[PATCH] Update packfile transfer protocol documentation

[PATCH] Update packfile transfer protocol documentation

[Scott Chacon]

The 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>
---

OK, I think I've incorporated all the comments from you and Shawn
here.  Let me know if that's cool or you need me to update anything
else.

 Documentation/technical/pack-protocol.txt         |  535 +++++++++++++++++++--
 Documentation/technical/protocol-capabilities.txt |  186 +++++++
 Documentation/technical/protocol-common.txt       |   96 ++++
 3 files changed, 776 insertions(+), 41 deletions(-)
 create mode 100644 Documentation/technical/protocol-capabilities.txt
 create mode 100644 Documentation/technical/protocol-common.txt

diff --git a/Documentation/technical/pack-protocol.txt
b/Documentation/technical/pack-protocol.txt
index 9cd48b4..4b476da 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -1,41 +1,494 @@
-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 protocol starts off by sending the command and repository
+on the wire using the pkt-line format, followed by a null byte and a
+hostname paramater, terminated by a null 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 known 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 SHOULD use lowercase for SHA1, both MUST treat SHA1
+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 has the objects that the client needs.
+The client will also send a list of the capabilities it supports 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
+signaled 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 multi_ack:
+
+ * 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 done some negotiation about what
+the minimal amount of data that can 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 it's 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 packet.  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 packet 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 doing something like creating
+a new branch 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 client sends a
+report if 'report-status' capability was sent to the server.
+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 be 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
+----
diff --git a/Documentation/technical/protocol-capabilities.txt
b/Documentation/technical/protocol-capabilities.txt
new file mode 100644
index 0000000..f4bf986
--- /dev/null
+++ b/Documentation/technical/protocol-capabilities.txt
@@ -0,0 +1,186 @@
+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 null 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
+can support. The client SHOULD NOT ask for capabilities the server
+did not say it supports.
+
+Server MUST ignore capabilities it does not understand.  Server MUST
+NOT ignore capabilities that client requested and server advertised.
+
+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 implies that the server can send 'thin' packs, packs
+which do not contain base objects; if those base objects are available
+on client side. Client requests 'thin-pack' capability when it
+understands how to "thicken" them adding required delta bases making
+them self contained.
+
+Client MUST NOT request 'thin-pack' capability if it cannot turn thin
+packs into proper independent packs.
+
+
+side-band, side-band-64k
+------------------------
+
+This 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 SHA-1.  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 all-zeroed SHA-1 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 zeroed SHA-1 values
+to delete references.
+
diff --git a/Documentation/technical/protocol-common.txt
b/Documentation/technical/protocol-common.txt
new file mode 100644
index 0000000..2dca642
--- /dev/null
+++ b/Documentation/technical/protocol-common.txt
@@ -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 hierarichal 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"            ""
+----
-- 
1.6.5.2.340.g000a3e

Re: [PATCH] Update packfile transfer protocol documentation

[Sverre Rabbelier]

Heya,

On Wed, Nov 4, 2009 at 06:58, Scott Chacon <schacon@gmail.com> wrote:
> The technical documentation for the packfile protocol is both sparse and incorrect

Each time I read this line I read:

"This technical documentation for the packfile protocol is both sparse
and incorrect"

Perhaps if there is another resend after this you can change it to:

"The current technical documentation for the packfile protocol is both sparse"

-- 
Cheers,

Sverre Rabbelier

Re: [PATCH] Update packfile transfer protocol documentation

[Junio C Hamano]

Scott Chacon <schacon@gmail.com> writes:

> The 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.

Thanks.

In this round, my comments ended up being mostly "nit-picky", although I
spotted a few logic errors.

> +Git Transport
> +-------------
> +
> +The Git protocol starts off by sending the command and repository

"transport"

> +on the wire using the pkt-line format, followed by a null byte and a
> +hostname paramater, terminated by a null byte.

The name of the byte whose value is 0 is "NUL", not "NULL" (as you
correctly did in the "Reference Discovery" section).

> +Basically what the Git client is doing to connect to an 'upload-pack'
> ...
> +SSH Transport
> ...
> +It is basically equivalent to running this:

> +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" |

Oops...

> +      nc -v example.com 9418
> +   00887217a7c7e582c46cec22a130adf4b9d7d950fba0 HEAD\0multi_ack
> thin-pack side-band side-band-64k ofs-delta shallow no-progress
> include-tag

Oops...

