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Update packfile transfer protocol documentation 

The current technical documentation for the packfile protocol is both
sparse and incorrect.  This documents the fetch-pack/upload-pack and
send-pack/ receive-pack protocols much more fully.

Add documentation from Shawn's upcoming http-protocol docs that is
shared by the packfile protocol. protocol-common.txt describes ABNF
notation amendments, refname rules and the packet line format.

Add documentation on the various capabilities supported by the
upload-pack and receive-pack protocols. protocol-capabilities.txt
describes multi-ack, thin-pack, side-band[-64k], shallow, no-progress,
include-tag, ofs-delta, delete-refs and report-status.

Signed-off-by: Scott Chacon <schacon@gmail.com>
Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
maint
Scott Chacon 15 years ago committed by Junio C Hamano
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  1. 535
      Documentation/technical/pack-protocol.txt
  2. 187
      Documentation/technical/protocol-capabilities.txt
  3. 96
      Documentation/technical/protocol-common.txt

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Pack transfer protocols
=======================

There are two Pack push-pull protocols.

upload-pack (S) | fetch/clone-pack (C) protocol:

# Tell the puller what commits we have and what their names are
S: SHA1 name
S: ...
S: SHA1 name
S: # flush -- it's your turn
# Tell the pusher what commits we want, and what we have
C: want name
C: ..
C: want name
C: have SHA1
C: have SHA1
C: ...
C: # flush -- occasionally ask "had enough?"
S: NAK
C: have SHA1
C: ...
C: have SHA1
S: ACK
C: done
S: XXXXXXX -- packfile contents.

send-pack | receive-pack protocol.

# Tell the pusher what commits we have and what their names are
C: SHA1 name
C: ...
C: SHA1 name
C: # flush -- it's your turn
# Tell the puller what the pusher has
S: old-SHA1 new-SHA1 name
S: old-SHA1 new-SHA1 name
S: ...
S: # flush -- done with the list
S: XXXXXXX --- packfile contents.
Packfile transfer protocols
===========================

Git supports transferring data in packfiles over the ssh://, git:// and
file:// transports. There exist two sets of protocols, one for pushing
data from a client to a server and another for fetching data from a
server to a client. All three transports (ssh, git, file) use the same
protocol to transfer data.

The processes invoked in the canonical Git implementation are 'upload-pack'
on the server side and 'fetch-pack' on the client side for fetching data;
then 'receive-pack' on the server and 'send-pack' on the client for pushing
data. The protocol functions to have a server tell a client what is
currently on the server, then for the two to negotiate the smallest amount
of data to send in order to fully update one or the other.

Transports
----------
There are three transports over which the packfile protocol is
initiated. The Git transport is a simple, unauthenticated server that
takes the command (almost always 'upload-pack', though Git
servers can be configured to be globally writable, in which 'receive-
pack' initiation is also allowed) with which the client wishes to
communicate and executes it and connects it to the requesting
process.

In the SSH transport, the client just runs the 'upload-pack'
or 'receive-pack' process on the server over the SSH protocol and then
communicates with that invoked process over the SSH connection.

The file:// transport runs the 'upload-pack' or 'receive-pack'
process locally and communicates with it over a pipe.

Git Transport
-------------

The Git transport starts off by sending the command and repository
on the wire using the pkt-line format, followed by a NUL byte and a
hostname paramater, terminated by a NUL byte.

0032git-upload-pack /project.git\0host=myserver.com\0

--
git-proto-request = request-command SP pathname NUL [ host-parameter NUL ]
request-command = "git-upload-pack" / "git-receive-pack" /
"git-upload-archive" ; case sensitive
pathname = *( %x01-ff ) ; exclude NUL
host-parameter = "host=" hostname [ ":" port ]
--

Only host-parameter is allowed in the git-proto-request. Clients
MUST NOT attempt to send additional parameters. It is used for the
git-daemon name based virtual hosting. See --interpolated-path
option to git daemon, with the %H/%CH format characters.

Basically what the Git client is doing to connect to an 'upload-pack'
process on the server side over the Git protocol is this:

$ echo -e -n \
"0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" |
nc -v example.com 9418


SSH Transport
-------------

Initiating the upload-pack or receive-pack processes over SSH is
executing the binary on the server via SSH remote execution.
It is basically equivalent to running this:

$ ssh git.example.com "git-upload-pack '/project.git'"

For a server to support Git pushing and pulling for a given user over
SSH, that user needs to be able to execute one or both of those
commands via the SSH shell that they are provided on login. On some
systems, that shell access is limited to only being able to run those
two commands, or even just one of them.

