Subject: Git-aware HTTP transport

From: "Shawn O. Pearce" <[email-masked]>
Date: Mon, 25 Aug 2008 18:26:43 -0700

I spent some time on Friday kicking around how the fetch part of
an HTTP protocol could be implemented.  What I seem to have settled
on at this point in time is a more condensed version of the native
git protocol, batched into 256 commit blocks.

--8<--
Smart HTTP transfer protocols
=============================

Git supports two HTTP based transfer protocols.  A "dumb" protocol
which requires only a standard HTTP server on the server end of the
connection, and a "smart" protocol which requires a Git aware CGI
(or server module).  This document describes the "smart" protocol.

As a design feature smart clients can automatically translate and
upgrade "dumb" protocol URLs.  This permits all users to have the
same published URL, with the peers automatically choosing to use
the most efficient transport available to them.

Authentication
--------------

Standard HTTP authentication is used if authentication is required
to access a repository, and must be configured and enforced by the
HTTP server software itself.

Stateless
---------

The protocol, much like its underlying HTTP, is stateless, from the
perspective of the HTTP server side.  All state must be retained and
managed by the client.  This permits round-robin load-balancing on
the server side, among many other implementation details.

HTTP/1.1 Preference
-------------------

For performance reasons the HTTP/1.1 chunked transfer encoding
is used whenever possible to transfer variable length objects.
This avoids needing to produce large results in memory to compute
the proper content-length.

Detecting Smart Servers
-----------------------

HTTP clients can detect a smart Git-aware server by HEADing
$repo/backend.git-http and looking for a 302 redirect to the
repository's smart service URL:

	C: HEAD /path/to/repository.git/backend.git-http HTTP/1.1

	S: HTTP/1.1 302 Found
	S: Location: /git/path/to/repository.git

A dumb server would respond with a 304 Not Found (or 200 OK).

Smart servers may send a redirect to any URL that does not
contain query args (e.g. "foo?repo=path.git" is invalid).
The URL must be sufficient to provide the location of the
repository to the smart service code.

A valid redirect can be to yourself, for example:

	C: HEAD /path/to/repository.git/backend.git-http HTTP/1.1

	S: HTTP/1.1 302 Found
	S: Location: /path/to/repository.git/backend.git-http/.

All subsequent communcation for this transaction is done through
the smart service URL ($ssurl), not the original URL.

GET $ssurl/refs
---------------

Obtains the available refs from the remote repository.  The response
is a sequence of refs, one per Git packet line.  The final packet
line has a length of 0 to indicate the end.  This is basically
the same protocol that is used by the git-upload-pack service to
advertise the available refs.

	C: GET $ssurl/refs HTTP/1.1

	S: HTTP/1.1 200 OK
	S: Content-Type: application/x-git-refs
	S:
	S: 003295dcfa3633004da0049d3d0fa03f80589cbcaf31 HEAD
	S: 003e95dcfa3633004da0049d3d0fa03f80589cbcaf31 refs/heads/maint
	S: 003fd049f6c27a2244e12041955e262a404c7faba355 refs/heads/master
	S: 003b2cb58b79488a98d2721cea644875a8dd0026b115 refs/heads/pu
	S: 0000

POST $ssurl/upload-pack
-----------------------

Prepares an estimated minimal pack to transfer new objects to the
client.

The computation to select the minimal pack proceeds as follows
(c = client, s = server):

 init step:
 (c) Use /refs to obtain the advertised refs.
 (c) Place any object seen in /refs into set ADVERTISED.

 (c) Build a set, WANT, of the objects from ADVERTISED the client
     wants to fetch, based on what it saw from /refs.

