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494 lines
19 KiB
494 lines
19 KiB
Packfile transfer protocols |
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=========================== |
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Git supports transferring data in packfiles over the ssh://, git:// and |
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file:// transports. There exist two sets of protocols, one for pushing |
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data from a client to a server and another for fetching data from a |
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server to a client. All three transports (ssh, git, file) use the same |
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protocol to transfer data. |
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The processes invoked in the canonical Git implementation are 'upload-pack' |
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on the server side and 'fetch-pack' on the client side for fetching data; |
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then 'receive-pack' on the server and 'send-pack' on the client for pushing |
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data. The protocol functions to have a server tell a client what is |
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currently on the server, then for the two to negotiate the smallest amount |
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of data to send in order to fully update one or the other. |
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Transports |
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---------- |
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There are three transports over which the packfile protocol is |
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initiated. The Git transport is a simple, unauthenticated server that |
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takes the command (almost always 'upload-pack', though Git |
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servers can be configured to be globally writable, in which 'receive- |
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pack' initiation is also allowed) with which the client wishes to |
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communicate and executes it and connects it to the requesting |
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process. |
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In the SSH transport, the client just runs the 'upload-pack' |
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or 'receive-pack' process on the server over the SSH protocol and then |
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communicates with that invoked process over the SSH connection. |
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The file:// transport runs the 'upload-pack' or 'receive-pack' |
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process locally and communicates with it over a pipe. |
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Git Transport |
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------------- |
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The Git transport starts off by sending the command and repository |
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on the wire using the pkt-line format, followed by a NUL byte and a |
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hostname parameter, terminated by a NUL byte. |
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0032git-upload-pack /project.git\0host=myserver.com\0 |
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-- |
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git-proto-request = request-command SP pathname NUL [ host-parameter NUL ] |
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request-command = "git-upload-pack" / "git-receive-pack" / |
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"git-upload-archive" ; case sensitive |
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pathname = *( %x01-ff ) ; exclude NUL |
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host-parameter = "host=" hostname [ ":" port ] |
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-- |
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Only host-parameter is allowed in the git-proto-request. Clients |
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MUST NOT attempt to send additional parameters. It is used for the |
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git-daemon name based virtual hosting. See --interpolated-path |
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option to git daemon, with the %H/%CH format characters. |
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Basically what the Git client is doing to connect to an 'upload-pack' |
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process on the server side over the Git protocol is this: |
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$ echo -e -n \ |
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"0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" | |
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nc -v example.com 9418 |
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SSH Transport |
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------------- |
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Initiating the upload-pack or receive-pack processes over SSH is |
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executing the binary on the server via SSH remote execution. |
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It is basically equivalent to running this: |
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$ ssh git.example.com "git-upload-pack '/project.git'" |
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For a server to support Git pushing and pulling for a given user over |
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SSH, that user needs to be able to execute one or both of those |
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commands via the SSH shell that they are provided on login. On some |
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systems, that shell access is limited to only being able to run those |
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two commands, or even just one of them. |
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In an ssh:// format URI, it's absolute in the URI, so the '/' after |
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the host name (or port number) is sent as an argument, which is then |
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read by the remote git-upload-pack exactly as is, so it's effectively |
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an absolute path in the remote filesystem. |
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git clone ssh://user@example.com/project.git |
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| |
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v |
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ssh user@example.com "git-upload-pack '/project.git'" |
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In a "user@host:path" format URI, its relative to the user's home |
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directory, because the Git client will run: |
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git clone user@example.com:project.git |
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v |
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ssh user@example.com "git-upload-pack 'project.git'" |
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The exception is if a '~' is used, in which case |
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we execute it without the leading '/'. |
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ssh://user@example.com/~alice/project.git, |
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v |
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ssh user@example.com "git-upload-pack '~alice/project.git'" |
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A few things to remember here: |
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- The "command name" is spelled with dash (e.g. git-upload-pack), but |
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this can be overridden by the client; |
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- The repository path is always quoted with single quotes. |
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Fetching Data From a Server |
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=========================== |
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When one Git repository wants to get data that a second repository |
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has, the first can 'fetch' from the second. This operation determines |
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what data the server has that the client does not then streams that |
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data down to the client in packfile format. |
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Reference Discovery |
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------------------- |
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When the client initially connects the server will immediately respond |
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with a listing of each reference it has (all branches and tags) along |
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with the object name that each reference currently points to. |
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$ echo -e -n "0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" | |