In an ssh:// format URI, it's absolute in the URI, so the '/' after
the host name (or port number) is sent as an argument, which is then
read by the remote git-upload-pack exactly as is, so it's effectively
an absolute path in the remote filesystem.

git clone ssh://user@example.com/project.git
|
v
ssh user@example.com "git-upload-pack '/project.git'"

In a "user@host:path" format URI, its relative to the user's home
directory, because the Git client will run:

git clone user@example.com:project.git
|
v
ssh user@example.com "git-upload-pack 'project.git'"

The exception is if a '~' is used, in which case
we execute it without the leading '/'.

ssh://user@example.com/~alice/project.git,
|
v
ssh user@example.com "git-upload-pack '~alice/project.git'"

A few things to remember here:

- The "command name" is spelled with dash (e.g. git-upload-pack), but
this can be overridden by the client;

- The repository path is always quoted with single quotes.

Fetching Data From a Server
===========================

When one Git repository wants to get data that a second repository
has, the first can 'fetch' from the second. This operation determines
what data the server has that the client does not then streams that
data down to the client in packfile format.


Reference Discovery
-------------------

When the client initially connects the server will immediately respond
with a listing of each reference it has (all branches and tags) along
with the object name that each reference currently points to.

$ echo -e -n "0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" |
nc -v example.com 9418
00887217a7c7e582c46cec22a130adf4b9d7d950fba0 HEAD\0multi_ack thin-pack side-band side-band-64k ofs-delta shallow no-progress include-tag
00441d3fcd5ced445d1abc402225c0b8a1299641f497 refs/heads/integration
003f7217a7c7e582c46cec22a130adf4b9d7d950fba0 refs/heads/master
003cb88d2441cac0977faf98efc80305012112238d9d refs/tags/v0.9
003c525128480b96c89e6418b1e40909bf6c5b2d580f refs/tags/v1.0
003fe92df48743b7bc7d26bcaabfddde0a1e20cae47c refs/tags/v1.0^{}
0000

Server SHOULD terminate each non-flush line using LF ("\n") terminator;
client MUST NOT complain if there is no terminator.

The returned response is a pkt-line stream describing each ref and
its current value. The stream MUST be sorted by name according to
the C locale ordering.

If HEAD is a valid ref, HEAD MUST appear as the first advertised
ref. If HEAD is not a valid ref, HEAD MUST NOT appear in the
advertisement list at all, but other refs may still appear.

The stream MUST include capability declarations behind a NUL on the
first ref. The peeled value of a ref (that is "ref^{}") MUST be
immediately after the ref itself, if presented. A conforming server
MUST peel the ref if its an annotated tag.

----
advertised-refs = (no-refs / list-of-refs)
flush-pkt

no-refs = PKT-LINE(zero-id SP "capabilities^{}"
NUL capability-list LF)

list-of-refs = first-ref *other-ref
first-ref = PKT-LINE(obj-id SP refname
NUL capability-list LF)

other-ref = PKT-LINE(other-tip / other-peeled)
other-tip = obj-id SP refname LF
other-peeled = obj-id SP refname "^{}" LF

capability-list = capability *(SP capability)
capability = 1*(LC_ALPHA / DIGIT / "-" / "_")
LC_ALPHA = %x61-7A
----

Server and client MUST use lowercase for obj-id, both MUST treat obj-id
as case-insensitive.

See protocol-capabilities.txt for a list of allowed server capabilities
and descriptions.

Packfile Negotiation
--------------------
After reference and capabilities discovery, the client can decide
to terminate the connection by sending a flush-pkt, telling the
server it can now gracefully terminate (as happens with the ls-remote
command) or it can enter the negotiation phase, where the client and
server determine what the minimal packfile necessary for transport is.

Once the client has the initial list of references that the server
has, as well as the list of capabilities, it will begin telling the
server what objects it wants and what objects it has, so the server
can make a packfile that only contains the objects that the client needs.
The client will also send a list of the capabilities it wants to be in
effect, out of what the server said it could do with the first 'want' line.