It is perfectly fine to fold a long line in the document.  What I
earlier suggested was to state the fact that the document _did_ fold
a long line for typographical reasons and in the actual protocol it
is a single long line to the reader.

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

Do we need to say "known"?

> ...
> +  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 SHOULD use lowercase for SHA1, both MUST treat SHA1
> +as case-insensitive.

I hate to sound like a broken parrot, but ...

 - You do not have SHA1 in the above list; say "obj-id".

 - Why is this SHOULD and not MUST?  It's not like you or somebody else
   have already written such a broken server or client and you have to
   grandfather it by defining what goes over the wire looser than the
   current practice.  I think this is taking the "be liberal in what you
   accept" mantra too literally without real reason for doing so.

> +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 has the objects that the client needs.

"only has" -> "only contains", perhaps, as the above has too many has
already?

> +The client will also send a list of the capabilities it supports out
> +of what the server said it could do with the first 'want' line.

It is not _wrong_ per-se, but it is not about client "supporting".  It is
asking these particular protocol extensions enabled.  How about...

	.. will also send a list of the capabilities it wants to be in
        effect, out of ...

> +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
> +signaled by the client.

Perhaps "signaled" -> "chosen"?

> +In multi_ack mode:
> ...
> +In multi_ack_detailed mode:
> ...
> +Without multi_ack:
> ...

The last one is "without multi_ack or multi_ack_detailed", isn't it?
I think the order you described these three is sensible (from the most
commonly deployed to merely describing a historical practice).

> +Packfile Data
> +-------------
> +
> +Now that the client and server have done some negotiation about what
> +the minimal amount of data that can be sent to the client is, the server
> +will construct and send the required data in packfile format.

Perhaps "done some" -> "finished"?

Is it "can be sent" or "needs to be sent"?

> +Pushing Data To a Server
> ...
> +references to be complete.  Once all the data is received and validated,
> +the server will then update it's references to what the client specified.

"it's" -> "its".

> +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 packet.  The only

I slightly prefer "a flush packet" spelled out like this, but please be
consistent either way.  You have only two instances of spelled-out "flush
packet" around this section, and everywhere else in this patch you used
"flush-pkt" (I am talking about explanation prose, not ABNF, in which you
consistently used "flush-pkt" everywhere), so changing these "flush
packet" to "flush-pkt" may be easier to change and less error prone.

> +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 doing something like creating
> +a new branch that points to an existing obj-id.

"doing something like" is redundant.

> +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.

Strictly speaking, "unpack it" is a wrong thing to say in a protocol
specification document.  The only requirement is for the server to make
objects in it accessible before the ref update takes place.  The objects
need to be accessible when hooks run.

I was about to suggest "store objects in it in the repository" instead,
but in the status report we do use a successful "unpack" to mean "made
objects accessible successfully", so it probably is Ok as-is.  At least I
cannot think of a better way to rephrase this part.

> +Report Status
> +-------------
> +
> +After receiving the pack data from the sender, the client sends a

"the client sends" -> "the server sends".

Even though it makes me a bit uneasy to describe these entities "server vs
client" (I'd prefer "sender" vs "receiver"), the entire document is
written based on the assumption that "clients" fetch from or push to "the
server", so let's be consistent.

> +report if 'report-status' capability was sent to the server.
> +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 be either 'ok [refname]' if the

Sorry for asking a language question but "be"?

> +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

Oops.

> diff --git a/Documentation/technical/protocol-capabilities.txt
> b/Documentation/technical/protocol-capabilities.txt
> new file mode 100644
> index 0000000..f4bf986
> --- /dev/null
> +++ b/Documentation/technical/protocol-capabilities.txt
> @@ -0,0 +1,186 @@
> +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 null byte and then a list of space delimited server capabilities.

"null" -> "NUL"

> +Client will then send a space separated list of capabilities it
> +can support. The client SHOULD NOT ask for capabilities the server
> +did not say it supports.
> +
> +Server MUST ignore capabilities it does not understand.  Server MUST
> +NOT ignore capabilities that client requested and server advertised.
> +

I hate to sound like a broken parrot, but ...

 - It is more like "Client will ... it wants enabled", not "it can
   support".