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nc -v example.com 9418 |
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00887217a7c7e582c46cec22a130adf4b9d7d950fba0 HEAD\0multi_ack thin-pack side-band side-band-64k ofs-delta shallow no-progress include-tag |
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00441d3fcd5ced445d1abc402225c0b8a1299641f497 refs/heads/integration |
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003f7217a7c7e582c46cec22a130adf4b9d7d950fba0 refs/heads/master |
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003cb88d2441cac0977faf98efc80305012112238d9d refs/tags/v0.9 |
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003c525128480b96c89e6418b1e40909bf6c5b2d580f refs/tags/v1.0 |
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003fe92df48743b7bc7d26bcaabfddde0a1e20cae47c refs/tags/v1.0^{} |
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0000 |
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Server SHOULD terminate each non-flush line using LF ("\n") terminator; |
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client MUST NOT complain if there is no terminator. |
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The returned response is a pkt-line stream describing each ref and |
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its current value. The stream MUST be sorted by name according to |
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the C locale ordering. |
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If HEAD is a valid ref, HEAD MUST appear as the first advertised |
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ref. If HEAD is not a valid ref, HEAD MUST NOT appear in the |
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advertisement list at all, but other refs may still appear. |
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The stream MUST include capability declarations behind a NUL on the |
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first ref. The peeled value of a ref (that is "ref^{}") MUST be |
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immediately after the ref itself, if presented. A conforming server |
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MUST peel the ref if it's an annotated tag. |
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---- |
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advertised-refs = (no-refs / list-of-refs) |
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flush-pkt |
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no-refs = PKT-LINE(zero-id SP "capabilities^{}" |
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NUL capability-list LF) |
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list-of-refs = first-ref *other-ref |
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first-ref = PKT-LINE(obj-id SP refname |
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NUL capability-list LF) |
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other-ref = PKT-LINE(other-tip / other-peeled) |
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other-tip = obj-id SP refname LF |
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other-peeled = obj-id SP refname "^{}" LF |
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capability-list = capability *(SP capability) |
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capability = 1*(LC_ALPHA / DIGIT / "-" / "_") |
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LC_ALPHA = %x61-7A |
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---- |
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Server and client MUST use lowercase for obj-id, both MUST treat obj-id |
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as case-insensitive. |
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See protocol-capabilities.txt for a list of allowed server capabilities |
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and descriptions. |
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Packfile Negotiation |
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-------------------- |
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After reference and capabilities discovery, the client can decide |
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to terminate the connection by sending a flush-pkt, telling the |
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server it can now gracefully terminate (as happens with the ls-remote |
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command) or it can enter the negotiation phase, where the client and |
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server determine what the minimal packfile necessary for transport is. |
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Once the client has the initial list of references that the server |
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has, as well as the list of capabilities, it will begin telling the |
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server what objects it wants and what objects it has, so the server |
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can make a packfile that only contains the objects that the client needs. |
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The client will also send a list of the capabilities it wants to be in |
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effect, out of what the server said it could do with the first 'want' line. |
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---- |
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upload-request = want-list |
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have-list |
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compute-end |
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want-list = first-want |
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*additional-want |
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flush-pkt |
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first-want = PKT-LINE("want" SP obj-id SP capability-list LF) |
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additional-want = PKT-LINE("want" SP obj-id LF) |
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have-list = *have-line |
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have-line = PKT-LINE("have" SP obj-id LF) |
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compute-end = flush-pkt / PKT-LINE("done") |
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---- |
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Clients MUST send all the obj-ids it wants from the reference |
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discovery phase as 'want' lines. Clients MUST send at least one |
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'want' command in the request body. Clients MUST NOT mention an |
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obj-id in a 'want' command which did not appear in the response |
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obtained through ref discovery. |
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If client is requesting a shallow clone, it will now send a 'deepen' |
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line with the depth it is requesting. |
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Once all the "want"s (and optional 'deepen') are transferred, |
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clients MUST send a flush-pkt. If the client has all the references |
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on the server, client flushes and disconnects. |
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TODO: shallow/unshallow response and document the deepen command in the ABNF. |
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Now the client will send a list of the obj-ids it has using 'have' |
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lines. In multi_ack mode, the canonical implementation will send up |
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to 32 of these at a time, then will send a flush-pkt. The canonical |
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implementation will skip ahead and send the next 32 immediately, |
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so that there is always a block of 32 "in-flight on the wire" at a |
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time. |
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If the server reads 'have' lines, it then will respond by ACKing any |
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of the obj-ids the client said it had that the server also has. The |
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server will ACK obj-ids differently depending on which ack mode is |
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chosen by the client. |
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In multi_ack mode: |
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* the server will respond with 'ACK obj-id continue' for any common |
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commits. |
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* once the server has found an acceptable common base commit and is |
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ready to make a packfile, it will blindly ACK all 'have' obj-ids |
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back to the client. |
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* the server will then send a 'NACK' and then wait for another response |
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from the client - either a 'done' or another list of 'have' lines. |
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In multi_ack_detailed mode: |
