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Differential D2035

sshpeer: document the handshake mechanism
ClosedPublic

Authored by indygreg on Feb 4 2018, 10:35 PM.

Details

Reviewers
lothiraldan
yuja
Group Reviewers
hg-reviewers
Commits
rHGa622a927fe03: sshpeer: document the handshake mechanism
Summary

The mechanism by which SSH peers establish connections with remotes
is wonky and requires a bit of code archeology to understand. While
it is already documented in hg help internals.wireproto, it helps
to have documentation in the code as well.

Diff Detail

Repository
rHG Mercurial
Lint
Automatic diff as part of commit; lint not applicable.
Unit
Automatic diff as part of commit; unit tests not applicable.

Event Timeline

indygreg created this revision.Feb 4 2018, 10:35 PM
lothiraldan accepted this revision.Feb 5 2018, 8:19 AM
indygreg updated this revision to Diff 5230.Feb 5 2018, 12:15 PM
indygreg updated this revision to Diff 5236.Feb 5 2018, 5:21 PM
yuja accepted this revision.Feb 6 2018, 8:07 AM
This revision is now accepted and ready to land.Feb 6 2018, 8:07 AM
Closed by commit rHGa622a927fe03: sshpeer: document the handshake mechanism (authored by indygreg). · Explain WhyFeb 6 2018, 8:24 AM
This revision was automatically updated to reflect the committed changes.

Revision Contents
Changeset List

PathPackages
Mmercurial/sshpeer.py (33 lines)

Diff 5252

mercurial/sshpeer.py

return proc, stdin, stdout, stderr return proc, stdin, stdout, stderr
def _performhandshake(ui, stdin, stdout, stderr): def _performhandshake(ui, stdin, stdout, stderr):
def badresponse(): def badresponse():
msg = _('no suitable response from remote hg') msg = _('no suitable response from remote hg')
hint = ui.config('ui', 'ssherrorhint') hint = ui.config('ui', 'ssherrorhint')
raise error.RepoError(msg, hint=hint) raise error.RepoError(msg, hint=hint)
# The handshake consists of sending 2 wire protocol commands:
# ``hello`` and ``between``.
#
# The ``hello`` command (which was introduced in Mercurial 0.9.1)
# instructs the server to advertise its capabilities.
#
# The ``between`` command (which has existed in all Mercurial servers
# for as long as SSH support has existed), asks for the set of revisions
# between a pair of revisions.
#
# The ``between`` command is issued with a request for the null
# range. If the remote is a Mercurial server, this request will
# generate a specific response: ``1\n\n``. This represents the
# wire protocol encoded value for ``\n``. We look for ``1\n\n``
# in the output stream and know this is the response to ``between``
# and we're at the end of our handshake reply.
#
# The response to the ``hello`` command will be a line with the
# length of the value returned by that command followed by that
# value. If the server doesn't support ``hello`` (which should be
# rare), that line will be ``0\n``. Otherwise, the value will contain
# RFC 822 like lines. Of these, the ``capabilities:`` line contains
# the capabilities of the server.
#
# In addition to the responses to our command requests, the server
# may emit "banner" output on stdout. SSH servers are allowed to
# print messages to stdout on login. Issuing commands on connection
# allows us to flush this banner output from the server by scanning
# for output to our well-known ``between`` command. Of course, if
# the banner contains ``1\n\n``, this will throw off our detection.
requestlog = ui.configbool('devel', 'debug.peer-request') requestlog = ui.configbool('devel', 'debug.peer-request')
try: try:
pairsarg = '%s-%s' % ('0' * 40, '0' * 40) pairsarg = '%s-%s' % ('0' * 40, '0' * 40)
handshake = [ handshake = [
'hello\n', 'hello\n',
'between\n', 'between\n',
'pairs %d\n' % len(pairsarg), 'pairs %d\n' % len(pairsarg),
max_noise -= 1 max_noise -= 1
except IOError: except IOError:
badresponse() badresponse()
else: else:
badresponse() badresponse()
caps = set() caps = set()
for l in reversed(lines): for l in reversed(lines):
# Look for response to ``hello`` command. Scan from the back so
# we don't misinterpret banner output as the command reply.
if l.startswith('capabilities:'): if l.startswith('capabilities:'):
caps.update(l[:-1].split(':')[1].split()) caps.update(l[:-1].split(':')[1].split())
break break
return caps return caps
class sshpeer(wireproto.wirepeer): class sshpeer(wireproto.wirepeer):
def __init__(self, ui, url, proc, stdin, stdout, stderr, caps): def __init__(self, ui, url, proc, stdin, stdout, stderr, caps):