schacon · July 26, 2013 22:16 · otac0n · Jul 4, 2015
diff --git a/git-http-proto.txt b/git-http-proto.txt
 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 both protocols.

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


 URL Format
 ----------

 URLs for Git repositories accessed by HTTP use the standard HTTP
 URL syntax documented by RFC 1738, so they are of the form:

  http://<host>:<port>/<path>

 Within this documentation the placeholder $GIT_URL will stand for
 the http:// repository URL entered by the end-user.

 Both the "smart" and "dumb" HTTP protocols used by Git operate
 by appending additional path components onto the end of the user
 supplied $GIT_URL string.

 Clients MUST strip a trailing '/', if present, from the user supplied
 $GIT_URL string to prevent empty path tokens ('//') from appearing
 in any URL sent to a server.  Compatible clients must expand
 '$GIT_URL/info/refs' as 'foo/info/refs' and not 'foo//info/refs'.


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

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

 Because Git repositories are accessed by standard path components
 server administrators MAY use directory based permissions within
 their HTTP server to control repository access.

 Clients SHOULD support Basic authentication as described by RFC 2616.
 Servers SHOULD support Basic authentication by relying upon the
 HTTP server placed in front of the Git server software.

 Servers MUST NOT require HTTP cookies for the purposes of
 authentication or access control.

 Clients and servers MAY support other common forms of HTTP based
 authentication, such as Digest authentication.


 SSL
 ---

 Clients and servers SHOULD support SSL, particularly to protect
 passwords when relying on Basic HTTP authentication.


 Session State
 -------------

 The Git over HTTP protocol (much like HTTP itself) is stateless
 from the perspective of the HTTP server side.  All state must be
 retained and managed by the client process.  This permits simple
 round-robin load-balancing on the server side, without needing to
 worry about state mangement.

 Clients MUST NOT require state management on the server side in
 order to function correctly.

 Servers MUST NOT require HTTP cookies in order to function correctly.
 Clients MAY store and forward HTTP cookies during request processing
 as described by RFC 2616 (HTTP/1.1).  Servers SHOULD ignore any
 cookies sent by a client.


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

 Much (but not all) 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 SHOULD BE terminated by an LF, which if present MUST be
 included in the total length.

 A pkt-line MAY contain binary data, so implementors MUST ensure all
 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 MUST
 be treated as a message break or terminator in the payload.


 General Request Processing
 --------------------------

 Except where noted, all standard HTTP behavior SHOULD be assumed
 by both client and server.  This includes (but is not necessarily
 limited to):

 If there is no repository at $GIT_URL, the server MUST respond with
 the '404 Not Found' HTTP status code.

 If there is a repository at $GIT_URL, but access is not currently
 permitted, the server MUST respond with the '403 Forbidden' HTTP
 status code.

 Servers SHOULD support both HTTP 1.0 and HTTP 1.1.
 Servers SHOULD support chunked encoding for both
 request and response bodies.

 Clients SHOULD support both HTTP 1.0 and HTTP 1.1.
 Clients SHOULD support chunked encoding for both
 request and response bodies.

 Servers MAY return ETag and/or Last-Modified headers.

 Clients MAY revalidate cached entities by including If-Modified-Since
 and/or If-None-Match request headers.

 Servers MAY return '304 Not Modified' if the relevant headers appear
 in the request and the entity has not changed.  Clients MUST treat
 '304 Not Modified' identical to '200 OK' by reusing the cached entity.

 Clients MAY reuse a cached entity without revalidation if the
 Cache-Control and/or Expires header permits caching.  Clients and
 servers MUST follow RFC 2616 for cache controls.


 Discovering References
 ----------------------

 All HTTP clients MUST begin either a fetch or a push exchange by
 discovering the references available on the remote repository.