----
upload-request = want-list
have-list
compute-end

want-list = first-want
*additional-want
flush-pkt

first-want = PKT-LINE("want" SP obj-id SP capability-list LF)
additional-want = PKT-LINE("want" SP obj-id LF)

have-list = *have-line
have-line = PKT-LINE("have" SP obj-id LF)
compute-end = flush-pkt / PKT-LINE("done")
----

Clients MUST send all the obj-ids it wants from the reference
discovery phase as 'want' lines. Clients MUST send at least one
'want' command in the request body. Clients MUST NOT mention an
obj-id in a 'want' command which did not appear in the response
obtained through ref discovery.

If client is requesting a shallow clone, it will now send a 'deepen'
line with the depth it is requesting.

Once all the "want"s (and optional 'deepen') are transferred,
clients MUST send a flush-pkt. If the client has all the references
on the server, client flushes and disconnects.

TODO: shallow/unshallow response and document the deepen command in the ABNF.

Now the client will send a list of the obj-ids it has using 'have'
lines. In multi_ack mode, the canonical implementation will send up
to 32 of these at a time, then will send a flush-pkt. The canonical
implementation will skip ahead and send the next 32 immediately,
so that there is always a block of 32 "in-flight on the wire" at a
time.

If the server reads 'have' lines, it then will respond by ACKing any
of the obj-ids the client said it had that the server also has. The
server will ACK obj-ids differently depending on which ack mode is
chosen by the client.

In multi_ack mode:

* the server will respond with 'ACK obj-id continue' for any common
commits.

* once the server has found an acceptable common base commit and is
ready to make a packfile, it will blindly ACK all 'have' obj-ids
back to the client.

* the server will then send a 'NACK' and then wait for another response
from the client - either a 'done' or another list of 'have' lines.

In multi_ack_detailed mode:

* the server will differentiate the ACKs where it is signaling
that it is ready to send data with 'ACK obj-id ready' lines, and
signals the identified common commits with 'ACK obj-id common' lines.

Without either multi_ack or multi_ack_detailed:

* upload-pack sends "ACK obj-id" on the first common object it finds.
After that it says nothing until the client gives it a "done".

* upload-pack sends "NAK" on a flush-pkt if no common object
has been found yet. If one has been found, and thus an ACK
was already sent, its silent on the flush-pkt.

After the client has gotten enough ACK responses that it can determine
that the server has enough information to send an efficient packfile
(in the canonical implementation, this is determined when it has received
enough ACKs that it can color everything left in the --date-order queue
as common with the server, or the --date-order queue is empty), or the
client determines that it wants to give up (in the canonical implementation,
this is determined when the client sends 256 'have' lines without getting
any of them ACKed by the server - meaning there is nothing in common and
the server should just send all it's objects), then the client will send
a 'done' command. The 'done' command signals to the server that the client
is ready to receive it's packfile data.

However, the 256 limit *only* turns on in the canonical client
implementation if we have received at least one "ACK %s continue"
during a prior round. This helps to ensure that at least one common
ancestor is found before we give up entirely.

Once the 'done' line is read from the client, the server will either
send a final 'ACK obj-id' or it will send a 'NAK'. The server only sends
ACK after 'done' if there is at least one common base and multi_ack or
multi_ack_detailed is enabled. The server always sends NAK after 'done'
if there is no common base found.

Then the server will start sending it's packfile data.

----
server-response = *ack_multi ack / nak
ack_multi = PKT-LINE("ACK" SP obj-id ack_status LF)
ack_status = "continue" / "common" / "ready"
ack = PKT-LINE("ACK SP obj-id LF)
nak = PKT-LINE("NAK" LF)
----

A simple clone may look like this (with no 'have' lines):

----
C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d\0multi_ack \
side-band-64k ofs-delta\n
C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
C: 0032want 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
C: 0032want 74730d410fcb6603ace96f1dc55ea6196122532d\n
C: 0000
C: 0009done\n

S: 0008NAK\n
S: [PACKFILE]
----

An incremental update (fetch) response might look like this:

----
C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d\0multi_ack \
side-band-64k ofs-delta\n
C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
C: 0000
C: 0032have 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
C: [30 more have lines]
C: 0032have 74730d410fcb6603ace96f1dc55ea6196122532d\n
C: 0000

S: 003aACK 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01 continue\n
S: 003aACK 74730d410fcb6603ace96f1dc55ea6196122532d continue\n
S: 0008NAK\n

C: 0009done\n

S: 003aACK 74730d410fcb6603ace96f1dc55ea6196122532d\n
S: [PACKFILE]
----


Packfile Data
-------------

Now that the client and server have finished negotiation about what
the minimal amount of data that needs to be sent to the client is, the server
will construct and send the required data in packfile format.

