Subject: Re: Git-aware HTTP transport

From: "Shawn O. Pearce" <[email-masked]>
Date: Fri, 29 Aug 2008 10:39:54 -0700

Yet another draft follows.  I believe that I have covered all
comments with this draft.  But I welcome any additional ones,
as thus far it has been a very constructive process.

The updated protocol looks more like the current native protocol
does.  This should make it easier to reuse code between the two
protocol implementations.

--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.

HTTP Transport
--------------

All requests are encoded as HTTP POST requests to the smart service
URL, "$url/backend.git-http/$service".

All responses are encoded as 200 Ok responses, even if the server
side has "failed" the request.  Service specific success/failure
codes are embedded in the content.

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.

Content Type
------------

All requests/responses use "application/x-git" as the content type.
Action specific subtypes are specified by the parameter "service",
e.g. "application/x-git; service=upload-pack".

HTTP Redirects
--------------

If a POST request results in an HTTP 302 or 303 redirect response
clients should retry the request by updating the URL and POSTing
the same request to the new location.  Subsequent requests should
still be sent to the original URL.

This redirect behavior is unrelated to the in-payload redirect
that is described below in "Service show-ref".

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

HTTP clients can detect a smart Git-aware server by sending
a request to service "show-ref".

A Git-aware server will respond with a valid response.  Clients
must check the following properties to prevent being fooled by
misconfigured servers:

  * HTTP status code is 200.
  * Content-Type is "application/x-git; service=show-ref"
  * The body can be parsed without errors.  The length of
    each pkt-line must be 4 valid hex digits.

A dumb server will respond with a non-200 HTTP status code.
A misconfigured server may respond with a normal 200 status
code, but an incorrect content type, or an invalid leading
4 byte sequence for a pkt-line (e.g. "<htm" or "<!DO" are
not valid lengths).

pkt-line Format
---------------

Much of the payload is described around pkt-lines.

A pkt-line is a variable length binary string.  The first four bytes
of the line indicates the total length of the line, in hexadecimal.
The total length includes the 4 bytes used to denote the length.  A
line is usually terminated by an LF, which must be included in the
total length if present.

Binary data is permitted within a pkt-line so implementors should
ensure their pkt-line parsing/formatting routines are 8-bit clean.
The maximum length of a pkt-line's data is 65532 bytes (65536 - 4).

Examples (as C-style strings):

  pkt-line          actual value
  ---------------------------------
  "0006a\n"         "a\n"
  "0005a"           "a"
  "000bfoobar\n"    "foobar\n"
  "0004"            ""

A pkt-line with a length of 0 ("0000") is a special case and is
treated as a break or terminator in the payload.

Service show-ref
----------------

Obtains the available refs from the remote repository.

URL: $url/backend.git-http/show-ref
Content-Type: application/x-git; service=show-ref

The request is an empty body.

The response is a pkt-line with "refs", followed by zero
or more ref pkt-lines ("$id $name"), and a final pkt-line
with a length of 0:

	S: 0009refs
	S: 003295dcfa3633004da0049d3d0fa03f80589cbcaf31 HEAD
	S: 003e95dcfa3633004da0049d3d0fa03f80589cbcaf31 refs/heads/maint
	S: 003fd049f6c27a2244e12041955e262a404c7faba355 refs/heads/master
	S: 003b2cb58b79488a98d2721cea644875a8dd0026b115 refs/heads/pu
	S: 0000

The response may begin with an optional redirect to a new service
URL for the repository:

	S: 0028redirect http://s1.example.com/git/
	S: 0009refs
	S: 003295dcfa3633004da0049d3d0fa03f80589cbcaf31 HEAD
	S: 003fd049f6c27a2244e12041955e262a404c7faba355 refs/heads/master
	S: 0000

or be composed of only a redirect:

	S: 0028redirect http://s1.example.com/git/
	S: 0000

If a redirect is returned the client should update itself
to use the new URL as the location for future requests.
A server may use the redirect to request that the client
"pin" itself to a particular server for the remainder of
the current transaction.

The URL listed in any redirect should be the base URL
without any query args.  The client will automatically
append "/backend.git-http/$service" as it makes each
future request.

If no "refs" line was received in the response, but
a "redirect" was received, the client should retry
its request at the new location before giving up.

