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diff --git a/Documentation/technical/protocol-capabilities.txt b/Documentation/technical/protocol-capabilities.txt new file mode 100644 index 0000000..9dfade9 --- /dev/null +++ b/Documentation/technical/protocol-capabilities.txt @@ -0,0 +1,380 @@ +Git Protocol Capabilities +========================= + +NOTE: this document describes capabilities for versions 0 and 1 of the pack +protocol. For version 2, please refer to the link:protocol-v2.html[protocol-v2] +doc. + +Servers SHOULD support all capabilities defined in this document. + +On the very first line of the initial server response of either +receive-pack and upload-pack the first reference is followed by +a NUL byte and then a list of space delimited server capabilities. +These allow the server to declare what it can and cannot support +to the client. + +Client will then send a space separated list of capabilities it wants +to be in effect. The client MUST NOT ask for capabilities the server +did not say it supports. + +Server MUST diagnose and abort if capabilities it does not understand +was sent. Server MUST NOT ignore capabilities that client requested +and server advertised. As a consequence of these rules, server MUST +NOT advertise capabilities it does not understand. + +The 'atomic', 'report-status', 'report-status-v2', 'delete-refs', 'quiet', +and 'push-cert' capabilities are sent and recognized by the receive-pack +(push to server) process. + +The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized +by both upload-pack and receive-pack protocols. The 'agent' and 'session-id' +capabilities may optionally be sent in both 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 it's 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. + +multi_ack_detailed +------------------ +This is an extension of multi_ack that permits client to better +understand the server's in-memory state. See pack-protocol.txt, +section "Packfile Negotiation" for more information. + +no-done +------- +This capability should only be used with the smart HTTP protocol. If +multi_ack_detailed and no-done are both present, then the sender is +free to immediately send a pack following its first "ACK obj-id ready" +message. + +Without no-done in the smart HTTP protocol, the server session would +end and the client has to make another trip to send "done" before +the server can send the pack. no-done removes the last round and +thus slightly reduces latency. + +thin-pack +--------- + +A thin pack is one with deltas which reference base objects not +contained within the pack (but are known to exist at the receiving +end). This can reduce the network traffic significantly, but it +requires the receiving end to know how to "thicken" these packs by +adding the missing bases to the pack. + +The upload-pack server advertises 'thin-pack' when it can generate +and send a thin pack. A client requests the 'thin-pack' capability +when it understands how to "thicken" it, notifying the server that +it can receive such a pack. A client MUST NOT request the +'thin-pack' capability if it cannot turn a thin pack into a +self-contained pack. + +Receive-pack, on the other hand, is assumed by default to be able to +handle thin packs, but can ask the client not to use the feature by +advertising the 'no-thin' capability. A client MUST NOT send a thin +pack if the server advertises the 'no-thin' capability. + +The reasons for this asymmetry are historical. The receive-pack +program did not exist until after the invention of thin packs, so +historically the reference implementation of receive-pack always +understood thin packs. Adding 'no-thin' later allowed receive-pack +to disable the feature in a backwards-compatible manner. + + +side-band, side-band-64k +------------------------ + +This capability means that server can send, and client understand multiplexed +progress reports and error info interleaved with the packfile itself. + +These two options are mutually exclusive. A modern client always +favors 'side-band-64k'. + +Either mode indicates that the packfile data will be streamed broken +up into packets of up to either 1000 bytes in the case of 'side_band', +or 65520 bytes in the case of 'side_band_64k'. Each packet is made up +of a leading 4-byte pkt-line length of how much data is in the packet, +followed by a 1-byte stream code, followed by the actual data. + +The stream code can be one of: + + 1 - pack data + 2 - progress messages + 3 - fatal error message just before stream aborts + +The "side-band-64k" capability came about as a way for newer clients +that can handle much larger packets to request packets that are +actually crammed nearly full, while maintaining backward compatibility +for the older clients. + +Further, with side-band and its up to 1000-byte messages, it's actually +999 bytes of payload and 1 byte for the stream code. With side-band-64k, +same deal, you have up to 65519 bytes of data and 1 byte for the stream +code. + +The client MUST send only maximum of one of "side-band" and "side- +band-64k". Server MUST diagnose it as an error if client requests +both. + +ofs-delta +--------- + +Server can send, and client understand PACKv2 with delta referring to +its base by position in pack rather than by an obj-id. That is, they can +send/read OBJ_OFS_DELTA (aka type 6) in a packfile. + +agent +----- + +The server may optionally send a capability of the form `agent=X` to +notify the client that the server is running version `X`. The client may +optionally return its own agent string by responding with an `agent=Y` +capability (but it MUST NOT do so if the server did not mention the +agent capability). The `X` and `Y` strings may contain any printable +ASCII characters except space (i.e., the byte range 32 < x < 127), and +are typically of the form "package/version" (e.g., "git/1.8.3.1"). The +agent strings are purely informative for statistics and debugging +purposes, and MUST NOT be used to programmatically assume the presence +or absence of