 Dumb Clients
 ~~~~~~~~~~~~

 HTTP clients that only support the "dumb" protocol MUST discover
 references by making a request for the special info/refs file of
 the repository.

 Dumb HTTP clients MUST NOT include search/query parameters when
 fetching the info/refs file.  (That is, '?' must not appear in the
 requested URL.)

  C: GET $GIT_URL/info/refs HTTP/1.0

 	S: 200 OK
 	S:
 	S: 95dcfa3633004da0049d3d0fa03f80589cbcaf31	refs/heads/maint
 	S: d049f6c27a2244e12041955e262a404c7faba355	refs/heads/master
 	S: 2cb58b79488a98d2721cea644875a8dd0026b115	refs/tags/v1.0
 	S: a3c2e2402b99163d1d59756e5f207ae21cccba4c	refs/tags/v1.0^{}

 The Content-Type of the returned info/refs entity SHOULD be
 "text/plain; charset=utf-8", but MAY be any content type.
 Clients MUST NOT attempt to validate the returned Content-Type.
 Dumb servers MUST NOT return a return type starting with
 "application/x-git-".

 Cache-Control headers MAY be returned to disable caching of the
 returned entity.

 When examining the response clients SHOULD only examine the HTTP
 status code.  Valid responses are '200 OK', or '304 Not Modified'.

 The returned content is a UNIX formatted text file describing
 each ref and its known value.  The file SHOULD be sorted by name
 according to the C locale ordering.  The file SHOULD NOT include
 the default ref named 'HEAD'.

 	info_refs     = *( ref_record )
 	ref_record    = any_ref | peeled_ref

 	any_ref       = id HT name LF
 	peeled_ref    = id HT name LF
 	                id HT name "^{}" LF
 	id            = 40*HEX

 	HEX           = "0".."9" | "a".."f"
 	LF            = <US-ASCII LF, linefeed (10)>
 	HT            = <US-ASCII HT, horizontal-tab (9)>

 Smart Clients
 ~~~~~~~~~~~~~

 HTTP clients that support the "smart" protocol (or both the
 "smart" and "dumb" protocols) MUST discover references by making
 a paramterized request for the info/refs file of the repository.

 The request MUST contain exactly one query parameter,
 'service=$servicename', where $servicename MUST be the service
 name the client wishes to contact to complete the operation.
 The request MUST NOT contain additional query parameters.

 	C: GET $GIT_URL/info/refs?service=git-upload-pack HTTP/1.0

 	dumb server reply:
 	S: 200 OK
 	S:
 	S: 95dcfa3633004da0049d3d0fa03f80589cbcaf31	refs/heads/maint
 	S: d049f6c27a2244e12041955e262a404c7faba355	refs/heads/master
 	S: 2cb58b79488a98d2721cea644875a8dd0026b115	refs/tags/v1.0
 	S: a3c2e2402b99163d1d59756e5f207ae21cccba4c	refs/tags/v1.0^{}

 	smart server reply:
 	S: 200 OK
 	S: Content-Type: application/x-git-upload-pack-advertisement
 	S: Cache-Control: no-cache
 	S:
 	S: ....# service=git-upload-pack
 	S: ....95dcfa3633004da0049d3d0fa03f80589cbcaf31 refs/heads/maint\0 multi_ack
 	S: ....d049f6c27a2244e12041955e262a404c7faba355 refs/heads/master
 	S: ....2cb58b79488a98d2721cea644875a8dd0026b115 refs/tags/v1.0
 	S: ....a3c2e2402b99163d1d59756e5f207ae21cccba4c refs/tags/v1.0^{}

 Dumb Server Response
 ^^^^^^^^^^^^^^^^^^^^
 Dumb servers MUST respond with the dumb server reply format.

 See the prior section under dumb clients for a more detailed
 description of the dumb server response.

 Smart Server Response
 ^^^^^^^^^^^^^^^^^^^^^
 Smart servers MUST respond with the smart server reply format.

 If the server does not recognize the requested service name, or the
 requested service name has been disabled by the server administrator,
 the server MUST respond with the '403 Forbidden' HTTP status code.

 Cache-Control headers SHOULD be used to disable caching of the
 returned entity.

 The Content-Type MUST be 'application/x-$servicename-advertisement'.
 Clients SHOULD fall back to the dumb protocol if another content
 type is returned.  When falling back to the dumb protocol clients
 SHOULD NOT make an additional request to $GIT_URL/info/refs, but
 instead SHOULD use the response already in hand.  Clients MUST NOT
 continue if they do not support the dumb protocol.

