wireproto: define and implement HTTP handshake to upgrade protocol
When clients connect to repositories over HTTP, they issue a request
to the well-known URL "?cmd=capabilities" to fetch the repository
capabilities. This is the handshake portion of the HTTP protocol.
This commit defines a mechanism to use that HTTP request to return
information about modern server features.
If a client sends an X-HgUpgrade-* header containing a list of
client-supported API names, the server responds with a response
containing information about available services. This includes
the normal capabilities string. So if the server doesn't support
any newer services, the client can easily fall back.
By advertising supported services from clients, server operators
can see and log what client support exists in the wild. This will
also help with debugging.
The response contains the base path to API services. We know there
are potential issues with the <repo>/api/ URL space conflicting with
hgwebdir and subrepos. By making the API URL dynamic from the
perspective of the client, the URL for APIs is not subject to backwards
compatibility concerns - at least as long as a ?cmd=capabilities request
is made.
We've also defined the ``cbor`` client capability for the X-HgProto-*
header. This MUST be sent in order to get the modern response from
"?cmd=capabilities". During implementation, I initially always sent
an application/mercurial-cbor response. However, the handshake
mechanism will be more future compatible if the client is in charge
of which formats to request. We already perform content negotiation
from X-HgProto-*, so keying off this for the capabilities response
feels appropriate.
In addition, I initially used application/cbor. However, it is
conceivable that a non-Mercurial server could serve application/cbor.
To rule out this possibility, I've invented a new media type that
is Mercurial specific and can't be confused for generic CBOR.
Differential Revision: https://phab.mercurial-scm.org/D3242
#require serve
$ hg init server
$ hg serve -R server -p $HGPORT -d --pid-file=hg1.pid -E ../error.log
$ cat hg1.pid >> $DAEMON_PIDS
Check basic fetching
$ hg debugdownload "http://localhost:$HGPORT/?cmd=lookup&key=tip"
1 0000000000000000000000000000000000000000
$ hg debugdownload -o null.txt "http://localhost:$HGPORT/?cmd=lookup&key=null"
$ cat null.txt
1 0000000000000000000000000000000000000000
Check the request is made from the usual Mercurial logic
(rev details, give different content if the request has a Mercurial user agent)
$ get-with-headers.py --headeronly "localhost:$HGPORT" "rev/tip" content-type
200 Script output follows
content-type: text/html; charset=ascii
$ hg debugdownload "http://localhost:$HGPORT/rev/tip"
# HG changeset patch
# User
# Date 0 0
# Node ID 0000000000000000000000000000000000000000
Check other kind of compatible url
$ hg debugdownload ./null.txt
1 0000000000000000000000000000000000000000
Test largefile URL
------------------
$ cat << EOF >> $HGRCPATH
> [extensions]
> largefiles=
> EOF
$ killdaemons.py
$ rm -f error.log hg1.pid
$ hg serve -R server -p $HGPORT -d --pid-file=hg1.pid -E error.log
$ cat hg1.pid >> $DAEMON_PIDS
$ hg -R server debuglfput null.txt
a57b57b39ee4dc3da1e03526596007f480ecdbe8
$ hg --traceback debugdownload "largefile://a57b57b39ee4dc3da1e03526596007f480ecdbe8" --config paths.default=http://localhost:$HGPORT/
1 0000000000000000000000000000000000000000
from within a repository
$ hg clone http://localhost:$HGPORT/ client
no changes found
updating to branch default
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd client
$ hg path
default = http://localhost:$HGPORT/
$ hg debugdownload "largefile://a57b57b39ee4dc3da1e03526596007f480ecdbe8"
1 0000000000000000000000000000000000000000
$ cd ..