particular features. + +object-format +------------- + +This capability, which takes a hash algorithm as an argument, indicates +that the server supports the given hash algorithms. It may be sent +multiple times; if so, the first one given is the one used in the ref +advertisement. + +When provided by the client, this indicates that it intends to use the +given hash algorithm to communicate. The algorithm provided must be one +that the server supports. + +If this capability is not provided, it is assumed that the only +supported algorithm is SHA-1. + +symref +------ + +This parameterized capability is used to inform the receiver which symbolic ref +points to which ref; for example, "symref=HEAD:refs/heads/master" tells the +receiver that HEAD points to master. This capability can be repeated to +represent multiple symrefs. + +Servers SHOULD include this capability for the HEAD symref if it is one of the +refs being sent. + +Clients MAY use the parameters from this capability to select the proper initial +branch when cloning a repository. + +shallow +------- + +This capability adds "deepen", "shallow" and "unshallow" commands to +the fetch-pack/upload-pack protocol so clients can request shallow +clones. + +deepen-since +------------ + +This capability adds "deepen-since" command to fetch-pack/upload-pack +protocol so the client can request shallow clones that are cut at a +specific time, instead of depth. Internally it's equivalent of doing +"rev-list --max-age=<timestamp>" on the server side. "deepen-since" +cannot be used with "deepen". + +deepen-not +---------- + +This capability adds "deepen-not" command to fetch-pack/upload-pack +protocol so the client can request shallow clones that are cut at a +specific revision, instead of depth. Internally it's equivalent of +doing "rev-list --not <rev>" on the server side. "deepen-not" +cannot be used with "deepen", but can be used with "deepen-since". + +deepen-relative +--------------- + +If this capability is requested by the client, the semantics of +"deepen" command is changed. The "depth" argument is the depth from +the current shallow boundary, instead of the depth from remote refs. + +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 receive-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. + +report-status-v2 +---------------- + +Capability 'report-status-v2' extends capability 'report-status' by +adding new "option" directives in order to support reference rewritten by +the "proc-receive" hook. The "proc-receive" hook may handle a command +for a pseudo-reference which may create or update a reference with +different name, new-oid, and old-oid. While the capability +'report-status' cannot report for such case. See pack-protocol.txt +for details. + +delete-refs +----------- + +If the server sends back the 'delete-refs' capability, it means that +it is capable of accepting a zero-id value as the target +value of a reference update. It is not sent back by the client, it +simply informs the client that it can be sent zero-id values +to delete references. + +quiet +----- + +If the receive-pack server advertises the 'quiet' capability, it is +capable of silencing human-readable progress output which otherwise may +be shown when processing the received pack. A send-pack client should +respond with the 'quiet' capability to suppress server-side progress +reporting if the local progress reporting is also being suppressed +(e.g., via `push -q`, or if stderr does not go to a tty). + +atomic +------ + +If the server sends the 'atomic' capability it is capable of accepting +atomic pushes. If the pushing client requests this capability, the server +will update the refs in one atomic transaction. Either all refs are +updated or none. + +push-options +------------ + +If the server sends the 'push-options' capability it is able to accept +push options after the update commands have been sent, but before the +packfile is streamed. If the pushing client requests this capability, +the server will pass the options to the pre- and post- receive hooks +that process this push request. + +allow-tip-sha1-in-want +---------------------- + +If the upload-pack server advertises this capability, fetch-pack may +send "want" lines with object names that exist at the server but are not +advertised by upload-pack. For historical reasons, the name of this +capability contains "sha1". Object names are always given using the +object format negotiated through the 'object-format' capability. + +allow-reachable-sha1-in-want +---------------------------- + +If the upload-pack server advertises this capability, fetch-pack may +send "want" lines with object names that exist at the server but are not +advertised by upload-pack. For historical reasons, the name of this +capability contains "sha1". Object names are always given using the +object format negotiated through the 'object-format' capability. + +push-cert=<nonce> +----------------- + +The receive-pack server that advertises this capability is willing +to accept a signed push certificate, and asks the <nonce> to be +included in the push certificate. A send-pack client MUST NOT +send a push-cert packet unless the receive-pack server advertises +this capability. + +filter +------ + +If the upload-pack server advertises the 'filter' capability, +fetch-pack may send "filter" commands to request a partial clone +or partial fetch and request that the server omit various objects +from the packfile. + +session-id=<session id> +----------------------- + +The server may advertise a session ID that can be used to identify this process +across multiple requests. The client may advertise its own session ID back to +the server as well. + +Session IDs should be unique to a given process. They must fit within a +packet-line, and must not contain non-printable or whitespace characters. The +current implementation uses trace2 session IDs (see +link:api-trace2.html[api-trace2] for details), but this may change and users of +the session ID should not rely on this fact. |