 Clients MUST validate the status code is either '200 OK' or
 '304 Not Modified'.

 Clients MUST validate the first five bytes of the response entity
 matches the regex "^[0-9a-f]{4}#".  If this test fails, clients
 MUST NOT continue.

 Clients MUST parse the entire response as a sequence of pkt-line
 records.

 Clients MUST verify the first pkt-line is "# service=$servicename".
 Servers MUST set $servicename to be the request parameter value.
 Servers SHOULD include an LF at the end of this line.
 Clients MUST ignore an LF at the end of the line.

 Servers MUST terminate the response with the magic "0000" end
 pkt-line marker.

 The returned response is a pkt-line stream describing each ref and
 its known value.  The stream SHOULD be sorted by name according to
 the C locale ordering.  The stream SHOULD include the default ref
 named 'HEAD' as the first ref.  The stream MUST include capability
 declarations behind a NUL on the first ref.

 	smart_reply    = PKT-LINE("# service=$servicename" LF)
 	                 ref_list
 	                 "0000"
 	ref_list       = empty_list | populated_list

 	empty_list     = PKT-LINE(id SP "capabilities^{}" NUL cap_list LF)

 	non_empty_list = PKT-LINE(id SP name NUL cap_list LF)
 	                 *ref_record

 	cap_list      = *(SP capability) SP
 	ref_record    = any_ref | peeled_ref

 	any_ref       = PKT-LINE(id SP name LF)
 	peeled_ref    = PKT-LINE(id SP name LF)
 	                PKT-LINE(id SP name "^{}" LF
 	id            = 40*HEX

 	HEX           = "0".."9" | "a".."f"
 	NL            = <US-ASCII NUL, null (0)>
 	LF            = <US-ASCII LF,  linefeed (10)>
 	SP            = <US-ASCII SP,  horizontal-tab (9)>


 Smart Service git-upload-pack
 ------------------------------
 This service reads from the remote repository.

 Clients MUST first perform ref discovery with
 '$GIT_URL/info/refs?service=git-upload-pack'.

 	C: POST $GIT_URL/git-upload-pack HTTP/1.0
 	C: Content-Type: application/x-git-upload-pack-request
 	C:
 	C: ....want 0a53e9ddeaddad63ad106860237bbf53411d11a7
 	C: ....have 441b40d833fdfa93eb2908e52742248faf0ee993
 	C: 0000

 	S: 200 OK
 	S: Content-Type: application/x-git-upload-pack-result
 	S: Cache-Control: no-cache
 	S:
 	S: ....ACK %s, continue
 	S: ....NAK

 Clients MUST NOT reuse or revalidate a cached reponse.
 Servers MUST include sufficient Cache-Control headers
 to prevent caching of the response.

 Servers SHOULD support all capabilities defined here.

 Clients MUST send at least one 'want' command in the request body.
 Clients MUST NOT reference an id in a 'want' command which did not
 appear in the response obtained through ref discovery.

 	compute_request   = want_list
 	                    have_list
 	                    request_end
 	request_end       = "0000" | "done"

 	want_list         = PKT-LINE(want NUL cap_list LF)
 	                    *(want_pkt)
 	want_pkt          = PKT-LINE(want LF)
 	want              = "want" SP id
 	cap_list          = *(SP capability) SP

 	have_list         = *PKT-LINE("have" SP id LF)

 	command           = create | delete | update
 	create            = 40*"0" SP new_id SP name
 	delete            = old_id SP 40*"0" SP name
 	update            = old_id SP new_id SP name

 TODO: Document this further.
 TODO: Don't use uppercase for variable names below.

 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.

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

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

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

 (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 MUST only enter the queue once.

 one compute step:
 (c) Send one $GIT_URL/git-upload-pack request:

 	C: 0032want <WANT #1>...............................
 	C: 0032want <WANT #2>...............................
 	....
 	C: 0032have <COMMON #1>.............................
 	C: 0032have <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.

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

       * want
       * have

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

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

     The HAVE list is created by popping the first 32 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 "done" command to let
     the server know it won't proceed:

 	C: 0009done

  (s) Parse the git-upload-pack request:

      Verify all objects in WANT are directly reachable from refs.