 - Is this really "SHOULD NOT", as opposed to "MUST NOT"?  What are
   examples of plausible justifications for client implementations to
   violate this requirement and ask for a capability that it knows the
   server does not support?

 - Shouldn't server "MUST diagnose and abort" connection when client
   requests a protocol feature (i.e. capability) that it does not
   understand?

When the server advertises a capability it itself does not understand
and the client asked for it, then the client can legitimately ask for
it, and the server is not allowed to ignore it, but it must ignore it, so
it cannot satisfy this requirement.  We should spell out that the server
MUST NOT advertise capabilities it does not understand.

> +thin-pack
> +---------
> +
> +This capability implies that the server can send 'thin' packs, packs

Does it imply or explicitly state?  I think it is the latter.

> +which do not contain base objects; if those base objects are available
> +on client side. Client requests 'thin-pack' capability when it
> +understands how to "thicken" them adding required delta bases making
> +them self contained.

A single pack is sent over the wire and thicking it will create a single
pack self contained, hence "making it self contained".

> +Client MUST NOT request 'thin-pack' capability if it cannot turn thin
> +packs into proper independent packs.

"a think pack into a self contained pack"

> +side-band, side-band-64k
> +------------------------
> +
> +This means that server can send, and client understand multiplexed
> +progress reports and error info interleaved with the packfile itself.

Either "This" -> "This capability", or just begin the sentence with
"Server can send..." like your description of "ofs-delta".

> +delete-refs
> +-----------
> +
> +If the server sends back the 'delete-refs' capability, it means that
> +it is capable of accepting an all-zeroed SHA-1 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 zeroed SHA-1 values
> +to delete references.

You earlier called these "all-zeroed SHA-1 value"s "zero-id"s.  Be
consistent.

> diff --git a/Documentation/technical/protocol-common.txt
> b/Documentation/technical/protocol-common.txt
> new file mode 100644
> index 0000000..2dca642
> --- /dev/null
> +++ b/Documentation/technical/protocol-common.txt
> @@ -0,0 +1,96 @@
> ...
> +A refname is a hierarichal octet string beginning with "refs/" and

"hierarchical"

Re: [PATCH 2/2] pack documentation review updates

[Scott Chacon]

Hey,

On Wed, Nov 11, 2009 at 9:19 AM, Nanako Shiraishi <nanako3@lavabit.com> wrote:
> Update Scott's protocol document according to review comments given by Junio.
>
>  * name of "%00" byte is NUL not null;
>  * requestee sends current values of its refs, not "known values";
>  * consistently use "obj-id";
>  * requestor chooses the capabilities it wants to be in effect;
>  * pack data is sent after negotiation finishes, not just after "some of
>   it is done";
>  * what it has is called "its", not "it's";
>  * consistently use "flush-pkt";
>  * remove redundant "something like";
>  * status report for send-pack is sent from the receiver, not from client;
>  * don't unnecessarily say "SHOULD NOT" when existing servers can satisify
>   "MUST NOT";
>  * only one thin pack is sent and thickened;
>  * an all-zero value is defined as a zero-id, so use it.
>
> Signed-off-by: しらいし ななこ <nanako3@lavabit.com>
> ---

I was _just_ working on this yesterday and had gotten about halfway
through - I'm traveling a lot right now, so I'm a bit behind.  This
looks good, thanks for taking the time Nanako.

Signed-off-by: Scott Chacon <schacon@gmail.com>

Re: [PATCH 2/2] pack documentation review updates

[Junio C Hamano]

Scott Chacon <schacon@gmail.com> writes:

> On Wed, Nov 11, 2009 at 9:19 AM, Nanako Shiraishi <nanako3@lavabit.com> wrote:
>> Update Scott's protocol document according to review comments given by Junio.
>>
>>  * name of "%00" byte is NUL not null;
>> ...
>>  * don't unnecessarily say "SHOULD NOT" when existing servers can satisify
>>   "MUST NOT";
>> ...
>
> I was _just_ working on this yesterday and had gotten about halfway
> through - I'm traveling a lot right now, so I'm a bit behind.  This
> looks good, thanks for taking the time Nanako.
>
> Signed-off-by: Scott Chacon <schacon@gmail.com>

Thanks, both.

I had some doubts about the "SHOULD NOT"/"MUST" on obj-id myself, but if
you are Ok with the change, I wouldn't complain ;-).

Will squash this in.