See pack-format.txt for what the packfile itself actually looks like.

If 'side-band' or 'side-band-64k' capabilities have been specified by
the client, the server will send the packfile data multiplexed.

Each packet starting with the packet-line length of the amount of data
that follows, followed by a single byte specifying the sideband the
following data is coming in on.

In 'side-band' mode, it will send up to 999 data bytes plus 1 control
code, for a total of up to 1000 bytes in a pkt-line. In 'side-band-64k'
mode it will send up to 65519 data bytes plus 1 control code, for a
total of up to 65520 bytes in a pkt-line.

The sideband byte will be a '1', '2' or a '3'. Sideband '1' will contain
packfile data, sideband '2' will be used for progress information that the
client will generally print to stderr and sideband '3' is used for error
information.

If no 'side-band' capability was specified, the server will stream the
entire packfile without multiplexing.


Pushing Data To a Server
========================

Pushing data to a server will invoke the 'receive-pack' process on the
server, which will allow the client to tell it which references it should
update and then send all the data the server will need for those new
references to be complete. Once all the data is received and validated,
the server will then update its references to what the client specified.

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

The protocol itself contains no authentication mechanisms. That is to be
handled by the transport, such as SSH, before the 'receive-pack' process is
invoked. If 'receive-pack' is configured over the Git transport, those
repositories will be writable by anyone who can access that port (9418) as
that transport is unauthenticated.

Reference Discovery
-------------------

The reference discovery phase is done nearly the same way as it is in the
fetching protocol. Each reference obj-id and name on the server is sent
in packet-line format to the client, followed by a flush-pkt. The only
real difference is that the capability listing is different - the only
possible values are 'report-status', 'delete-refs' and 'ofs-delta'.

Reference Update Request and Packfile Transfer
----------------------------------------------

Once the client knows what references the server is at, it can send a
list of reference update requests. For each reference on the server
that it wants to update, it sends a line listing the obj-id currently on
the server, the obj-id the client would like to update it to and the name
of the reference.

This list is followed by a flush-pkt and then the packfile that should
contain all the objects that the server will need to complete the new
references.

----
update-request = command-list [pack-file]

command-list = PKT-LINE(command NUL capability-list LF)
*PKT-LINE(command LF)
flush-pkt

command = create / delete / update
create = zero-id SP new-id SP name
delete = old-id SP zero-id SP name
update = old-id SP new-id SP name

old-id = obj-id
new-id = obj-id

pack-file = "PACK" 28*(OCTET)
----

If the receiving end does not support delete-refs, the sending end MUST
NOT ask for delete command.

The pack-file MUST NOT be sent if the only command used is 'delete'.

A pack-file MUST be sent if either create or update command is used,
even if the server already has all the necessary objects. In this
case the client MUST send an empty pack-file. The only time this
is likely to happen is if the client is creating
a new branch or a tag that points to an existing obj-id.

The server will receive the packfile, unpack it, then validate each
reference that is being updated that it hasn't changed while the request
was being processed (the obj-id is still the same as the old-id), and
it will run any update hooks to make sure that the update is acceptable.
If all of that is fine, the server will then update the references.

Report Status
-------------

After receiving the pack data from the sender, the receiver sends a
report if 'report-status' capability is in effect.
It is a short listing of what happened in that update. It will first
list the status of the packfile unpacking as either 'unpack ok' or
'unpack [error]'. Then it will list the status for each of the references
that it tried to update. Each line is either 'ok [refname]' if the
update was successful, or 'ng [refname] [error]' if the update was not.

----
report-status = unpack-status
1*(command-status)
flush-pkt

unpack-status = PKT-LINE("unpack" SP unpack-result LF)
unpack-result = "ok" / error-msg

command-status = command-ok / command-fail
command-ok = PKT-LINE("ok" SP refname LF)
command-fail = PKT-LINE("ng" SP refname SP error-msg LF)

error-msg = 1*(OCTECT) ; where not "ok"
----

Updates can be unsuccessful for a number of reasons. The reference can have
changed since the reference discovery phase was originally sent, meaning
someone pushed in the meantime. The reference being pushed could be a
non-fast-forward reference and the update hooks or configuration could be
set to not allow that, etc. Also, some references can be updated while others
can be rejected.

