in this state, one command
    #    after the other.
    #
    # protov2-serving
    #    Server is in protocol version 2 serving mode.
    #
    # upgrade-initial
    #    The server is going to process an upgrade request.
    #
    # upgrade-v2-filter-legacy-handshake
    #    The protocol is being upgraded to version 2. The server is expecting
    #    the legacy handshake from version 1.
    #
    # upgrade-v2-finish
    #    The upgrade to version 2 of the protocol is imminent.
    #
    # shutdown
    #    The server is shutting down, possibly in reaction to a client event.
    #
    # And here are their transitions:
    #
    # protov1-serving -> shutdown
    #    When server receives an empty request or encounters another
    #    error.
    #
    # protov1-serving -> upgrade-initial
    #    An upgrade request line was seen.
    #
    # upgrade-initial -> upgrade-v2-filter-legacy-handshake
    #    Upgrade to version 2 in progress. Server is expecting to
    #    process a legacy handshake.
    #
    # upgrade-v2-filter-legacy-handshake -> shutdown
    #    Client did not fulfill upgrade handshake requirements.
    #
    # upgrade-v2-filter-legacy-handshake -> upgrade-v2-finish
    #    Client fulfilled version 2 upgrade requirements. Finishing that
    #    upgrade.
    #
    # upgrade-v2-finish -> protov2-serving
    #    Protocol upgrade to version 2 complete. Server can now speak protocol
    #    version 2.
    #
    # protov2-serving -> protov1-serving
    #    Ths happens by default since protocol version 2 is the same as
    #    version 1 except for the handshake.

    state = b'protov1-serving'
    proto = sshv1protocolhandler(ui, fin, fout)
    protoswitched = False

    while not ev.is_set():
        if state == b'protov1-serving':
            # Commands are issued on new lines.
            request = fin.readline()[:-1]

            # Empty lines signal to terminate the connection.
            if not request:
                state = b'shutdown'
                continue

            # It looks like a protocol upgrade request. Transition state to
            # handle it.
            if request.startswith(b'upgrade '):
                if protoswitched:
                    _sshv1respondooberror(
                        fout,
                        ui.ferr,
                        b'cannot upgrade protocols multiple times',
                    )
                    state = b'shutdown'
                    continue

                state = b'upgrade-initial'
                continue

            available = wireprotov1server.commands.commandavailable(
                request, proto
            )

            # This command isn't available. Send an empty response and go
            # back to waiting for a new command.
            if not available:
                _sshv1respondbytes(fout, b'')
                continue

            rsp = wireprotov1server.dispatch(repo, proto, request)
            repo.ui.fout.flush()
            repo.ui.ferr.flush()

            if isinstance(rsp, bytes):
                _sshv1respondbytes(fout, rsp)
            elif isinstance(rsp, wireprototypes.bytesresponse):
                _sshv1respondbytes(fout, rsp.data)
            elif isinstance(rsp, wireprototypes.streamres):
                _sshv1respondstream(fout, rsp)
            elif isinstance(rsp, wireprototypes.streamreslegacy):
                _sshv1respondstream(fout, rsp)
            elif isinstance(rsp, wireprototypes.pushres):
                _sshv1respondbytes(fout, b'')
                _sshv1respondbytes(fout, b'%d' % rsp.res)
            elif isinstance(rsp, wireprototypes.pusherr):
                _sshv1respondbytes(fout, rsp.res)
            elif isinstance(rsp, wireprototypes.ooberror):
                _sshv1respondooberror(fout, ui.ferr, rsp.message)
            else:
                raise error.ProgrammingError(
                    b'unhandled response type from '
                    b'wire protocol command: %s' % rsp
                )

        # For now, protocol version 2 serving just goes back to version 1.
        elif state == b'protov2-serving':
            state = b'protov1-serving'
            continue

        elif state == b'upgrade-initial':
            # We should never transition into this state if we've switched
            # protocols.
            assert not protoswitched
            assert proto.name == wireprototypes.SSHV1

            # Expected: upgrade <token> <capabilities>
            # If we get something else, the request is malformed. It could be
            # from a future client that has altered the upgrade line content.
            # We treat this as an unknown command.
            try:
                token, caps = request.split(b' ')[1:]
            except ValueError:
                _sshv1respondbytes(fout, b'')
                state = b'protov1-serving'
                continue

            # Send empty response if we don't support upgrading protocols.
            if not ui.configbool(b'experimental', b'sshserver.support-v2'):
                _sshv1respondbytes(fout, b'')
                state = b'protov1-serving'
                continue

            try:
                caps = urlreq.parseqs(caps)
            except ValueError:
                _sshv1respondbytes(fout, b'')
                state = b'protov1-serving'
                continue

            # We don't see an upgrade request to protocol version 2. Ignore
            # the upgrade request.
            wantedprotos = caps.get(b'proto', [b''])[0]
            if SSHV2 not in wantedprotos:
                _sshv1respondbytes(fout, b'')
                state = b'protov1-serving'
                continue

            # It looks like we can honor this upgrade request to protocol 2.
            # Filter the rest of the handshake protocol request lines.
            state = b'upgrade-v2-filter-legacy-handshake'
            continue

        elif state == b'upgrade-v2-filter-legacy-handshake':
            # Client should have sent legacy handshake after an ``upgrade``
            # request. Expected lines:
            #
            #    hello
            #    between
            #    pairs 81
            #    0000...-0000...

            ok = True
            for line in (b'hello', b'between', b'pairs 81'):
                request = fin.readline()[:-1]

                if request != line:
                    _sshv1respondooberror(
                        fout,
                        ui.ferr,
                        b'malformed handshake protocol: missing %s' % line,
                    )
                    ok = False
                    state = b'shutdown'
                    break

            if not ok:
                continue

            request = fin.read(81)
            if request != b'%s-%s' % (b'0' * 40, b'0' * 40):
                _sshv1respondooberror(
                    fout,
                    ui.ferr,
                    b'malformed handshake protocol: '
                    b'missing between argument value',
                )
                state = b'shutdown'
                continue

            state = b'upgrade-v2-finish'
            continue

        elif state == b'upgrade-v2-finish':
            # Send the upgrade response.
            fout.write(b'upgraded %s %s\n' % (token, SSHV2))
            servercaps = wireprotov1server.capabilities(repo, proto)
            rsp = b'capabilities: %s' % servercaps.data
            fout.write(b'%d\n%s\n' % (len(rsp), rsp))
            fout.flush()

            proto = sshv2protocolhandler(ui, fin, fout)
            protoswitched = True

            state = b'protov2-serving'
            continue

        elif state == b'shutdown':
            break

        else:
            raise error.ProgrammingError(
                b'unhandled ssh server state: %s' % state
            )


class sshserver(object):
    def __init__(self, ui, repo, logfh=None):
        self._ui = ui
        self._repo = repo
        self._fin, self._fout = ui.protectfinout()

        # Log write I/O to stdout and stderr if configured.
        if logfh:
            self._fout = util.makeloggingfileobject(
                logfh, self._fout, b'o', logdata=True
            )
            ui.ferr = util.makeloggingfileobject(
                logfh, ui.ferr, b'e', logdata=True
            )

    def serve_forever(self):
        self.serveuntil(threading.Event())
        self._ui.restorefinout(self._fin, self._fout)

    def serveuntil(self, ev):
        """Serve until a threading.Event is set."""
        _runsshserver(self._ui, self._repo, self._fin, self._fout, ev)