 	  The server MAY walk backwards through history or through
      the reflog to permit slightly stale requests.

      If no WANT objects are received, send an error:

 TODO: Define error if no want lines are requested.

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

 TODO: Define error if an invalid want is requested.

     Create an empty list, 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 git-upload-pack response:

     If the server has found a closed set of objects to pack or the
     request ends with "done", it replies with the pack.

 TODO: Document the pack based response
 	S: 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:

 TODO: Document the non-pack response

  (c) Parse the upload-pack response:

 TODO: Document parsing response

      Do another compute step.


 Smart Service git-receive-pack
 ------------------------------
 This service modifies the remote repository.

 Clients MUST first perform ref discovery with
 '$GIT_URL/info/refs?service=git-receive-pack'.

 	C: POST $GIT_URL/git-receive-pack HTTP/1.0
 	C: Content-Type: application/x-git-receive-pack-request
 	C:
 	C: ....0a53e9ddeaddad63ad106860237bbf53411d11a7 441b40d833fdfa93eb2908e52742248faf0ee993 refs/heads/maint\0 report-status
 	C: 0000
 	C: PACK....

 	S: 200 OK
 	S: Content-Type: application/x-git-receive-pack-result
 	S: Cache-Control: no-cache
 	S:
 	S: ....

 Clients MUST NOT reuse or revalidate a cached reponse.
 Servers MUST include sufficient Cache-Control headers
 to prevent caching of the response.

 Servers SHOULD support all capabilities defined here.

 Clients MUST send at least one command in the request body.
 Within the command portion of the request body clients SHOULD send
 the id obtained through ref discovery as old_id.

 	update_request    = command_list
 	                    "PACK" <binary data>

 	command_list      = PKT-LINE(command NUL cap_list LF)
 	                    *(command_pkt)
 	command_pkt       = PKT-LINE(command LF)
 	cap_list          = *(SP capability) SP

 	command           = create | delete | update
 	create            = 40*"0" SP new_id SP name
 	delete            = old_id SP 40*"0" SP name
 	update            = old_id SP new_id SP name

 TODO: Document this further.


 References
 ----------

 link:http://www.ietf.org/rfc/rfc1738.txt[RFC 1738: Uniform Resource Locators (URL)]
 link:http://www.ietf.org/rfc/rfc2616.txt[RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1]
	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 both protocols.

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


	URL Format
	----------

	URLs for Git repositories accessed by HTTP use the standard HTTP
	URL syntax documented by RFC 1738, so they are of the form:

	http://<host>:<port>/<path>

	Within this documentation the placeholder $GIT_URL will stand for
	the http:// repository URL entered by the end-user.

	Both the "smart" and "dumb" HTTP protocols used by Git operate
	by appending additional path components onto the end of the user
	supplied $GIT_URL string.

	Clients MUST strip a trailing '/', if present, from the user supplied
	$GIT_URL string to prevent empty path tokens ('//') from appearing
	in any URL sent to a server. Compatible clients must expand
	'$GIT_URL/info/refs' as 'foo/info/refs' and not 'foo//info/refs'.


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

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

	Because Git repositories are accessed by standard path components
	server administrators MAY use directory based permissions within
	their HTTP server to control repository access.

	Clients SHOULD support Basic authentication as described by RFC 2616.
	Servers SHOULD support Basic authentication by relying upon the
	HTTP server placed in front of the Git server software.

	Servers MUST NOT require HTTP cookies for the purposes of
	authentication or access control.

	Clients and servers MAY support other common forms of HTTP based
	authentication, such as Digest authentication.


	SSL
	---

	Clients and servers SHOULD support SSL, particularly to protect
	passwords when relying on Basic HTTP authentication.


	Session State
	-------------

	The Git over HTTP protocol (much like HTTP itself) is stateless
	from the perspective of the HTTP server side. All state must be
	retained and managed by the client process. This permits simple
	round-robin load-balancing on the server side, without needing to
	worry about state mangement.

	Clients MUST NOT require state management on the server side in
	order to function correctly.

