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187 lines
7.1 KiB
187 lines
7.1 KiB
Git Protocol Capabilities |
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========================= |
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Servers SHOULD support all capabilities defined in this document. |
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On the very first line of the initial server response of either |
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receive-pack and upload-pack the first reference is followed by |
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a NUL byte and then a list of space delimited server capabilities. |
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These allow the server to declare what it can and cannot support |
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to the client. |
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Client will then send a space separated list of capabilities it wants |
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to be in effect. The client MUST NOT ask for capabilities the server |
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did not say it supports. |
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Server MUST diagnose and abort if capabilities it does not understand |
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was sent. Server MUST NOT ignore capabilities that client requested |
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and server advertised. As a consequence of these rules, server MUST |
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NOT advertise capabilities it does not understand. |
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The 'report-status' and 'delete-refs' capabilities are sent and |
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recognized by the receive-pack (push to server) process. |
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The 'ofs-delta' capability is sent and recognized by both upload-pack |
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and receive-pack protocols. |
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All other capabilities are only recognized by the upload-pack (fetch |
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from server) process. |
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multi_ack |
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--------- |
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The 'multi_ack' capability allows the server to return "ACK obj-id |
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continue" as soon as it finds a commit that it can use as a common |
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base, between the client's wants and the client's have set. |
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By sending this early, the server can potentially head off the client |
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from walking any further down that particular branch of the client's |
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repository history. The client may still need to walk down other |
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branches, sending have lines for those, until the server has a |
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complete cut across the DAG, or the client has said "done". |
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Without multi_ack, a client sends have lines in --date-order until |
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the server has found a common base. That means the client will send |
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have lines that are already known by the server to be common, because |
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they overlap in time with another branch that the server hasn't found |
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a common base on yet. |
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For example suppose the client has commits in caps that the server |
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doesn't and the server has commits in lower case that the client |
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doesn't, as in the following diagram: |
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+---- u ---------------------- x |
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/ +----- y |
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/ / |
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a -- b -- c -- d -- E -- F |
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\ |
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+--- Q -- R -- S |
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If the client wants x,y and starts out by saying have F,S, the server |
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doesn't know what F,S is. Eventually the client says "have d" and |
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the server sends "ACK d continue" to let the client know to stop |
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walking down that line (so don't send c-b-a), but its not done yet, |
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it needs a base for x. The client keeps going with S-R-Q, until a |
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gets reached, at which point the server has a clear base and it all |
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ends. |
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Without multi_ack the client would have sent that c-b-a chain anyway, |
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interleaved with S-R-Q. |
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thin-pack |
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--------- |
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This capability means that the server can send a 'thin' pack, a pack |
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which does not contain base objects; if those base objects are available |
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on client side. Client requests 'thin-pack' capability when it |
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understands how to "thicken" it by adding required delta bases making |
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it self-contained. |
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Client MUST NOT request 'thin-pack' capability if it cannot turn a thin |
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pack into a self-contained pack. |
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side-band, side-band-64k |
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------------------------ |
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This capability means that server can send, and client understand multiplexed |
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progress reports and error info interleaved with the packfile itself. |
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These two options are mutually exclusive. A modern client always |
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favors 'side-band-64k'. |
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Either mode indicates that the packfile data will be streamed broken |
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up into packets of up to either 1000 bytes in the case of 'side_band', |
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or 65520 bytes in the case of 'side_band_64k'. Each packet is made up |
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of a leading 4-byte pkt-line length of how much data is in the packet, |
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followed by a 1-byte stream code, followed by the actual data. |
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The stream code can be one of: |
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1 - pack data |
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2 - progress messages |
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3 - fatal error message just before stream aborts |
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The "side-band-64k" capability came about as a way for newer clients |
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that can handle much larger packets to request packets that are |
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actually crammed nearly full, while maintaining backward compatibility |
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for the older clients. |
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Further, with side-band and its up to 1000-byte messages, it's actually |
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999 bytes of payload and 1 byte for the stream code. With side-band-64k, |
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same deal, you have up to 65519 bytes of data and 1 byte for the stream |
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code. |
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The client MUST send only maximum of one of "side-band" and "side- |
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band-64k". Server MUST diagnose it as an error if client requests |
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both. |
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ofs-delta |
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--------- |
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Server can send, and client understand PACKv2 with delta refering to |
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its base by position in pack rather than by an obj-id. That is, they can |
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send/read OBJ_OFS_DELTA (aka type 6) in a packfile. |
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shallow |
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------- |
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This capability adds "deepen", "shallow" and "unshallow" commands to |
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the fetch-pack/upload-pack protocol so clients can request shallow |
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clones. |
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no-progress |
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----------- |
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The client was started with "git clone -q" or something, and doesn't |
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want that side band 2. Basically the client just says "I do not |
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wish to receive stream 2 on sideband, so do not send it to me, and if |
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you did, I will drop it on the floor anyway". However, the sideband |
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channel 3 is still used for error responses. |
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include-tag |
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----------- |
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The 'include-tag' capability is about sending annotated tags if we are |
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sending objects they point to. If we pack an object to the client, and |
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a tag object points exactly at that object, we pack the tag object too. |
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In general this allows a client to get all new annotated tags when it |
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fetches a branch, in a single network connection. |
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Clients MAY always send include-tag, hardcoding it into a request when |
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the server advertises this capability. The decision for a client to |
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request include-tag only has to do with the client's desires for tag |
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data, whether or not a server had advertised objects in the |
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refs/tags/* namespace. |
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Servers MUST pack the tags if their referrant is packed and the client |
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has requested include-tags. |
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Clients MUST be prepared for the case where a server has ignored |
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include-tag and has not actually sent tags in the pack. In such |
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cases the client SHOULD issue a subsequent fetch to acquire the tags |
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that include-tag would have otherwise given the client. |
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The server SHOULD send include-tag, if it supports it, regardless |
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of whether or not there are tags available. |
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report-status |
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------------- |
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The upload-pack process can receive a 'report-status' capability, |
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which tells it that the client wants a report of what happened after |
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a packfile upload and reference update. If the pushing client requests |
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this capability, after unpacking and updating references the server |
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will respond with whether the packfile unpacked successfully and if |
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each reference was updated successfully. If any of those were not |
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successful, it will send back an error message. See pack-protocol.txt |
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for example messages. |
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delete-refs |
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----------- |
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If the server sends back the 'delete-refs' capability, it means that |
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it is capable of accepting an zero-id value as the target |
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value of a reference update. It is not sent back by the client, it |
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simply informs the client that it can be sent zero-id values |
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to delete references.
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