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* the server will differentiate the ACKs where it is signaling |
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that it is ready to send data with 'ACK obj-id ready' lines, and |
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signals the identified common commits with 'ACK obj-id common' lines. |
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Without either multi_ack or multi_ack_detailed: |
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* upload-pack sends "ACK obj-id" on the first common object it finds. |
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After that it says nothing until the client gives it a "done". |
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* upload-pack sends "NAK" on a flush-pkt if no common object |
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has been found yet. If one has been found, and thus an ACK |
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was already sent, it's silent on the flush-pkt. |
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After the client has gotten enough ACK responses that it can determine |
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that the server has enough information to send an efficient packfile |
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(in the canonical implementation, this is determined when it has received |
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enough ACKs that it can color everything left in the --date-order queue |
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as common with the server, or the --date-order queue is empty), or the |
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client determines that it wants to give up (in the canonical implementation, |
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this is determined when the client sends 256 'have' lines without getting |
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any of them ACKed by the server - meaning there is nothing in common and |
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the server should just send all of its objects), then the client will send |
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a 'done' command. The 'done' command signals to the server that the client |
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is ready to receive its packfile data. |
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However, the 256 limit *only* turns on in the canonical client |
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implementation if we have received at least one "ACK %s continue" |
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during a prior round. This helps to ensure that at least one common |
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ancestor is found before we give up entirely. |
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Once the 'done' line is read from the client, the server will either |
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send a final 'ACK obj-id' or it will send a 'NAK'. The server only sends |
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ACK after 'done' if there is at least one common base and multi_ack or |
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multi_ack_detailed is enabled. The server always sends NAK after 'done' |
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if there is no common base found. |
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Then the server will start sending its packfile data. |
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---- |
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server-response = *ack_multi ack / nak |
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ack_multi = PKT-LINE("ACK" SP obj-id ack_status LF) |
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ack_status = "continue" / "common" / "ready" |
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ack = PKT-LINE("ACK SP obj-id LF) |
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nak = PKT-LINE("NAK" LF) |
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---- |
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A simple clone may look like this (with no 'have' lines): |
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---- |
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C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d\0multi_ack \ |
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side-band-64k ofs-delta\n |
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C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n |
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C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n |
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C: 0032want 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n |
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C: 0032want 74730d410fcb6603ace96f1dc55ea6196122532d\n |
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C: 0000 |
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C: 0009done\n |
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S: 0008NAK\n |
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S: [PACKFILE] |
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---- |
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An incremental update (fetch) response might look like this: |
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---- |
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C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d\0multi_ack \ |
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side-band-64k ofs-delta\n |
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C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n |
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C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n |
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C: 0000 |
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C: 0032have 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n |
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C: [30 more have lines] |
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C: 0032have 74730d410fcb6603ace96f1dc55ea6196122532d\n |
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C: 0000 |
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S: 003aACK 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01 continue\n |
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S: 003aACK 74730d410fcb6603ace96f1dc55ea6196122532d continue\n |
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S: 0008NAK\n |
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C: 0009done\n |
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S: 0031ACK 74730d410fcb6603ace96f1dc55ea6196122532d\n |
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S: [PACKFILE] |
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---- |
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Packfile Data |
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------------- |
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Now that the client and server have finished negotiation about what |
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the minimal amount of data that needs to be sent to the client is, the server |
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will construct and send the required data in packfile format. |
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See pack-format.txt for what the packfile itself actually looks like. |
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If 'side-band' or 'side-band-64k' capabilities have been specified by |
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the client, the server will send the packfile data multiplexed. |
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Each packet starting with the packet-line length of the amount of data |
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that follows, followed by a single byte specifying the sideband the |
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following data is coming in on. |
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In 'side-band' mode, it will send up to 999 data bytes plus 1 control |
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code, for a total of up to 1000 bytes in a pkt-line. In 'side-band-64k' |
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mode it will send up to 65519 data bytes plus 1 control code, for a |
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total of up to 65520 bytes in a pkt-line. |
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The sideband byte will be a '1', '2' or a '3'. Sideband '1' will contain |
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packfile data, sideband '2' will be used for progress information that the |
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client will generally print to stderr and sideband '3' is used for error |
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information. |
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If no 'side-band' capability was specified, the server will stream the |
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entire packfile without multiplexing. |
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Pushing Data To a Server |
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======================== |
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Pushing data to a server will invoke the 'receive-pack' process on the |
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server, which will allow the client to tell it which references it should |