	Servers MUST NOT require HTTP cookies in order to function correctly.
	Clients MAY store and forward HTTP cookies during request processing
	as described by RFC 2616 (HTTP/1.1). Servers SHOULD ignore any
	cookies sent by a client.


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

	Much (but not all) 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 SHOULD BE terminated by an LF, which if present MUST be
	included in the total length.

	A pkt-line MAY contain binary data, so implementors MUST ensure all
	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 MUST
	be treated as a message break or terminator in the payload.


	General Request Processing
	--------------------------

	Except where noted, all standard HTTP behavior SHOULD be assumed
	by both client and server. This includes (but is not necessarily
	limited to):

	If there is no repository at $GIT_URL, the server MUST respond with
	the '404 Not Found' HTTP status code.

	If there is a repository at $GIT_URL, but access is not currently
	permitted, the server MUST respond with the '403 Forbidden' HTTP
	status code.

	Servers SHOULD support both HTTP 1.0 and HTTP 1.1.
	Servers SHOULD support chunked encoding for both
	request and response bodies.

	Clients SHOULD support both HTTP 1.0 and HTTP 1.1.
	Clients SHOULD support chunked encoding for both
	request and response bodies.

	Servers MAY return ETag and/or Last-Modified headers.

	Clients MAY revalidate cached entities by including If-Modified-Since
	and/or If-None-Match request headers.

	Servers MAY return '304 Not Modified' if the relevant headers appear
	in the request and the entity has not changed. Clients MUST treat
	'304 Not Modified' identical to '200 OK' by reusing the cached entity.

	Clients MAY reuse a cached entity without revalidation if the
	Cache-Control and/or Expires header permits caching. Clients and
	servers MUST follow RFC 2616 for cache controls.


	Discovering References
	----------------------

	All HTTP clients MUST begin either a fetch or a push exchange by
	discovering the references available on the remote repository.

	Dumb Clients
	~~~~~~~~~~~~

	HTTP clients that only support the "dumb" protocol MUST discover
	references by making a request for the special info/refs file of
	the repository.

	Dumb HTTP clients MUST NOT include search/query parameters when
	fetching the info/refs file. (That is, '?' must not appear in the
	requested URL.)

	C: GET $GIT_URL/info/refs HTTP/1.0

	S: 200 OK
	S:
	S: 95dcfa3633004da0049d3d0fa03f80589cbcaf31 refs/heads/maint
	S: d049f6c27a2244e12041955e262a404c7faba355 refs/heads/master
	S: 2cb58b79488a98d2721cea644875a8dd0026b115 refs/tags/v1.0
	S: a3c2e2402b99163d1d59756e5f207ae21cccba4c refs/tags/v1.0^{}

	The Content-Type of the returned info/refs entity SHOULD be
	"text/plain; charset=utf-8", but MAY be any content type.
	Clients MUST NOT attempt to validate the returned Content-Type.
	Dumb servers MUST NOT return a return type starting with
	"application/x-git-".

	Cache-Control headers MAY be returned to disable caching of the
	returned entity.

	When examining the response clients SHOULD only examine the HTTP
	status code. Valid responses are '200 OK', or '304 Not Modified'.

	The returned content is a UNIX formatted text file describing
	each ref and its known value. The file SHOULD be sorted by name
	according to the C locale ordering. The file SHOULD NOT include
	the default ref named 'HEAD'.

	info_refs = *( ref_record )
	ref_record = any_ref \| peeled_ref

	any_ref = id HT name LF
	peeled_ref = id HT name LF
	id HT name "^{}" LF
	id = 40*HEX

	HEX = "0".."9" \| "a".."f"
	LF = <US-ASCII LF, linefeed (10)>
	HT = <US-ASCII HT, horizontal-tab (9)>

	Smart Clients
	~~~~~~~~~~~~~

	HTTP clients that support the "smart" protocol (or both the
	"smart" and "dumb" protocols) MUST discover references by making
	a paramterized request for the info/refs file of the repository.

	The request MUST contain exactly one query parameter,
	'service=$servicename', where $servicename MUST be the service
	name the client wishes to contact to complete the operation.
	The request MUST NOT contain additional query parameters.

