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diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt new file mode 100644 index 0000000..369f91d --- /dev/null +++ b/Documentation/technical/pack-protocol.txt @@ -0,0 +1,494 @@ +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 parameter, 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 it's 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, it's 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 of its objects), then the client will send +a 'done' command. The 'done' command signals to the server that the client +is ready to receive its 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 its 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: 0031ACK 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: 000eunpack ok\n + S: 0018ok refs/heads/debug\n + S: 002ang refs/heads/master non-fast-forward\n +---- |