An example client/server communication might look like this:

----
S: 007c74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/local\0report-status delete-refs ofs-delta\n
S: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe refs/heads/debug\n
S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/master\n
S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/team\n
S: 0000

C: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe 74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/debug\n
C: 003e74730d410fcb6603ace96f1dc55ea6196122532d 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a refs/heads/master\n
C: 0000
C: [PACKDATA]

S: 000aunpack ok\n
S: 0014ok refs/heads/debug\n
S: 0026ng refs/heads/master non-fast-forward\n
----

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Git Protocol Capabilities
=========================

Servers SHOULD support all capabilities defined in this document.

On the very first line of the initial server response of either
receive-pack and upload-pack the first reference is followed by
a NUL byte and then a list of space delimited server capabilities.
These allow the server to declare what it can and cannot support
to the client.

Client will then send a space separated list of capabilities it wants
to be in effect. The client MUST NOT ask for capabilities the server
did not say it supports.

Server MUST diagnose and abort if capabilities it does not understand
was sent. Server MUST NOT ignore capabilities that client requested
and server advertised. As a consequence of these rules, server MUST
NOT advertise capabilities it does not understand.

The 'report-status' and 'delete-refs' capabilities are sent and
recognized by the receive-pack (push to server) process.

The 'ofs-delta' capability is sent and recognized by both upload-pack
and receive-pack protocols.

All other capabilities are only recognized by the upload-pack (fetch
from server) process.

multi_ack
---------

The 'multi_ack' capability allows the server to return "ACK obj-id
continue" as soon as it finds a commit that it can use as a common
base, between the client's wants and the client's have set.

By sending this early, the server can potentially head off the client
from walking any further down that particular branch of the client's
repository history. The client may still need to walk down other
branches, sending have lines for those, until the server has a
complete cut across the DAG, or the client has said "done".

Without multi_ack, a client sends have lines in --date-order until
the server has found a common base. That means the client will send
have lines that are already known by the server to be common, because
they overlap in time with another branch that the server hasn't found
a common base on yet.

For example suppose the client has commits in caps that the server
doesn't and the server has commits in lower case that the client
doesn't, as in the following diagram:

+---- u ---------------------- x
/ +----- y
/ /
a -- b -- c -- d -- E -- F
\
+--- Q -- R -- S

If the client wants x,y and starts out by saying have F,S, the server
doesn't know what F,S is. Eventually the client says "have d" and
the server sends "ACK d continue" to let the client know to stop
walking down that line (so don't send c-b-a), but its not done yet,
it needs a base for x. The client keeps going with S-R-Q, until a
gets reached, at which point the server has a clear base and it all
ends.

Without multi_ack the client would have sent that c-b-a chain anyway,
interleaved with S-R-Q.

thin-pack
---------

This capability means that the server can send a 'thin' pack, a pack
which does not contain base objects; if those base objects are available
on client side. Client requests 'thin-pack' capability when it
understands how to "thicken" it by adding required delta bases making
it self-contained.

Client MUST NOT request 'thin-pack' capability if it cannot turn a thin
pack into a self-contained pack.


side-band, side-band-64k
------------------------

This capability means that server can send, and client understand multiplexed
progress reports and error info interleaved with the packfile itself.

These two options are mutually exclusive. A modern client always
favors 'side-band-64k'.

Either mode indicates that the packfile data will be streamed broken
up into packets of up to either 1000 bytes in the case of 'side_band',
or 65520 bytes in the case of 'side_band_64k'. Each packet is made up
of a leading 4-byte pkt-line length of how much data is in the packet,
followed by a 1-byte stream code, followed by the actual data.

The stream code can be one of:

1 - pack data
2 - progress messages
3 - fatal error message just before stream aborts

The "side-band-64k" capability came about as a way for newer clients
that can handle much larger packets to request packets that are
actually crammed nearly full, while maintaining backward compatibility
for the older clients.

Further, with side-band and its up to 1000-byte messages, it's actually
999 bytes of payload and 1 byte for the stream code. With side-band-64k,
same deal, you have up to 65519 bytes of data and 1 byte for the stream
code.