	C: GET $GIT_URL/info/refs?service=git-upload-pack HTTP/1.0

	dumb server reply:
	S: 200 OK
	S:
	S: 95dcfa3633004da0049d3d0fa03f80589cbcaf31 refs/heads/maint
	S: d049f6c27a2244e12041955e262a404c7faba355 refs/heads/master
	S: 2cb58b79488a98d2721cea644875a8dd0026b115 refs/tags/v1.0
	S: a3c2e2402b99163d1d59756e5f207ae21cccba4c refs/tags/v1.0^{}

	smart server reply:
	S: 200 OK
	S: Content-Type: application/x-git-upload-pack-advertisement
	S: Cache-Control: no-cache
	S:
	S: ....# service=git-upload-pack
	S: ....95dcfa3633004da0049d3d0fa03f80589cbcaf31 refs/heads/maint\0 multi_ack
	S: ....d049f6c27a2244e12041955e262a404c7faba355 refs/heads/master
	S: ....2cb58b79488a98d2721cea644875a8dd0026b115 refs/tags/v1.0
	S: ....a3c2e2402b99163d1d59756e5f207ae21cccba4c refs/tags/v1.0^{}

	Dumb Server Response
	^^^^^^^^^^^^^^^^^^^^
	Dumb servers MUST respond with the dumb server reply format.

	See the prior section under dumb clients for a more detailed
	description of the dumb server response.

	Smart Server Response
	^^^^^^^^^^^^^^^^^^^^^
	Smart servers MUST respond with the smart server reply format.

	If the server does not recognize the requested service name, or the
	requested service name has been disabled by the server administrator,
	the server MUST respond with the '403 Forbidden' HTTP status code.

	Cache-Control headers SHOULD be used to disable caching of the
	returned entity.

	The Content-Type MUST be 'application/x-$servicename-advertisement'.
	Clients SHOULD fall back to the dumb protocol if another content
	type is returned. When falling back to the dumb protocol clients
	SHOULD NOT make an additional request to $GIT_URL/info/refs, but
	instead SHOULD use the response already in hand. Clients MUST NOT
	continue if they do not support the dumb protocol.

	Clients MUST validate the status code is either '200 OK' or
	'304 Not Modified'.

	Clients MUST validate the first five bytes of the response entity
	matches the regex "^[0-9a-f]{4}#". If this test fails, clients
	MUST NOT continue.

	Clients MUST parse the entire response as a sequence of pkt-line
	records.

	Clients MUST verify the first pkt-line is "# service=$servicename".
	Servers MUST set $servicename to be the request parameter value.
	Servers SHOULD include an LF at the end of this line.
	Clients MUST ignore an LF at the end of the line.

	Servers MUST terminate the response with the magic "0000" end
	pkt-line marker.

	The returned response is a pkt-line stream describing each ref and
	its known value. The stream SHOULD be sorted by name according to
	the C locale ordering. The stream SHOULD include the default ref
	named 'HEAD' as the first ref. The stream MUST include capability
	declarations behind a NUL on the first ref.

	smart_reply = PKT-LINE("# service=$servicename" LF)
	ref_list
	"0000"
	ref_list = empty_list \| populated_list

	empty_list = PKT-LINE(id SP "capabilities^{}" NUL cap_list LF)

	non_empty_list = PKT-LINE(id SP name NUL cap_list LF)
	*ref_record

	cap_list = *(SP capability) SP
	ref_record = any_ref \| peeled_ref

	any_ref = PKT-LINE(id SP name LF)
	peeled_ref = PKT-LINE(id SP name LF)
	PKT-LINE(id SP name "^{}" LF
	id = 40*HEX

	HEX = "0".."9" \| "a".."f"
	NL = <US-ASCII NUL, null (0)>
	LF = <US-ASCII LF, linefeed (10)>
	SP = <US-ASCII SP, horizontal-tab (9)>


	Smart Service git-upload-pack
	------------------------------
	This service reads from the remote repository.

	Clients MUST first perform ref discovery with
	'$GIT_URL/info/refs?service=git-upload-pack'.