Service upload-pack
-------------------

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

URL: $url/backend.git-http/upload-pack
Content-Type: application/x-git; service=upload-pack

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

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

 (c) Build a set, WANT, of the objects from ADVERTISED the client
     wants to fetch, based on what it saw from show-ref.
 (c) Build an empty set, COMMON, to hold the objects that are later
     determined to be on both ends.

 (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 an upload-pack request:

	C: 001bcapability include-tag
	C: 0019capability thin-pack
	....
	C: 0032want <WANT #1>...............................
	C: 0032want <WANT #2>...............................
	....
	C: 0034common <COMMON #1>.............................
	C: 0034common <COMMON #2>.............................
	....
	C: 0032have <HAVE #1>...............................
	C: 0032have <HAVE #2>...............................
	....
	C: 0000

     The stream is organized into "commands", with each command
     appearing by itself in a pkt-line.  Within a command line
     the text leading up to the first space is the command name,
     and the remainder of the line to the first LF is the value.
     Command lines are terminated with an LF as the last byte of
     the pkt-line value.

     Servers must ignore commands which they do not recognize.
     This permits newer clients to transmit additional data to
     an unknown server, in case the server is new enough to use
     the additional information.

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

       * capability
       * want
       * common
       * have
       * give-up

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

     The "capability" command requests a single protocol feature
     to be enabled by the server.  Typically these are used to
     describe aspects of the pack that will be returned.  See
     below for more details on the current capabilities.

     A single "want", "common", or "have" command has one hex
     formatted SHA-1 as its value.  Multiple SHA-1s can be sent
     by sending multiple commands.

     The HAVE list is created by popping the first 64 commits
     from C_PENDING.  Less can be supplied if C_PENDING empties.

     If the client has sent 256 HAVE commits and has not yet
     received one of those back from S_COMMON, or the client
     has emptied C_PENDING it should include a "give-up"
     command to let the server know it won't proceed:

	C: 000cgive-up

  (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 no WANT objects are received, send an error:

	S: 0019status error no want

      If any WANT object is not reachable, send an error:

	S: 001estatus error invalid want

     Create an empty list, S_COMMON.

     If 'common' was sent:

     Load all objects into S_COMMON.  If an object appears in
     'common' but the server does not have the object locally
     an error should be returned:

	S: 001estatus error invalid 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:

     If the server has found a closed set of objects to pack or the
     request contains "give-up", it replies with the pack and the
     enabled capabilities.  The set of enabled capabilities is limited
     to the intersection of what the client requested and what the
     server supports.

	S: 0010status pack
	C: 001bcapability include-tag
	C: 0019capability thin-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.

     Here a "closed set of objects" is defined to have at least
     one path from every WANT to at least one COMMON object.

     If the server needs more information, it replies with a
     status continue response:

	S: 0014status continue
	S: 0034common <S_COMMON #1>...........................
	S: 0034common <S_COMMON #2>...........................
	...
	S: 0000

     The stream formatting rules are the same as the request.

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

  (c) Parse the upload-pack response:

      If the status pkt-line is "status pack:"

      Process the pack stream and update the local refs.

      If the status pkt-line is "status continue":

      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 step.

Capability include-tag
~~~~~~~~~~~~~~~~~~~~~~

When packing an object that an annotated tag points at, include
the tag object too.  Clients can request this if they want to
fetch tags, but don't know which tags they will need until after
they receive the branch data.  By enabling include-tag an entire
call to upload-pack can be avoided.

Capability thin-pack
~~~~~~~~~~~~~~~~~~~~

When packing a deltified object the base is not included if the
base is reachable from an object listed in the COMMON set by the
client.  This reduces the bandwidth required to transfer, but it
does slightly increase processing time for the client to save the
pack to disk.

Service receive-pack
--------------------

Uploads a pack and updates refs.

URL: $url/backend.git-http/receive-pack
Content-Type: application/x-git; service=receive-pack

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: 006395dcfa3633004da0049d3d0fa03f80589cbcaf31 d049f6c27a2244e12041955e262a404c7faba355 refs/heads/maint
	C: 0000
	C: PACK...

	S: 0005
	S: ...<output of receive-pack>...

The capabilities are handled exactly as in the fetch protocol,
however the server may reject a pack and its associated commands
if an invalid capability request is made by the client, or the
client has assumed a pack capability that the server does not
have support for.  In the latter case the server must still send
the capabilities key in the response so the client can correct
itself and try again.

-- 
Shawn.