Mercurial > hg > mercurial-source
changeset 36725:a622a927fe03
sshpeer: document the handshake mechanism
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.
Differential Revision: https://phab.mercurial-scm.org/D2035
author | Gregory Szorc <gregory.szorc@gmail.com> |
---|---|
date | Sun, 04 Feb 2018 14:44:04 -0800 |
parents | 80a2b8ae42a1 |
children | 556218e08e25 |
files | mercurial/sshpeer.py |
diffstat | 1 files changed, 33 insertions(+), 0 deletions(-) [+] |
line wrap: on
line diff
--- a/mercurial/sshpeer.py +++ b/mercurial/sshpeer.py @@ -162,6 +162,37 @@ hint = ui.config('ui', 'ssherrorhint') 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') try: @@ -205,6 +236,8 @@ caps = set() 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:'): caps.update(l[:-1].split(':')[1].split()) break