	C: POST $GIT_URL/git-upload-pack HTTP/1.0
	C: Content-Type: application/x-git-upload-pack-request
	C:
	C: ....want 0a53e9ddeaddad63ad106860237bbf53411d11a7
	C: ....have 441b40d833fdfa93eb2908e52742248faf0ee993
	C: 0000

	S: 200 OK
	S: Content-Type: application/x-git-upload-pack-result
	S: Cache-Control: no-cache
	S:
	S: ....ACK %s, continue
	S: ....NAK

	Clients MUST NOT reuse or revalidate a cached reponse.
	Servers MUST include sufficient Cache-Control headers
	to prevent caching of the response.

	Servers SHOULD support all capabilities defined here.

	Clients MUST send at least one 'want' command in the request body.
	Clients MUST NOT reference an id in a 'want' command which did not
	appear in the response obtained through ref discovery.

	compute_request = want_list
	have_list
	request_end
	request_end = "0000" \| "done"

	want_list = PKT-LINE(want NUL cap_list LF)
	*(want_pkt)
	want_pkt = PKT-LINE(want LF)
	want = "want" SP id
	cap_list = *(SP capability) SP

	have_list = *PKT-LINE("have" SP id LF)

	command = create \| delete \| update
	create = 40*"0" SP new_id SP name
	delete = old_id SP 40*"0" SP name
	update = old_id SP new_id SP name

	TODO: Document this further.
	TODO: Don't use uppercase for variable names below.

	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.

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

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

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

	(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 MUST only enter the queue once.

	one compute step:
	(c) Send one $GIT_URL/git-upload-pack request:

	C: 0032want <WANT #1>...............................
	C: 0032want <WANT #2>...............................
	....
	C: 0032have <COMMON #1>.............................
	C: 0032have <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.

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

	* want
	* have

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

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

	The HAVE list is created by popping the first 32 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 "done" command to let
	the server know it won't proceed:

	C: 0009done

	(s) Parse the git-upload-pack request:

	Verify all objects in WANT are directly reachable from refs.

	The server MAY walk backwards through history or through
	the reflog to permit slightly stale requests.

	If no WANT objects are received, send an error:

	TODO: Define error if no want lines are requested.

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

	TODO: Define error if an invalid want is requested.

	Create an empty list, 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 git-upload-pack response:

	If the server has found a closed set of objects to pack or the
	request ends with "done", it replies with the pack.

	TODO: Document the pack based response
	S: 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:

	TODO: Document the non-pack response

	(c) Parse the upload-pack response:

	TODO: Document parsing response

	Do another compute step.


	Smart Service git-receive-pack
	------------------------------
	This service modifies the remote repository.

	Clients MUST first perform ref discovery with
	'$GIT_URL/info/refs?service=git-receive-pack'.

	C: POST $GIT_URL/git-receive-pack HTTP/1.0
	C: Content-Type: application/x-git-receive-pack-request
	C:
	C: ....0a53e9ddeaddad63ad106860237bbf53411d11a7 441b40d833fdfa93eb2908e52742248faf0ee993 refs/heads/maint\0 report-status
	C: 0000
	C: PACK....

	S: 200 OK
	S: Content-Type: application/x-git-receive-pack-result
	S: Cache-Control: no-cache
	S:
	S: ....

	Clients MUST NOT reuse or revalidate a cached reponse.
	Servers MUST include sufficient Cache-Control headers
	to prevent caching of the response.

	Servers SHOULD support all capabilities defined here.

	Clients MUST send at least one command in the request body.
	Within the command portion of the request body clients SHOULD send
	the id obtained through ref discovery as old_id.

	update_request = command_list
	"PACK" <binary data>

	command_list = PKT-LINE(command NUL cap_list LF)
	*(command_pkt)
	command_pkt = PKT-LINE(command LF)
	cap_list = *(SP capability) SP

	command = create \| delete \| update
	create = 40*"0" SP new_id SP name
	delete = old_id SP 40*"0" SP name
	update = old_id SP new_id SP name

	TODO: Document this further.


	References
	----------

	link:http://www.ietf.org/rfc/rfc1738.txt[RFC 1738: Uniform Resource Locators (URL)]
	link:http://www.ietf.org/rfc/rfc2616.txt[RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1]