The client MUST send only maximum of one of "side-band" and "side-
band-64k". Server MUST diagnose it as an error if client requests
both.

ofs-delta
---------

Server can send, and client understand PACKv2 with delta refering to
its base by position in pack rather than by an obj-id. That is, they can
send/read OBJ_OFS_DELTA (aka type 6) in a packfile.

shallow
-------

This capability adds "deepen", "shallow" and "unshallow" commands to
the fetch-pack/upload-pack protocol so clients can request shallow
clones.

no-progress
-----------

The client was started with "git clone -q" or something, and doesn't
want that side band 2. Basically the client just says "I do not
wish to receive stream 2 on sideband, so do not send it to me, and if
you did, I will drop it on the floor anyway". However, the sideband
channel 3 is still used for error responses.

include-tag
-----------

The 'include-tag' capability is about sending annotated tags if we are
sending objects they point to. If we pack an object to the client, and
a tag object points exactly at that object, we pack the tag object too.
In general this allows a client to get all new annotated tags when it
fetches a branch, in a single network connection.

Clients MAY always send include-tag, hardcoding it into a request when
the server advertises this capability. The decision for a client to
request include-tag only has to do with the client's desires for tag
data, whether or not a server had advertised objects in the
refs/tags/* namespace.

Servers MUST pack the tags if their referrant is packed and the client
has requested include-tags.

Clients MUST be prepared for the case where a server has ignored
include-tag and has not actually sent tags in the pack. In such
cases the client SHOULD issue a subsequent fetch to acquire the tags
that include-tag would have otherwise given the client.

The server SHOULD send include-tag, if it supports it, regardless
of whether or not there are tags available.

report-status
-------------

The upload-pack process can receive a 'report-status' capability,
which tells it that the client wants a report of what happened after
a packfile upload and reference update. If the pushing client requests
this capability, after unpacking and updating references the server
will respond with whether the packfile unpacked successfully and if
each reference was updated successfully. If any of those were not
successful, it will send back an error message. See pack-protocol.txt
for example messages.

delete-refs
-----------

If the server sends back the 'delete-refs' capability, it means that
it is capable of accepting an zero-id value as the target
value of a reference update. It is not sent back by the client, it
simply informs the client that it can be sent zero-id values
to delete references.

96
Documentation/technical/protocol-common.txt
Unescape View File

@ -0,0 +1,96 @@ @@ -0,0 +1,96 @@
Documentation Common to Pack and Http Protocols
===============================================

ABNF Notation
-------------

ABNF notation as described by RFC 5234 is used within the protocol documents,
except the following replacement core rules are used:
----
HEXDIG = DIGIT / "a" / "b" / "c" / "d" / "e" / "f"
----

We also define the following common rules:
----
NUL = %x00
zero-id = 40*"0"
obj-id = 40*(HEXDIGIT)

refname = "HEAD"
refname /= "refs/" <see discussion below>
----

A refname is a hierarchical octet string beginning with "refs/" and
not violating the 'git-check-ref-format' command's validation rules.
More specifically, they:

. They can include slash `/` for hierarchical (directory)
grouping, but no slash-separated component can begin with a
dot `.`.

. They must contain at least one `/`. This enforces the presence of a
category like `heads/`, `tags/` etc. but the actual names are not
restricted.

. They cannot have two consecutive dots `..` anywhere.

. They cannot have ASCII control characters (i.e. bytes whose
values are lower than \040, or \177 `DEL`), space, tilde `~`,
caret `{caret}`, colon `:`, question-mark `?`, asterisk `*`,
or open bracket `[` anywhere.

. They cannot end with a slash `/` nor a dot `.`.

. They cannot end with the sequence `.lock`.

. They cannot contain a sequence `@{`.

. They cannot contain a `\\`.


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, the pkt-len, indicates the total length of the line,
in hexadecimal. The pkt-len includes the 4 bytes used to contain
the length's hexadecimal representation.

A pkt-line MAY contain binary data, so implementors MUST ensure
pkt-line parsing/formatting routines are 8-bit clean.

A non-binary line SHOULD BE terminated by an LF, which if present
MUST be included in the total length.

The maximum length of a pkt-line's data component is 65520 bytes.
Implementations MUST NOT send pkt-line whose length exceeds 65524
(65520 bytes of payload + 4 bytes of length data).

Implementations SHOULD NOT send an empty pkt-line ("0004").

A pkt-line with a length field of 0 ("0000"), called a flush-pkt,
is a special case and MUST be handled differently than an empty
pkt-line ("0004").

----
pkt-line = data-pkt / flush-pkt

data-pkt = pkt-len pkt-payload
pkt-len = 4*(HEXDIG)
pkt-payload = (pkt-len - 4)*(OCTET)

flush-pkt = "0000"
----

Examples (as C-style strings):

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