 (c) Start a queue, C_PENDING, ordered by commit time (popping newest
     first).  Add all client refs.  When a commit is popped from the
     queue its parents should be automatically inserted back.  Commits
     should only enter the queue once.

 one compute step:
 (c) Send a /upload-pack request:

	C: POST $ssurl/upload-pack HTTP/1.1
	C: Content-Type: application/x-git-uploadpack
	C: Content-Length: ...
	C:
	C: 0009want
	C: 0xxx<WANT list>
	C: 000bcommon
	C: 0xxx<COMMON list>
	C: 0009have
	C: 0xxx<HAVE list>
	C: 0000

     The stream is organized into "sections", where each section is
     composed of two git pkt-lines.  The first pkt-line provides the
     name of the section ("want", "have", "common").  The second
     pkt-line has the binary SHA-1 ids which compose that section.

     The "want" section is required.  The other sections ("have",
     "common") are optional.  A missing "want" section should be
     answered with a "400 Bad Request".

     Sections must appear in the following order, if they appear
     at all in the request stream:

       * want
       * common
       * have

     Each section may appear multiple times.  Client implementions
     are encouraged to use as few sections as possible, however the
     limit of 64k per pkt-line limits the number of ids to 3,276 per
     section entry.

     The stream is terminated by a pkt-line flush ("0000").

     The HAVE list is created by popping the first 256 commits
     from C_PENDING.  Less can be supplied if C_PENDING empties.

  (s) Parse the /upload-pack request.

      Verify all objects in WANT are reachable from refs.  As
      this may require walking backwards through history to
      the very beginning on invalid requests the server may
      use a reasonable limit of commits (e.g. 1000) walked
      beyond any ref tip before giving up.

      If any WANT object is not reachable, send a 409 error:

	S: HTTP/1.1 409 Conflict
	S: Content-Type: application/x-git-error
	S:
	S: %s not reachable

     Create an empty list, S_COMMON.

     If 'common' was sent:

     Load all objects into S_COMMON.

     If 'have' was sent:

     Loop through the objects in the order supplied by the client.
     For each object, if the server has the object reachable from
     a ref, add it to S_COMMON.  If a commit is added to S_COMMON,
     do not add any ancestors, even if they also appear in HAVE.

  (s) Send the /upload-pack response:

	S: HTTP/1.1 200 OK
	S: Content-Type: application/x-git-uploadpack

	S: 000bcommon
	S: 0xxx<S_COMMON list>
	S: 0000

     The stream formatting rules are the same as the request.

     The section "common" details the contents of S_COMMON,
     that is all objects from HAVE that the server also has.

     If the server has found a closed set of objects to pack,
     it replies with the pack and not x-git-uploadpack response.

	S: HTTP/1.1 200 OK
	S: Content-Type: application/x-git-pack

	S: 000c.PACK...

     The returned stream is the side-band-64k protocol supported
     by the git-upload-pack service, and the pack is embedded into
     stream 1.  Progress messages from the server side may appear
     in stream 2.

  (c) Parse the /upload-pack response:

      If the Content-Type is application/x-git-uploadpack:

      Reset COMMON to the items in S_COMMON.  The new S_COMMON
      should be a superset of the existing COMMON set.

      Remove all items in S_COMMON, and all of their ancestors,
      from PENDING.

      Do another /compute-common step.

      If the Content-Type is application/x-git-pack:

      Process the pack stream and update the local refs.

POST $ssurl/receive-pack
------------------------

TBD: Still a work in progress.

Uploads a pack and updates refs.  The start of the stream is the
commands to update the refs and the remainder of the stream is the
pack file itself.  See git-receive-pack and its network protocol
in pack-protocol.txt, as this is essentially the same.

	C: POST /path/to/repository.git/receive-pack HTTP/1.0
	C: Content-Type: application/x-git-receivepack
	C: Transfer-Encoding: chunked
	C:
	C: 103
	C: 006395dcfa3633004da0049d3d0fa03f80589cbcaf31 d049f6c27a2244e12041955e262a404c7faba355 refs/heads/maint
	C: 4
	C: 0000
	C: 12
	C: PACK
	...
	C: 0

	S: HTTP/1.0 200 OK
	S: Content-type: application/x-git-receive-pack-status
	S: Transfer-Encoding: chunked
	S:
	S: ...<output of receive-pack>...

-- 
Shawn.