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update and then send all the data the server will need for those new |
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references to be complete. Once all the data is received and validated, |
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the server will then update its references to what the client specified. |
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Authentication |
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-------------- |
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The protocol itself contains no authentication mechanisms. That is to be |
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handled by the transport, such as SSH, before the 'receive-pack' process is |
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invoked. If 'receive-pack' is configured over the Git transport, those |
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repositories will be writable by anyone who can access that port (9418) as |
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that transport is unauthenticated. |
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Reference Discovery |
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------------------- |
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The reference discovery phase is done nearly the same way as it is in the |
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fetching protocol. Each reference obj-id and name on the server is sent |
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in packet-line format to the client, followed by a flush-pkt. The only |
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real difference is that the capability listing is different - the only |
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possible values are 'report-status', 'delete-refs' and 'ofs-delta'. |
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Reference Update Request and Packfile Transfer |
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---------------------------------------------- |
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Once the client knows what references the server is at, it can send a |
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list of reference update requests. For each reference on the server |
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that it wants to update, it sends a line listing the obj-id currently on |
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the server, the obj-id the client would like to update it to and the name |
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of the reference. |
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This list is followed by a flush-pkt and then the packfile that should |
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contain all the objects that the server will need to complete the new |
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references. |
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---- |
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update-request = command-list [pack-file] |
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command-list = PKT-LINE(command NUL capability-list LF) |
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*PKT-LINE(command LF) |
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flush-pkt |
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command = create / delete / update |
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create = zero-id SP new-id SP name |
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delete = old-id SP zero-id SP name |
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update = old-id SP new-id SP name |
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old-id = obj-id |
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new-id = obj-id |
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pack-file = "PACK" 28*(OCTET) |
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---- |
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If the receiving end does not support delete-refs, the sending end MUST |
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NOT ask for delete command. |
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The pack-file MUST NOT be sent if the only command used is 'delete'. |
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A pack-file MUST be sent if either create or update command is used, |
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even if the server already has all the necessary objects. In this |
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case the client MUST send an empty pack-file. The only time this |
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is likely to happen is if the client is creating |
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a new branch or a tag that points to an existing obj-id. |
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The server will receive the packfile, unpack it, then validate each |
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reference that is being updated that it hasn't changed while the request |
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was being processed (the obj-id is still the same as the old-id), and |
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it will run any update hooks to make sure that the update is acceptable. |
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If all of that is fine, the server will then update the references. |
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Report Status |
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------------- |
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After receiving the pack data from the sender, the receiver sends a |
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report if 'report-status' capability is in effect. |
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It is a short listing of what happened in that update. It will first |
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list the status of the packfile unpacking as either 'unpack ok' or |
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'unpack [error]'. Then it will list the status for each of the references |
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that it tried to update. Each line is either 'ok [refname]' if the |
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update was successful, or 'ng [refname] [error]' if the update was not. |
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---- |
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report-status = unpack-status |
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1*(command-status) |
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flush-pkt |
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unpack-status = PKT-LINE("unpack" SP unpack-result LF) |
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unpack-result = "ok" / error-msg |
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command-status = command-ok / command-fail |
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command-ok = PKT-LINE("ok" SP refname LF) |
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command-fail = PKT-LINE("ng" SP refname SP error-msg LF) |
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error-msg = 1*(OCTECT) ; where not "ok" |
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---- |
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Updates can be unsuccessful for a number of reasons. The reference can have |
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changed since the reference discovery phase was originally sent, meaning |
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someone pushed in the meantime. The reference being pushed could be a |
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non-fast-forward reference and the update hooks or configuration could be |
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set to not allow that, etc. Also, some references can be updated while others |
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can be rejected. |
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An example client/server communication might look like this: |
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---- |
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S: 007c74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/local\0report-status delete-refs ofs-delta\n |
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S: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe refs/heads/debug\n |
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S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/master\n |
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S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/team\n |
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S: 0000 |
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C: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe 74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/debug\n |
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C: 003e74730d410fcb6603ace96f1dc55ea6196122532d 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a refs/heads/master\n |
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C: 0000 |
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C: [PACKDATA] |
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S: 000eunpack ok\n |
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S: 0018ok refs/heads/debug\n |
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S: 002ang refs/heads/master non-fast-forward\n |
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----
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