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https://github.com/git/git.git
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68ee628932
With uploadpack.allowReachableSHA1InWant configuration option set on the server side, "git fetch" can make a request with a "want" line that names an object that has not been advertised (likely to have been obtained out of band or from a submodule pointer). Only objects reachable from the branch tips, i.e. the union of advertised branches and branches hidden by transfer.hideRefs, will be processed. Note that there is an associated cost of having to walk back the history to check the reachability. This feature can be used when obtaining the content of a certain commit, for which the sha1 is known, without the need of cloning the whole repository, especially if a shallow fetch is used. Useful cases are e.g. repositories containing large files in the history, fetching only the needed data for a submodule checkout, when sharing a sha1 without telling which exact branch it belongs to and in Gerrit, if you think in terms of commits instead of change numbers. (The Gerrit case has already been solved through allowTipSHA1InWant as every Gerrit change has a ref.) Signed-off-by: Fredrik Medley <fredrik.medley@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
277 lines
11 KiB
Text
277 lines
11 KiB
Text
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 'atomic', 'report-status', 'delete-refs', 'quiet', and 'push-cert'
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capabilities are sent and recognized by the receive-pack (push to server)
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process.
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The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
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by both upload-pack and receive-pack protocols. The 'agent' capability
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may optionally be sent in both 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 it's 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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multi_ack_detailed
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------------------
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This is an extension of multi_ack that permits client to better
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understand the server's in-memory state. See pack-protocol.txt,
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section "Packfile Negotiation" for more information.
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no-done
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-------
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This capability should only be used with the smart HTTP protocol. If
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multi_ack_detailed and no-done are both present, then the sender is
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free to immediately send a pack following its first "ACK obj-id ready"
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message.
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Without no-done in the smart HTTP protocol, the server session would
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end and the client has to make another trip to send "done" before
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the server can send the pack. no-done removes the last round and
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thus slightly reduces latency.
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thin-pack
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---------
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A thin pack is one with deltas which reference base objects not
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contained within the pack (but are known to exist at the receiving
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end). This can reduce the network traffic significantly, but it
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requires the receiving end to know how to "thicken" these packs by
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adding the missing bases to the pack.
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The upload-pack server advertises 'thin-pack' when it can generate
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and send a thin pack. A client requests the 'thin-pack' capability
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when it understands how to "thicken" it, notifying the server that
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it can receive such a pack. A client MUST NOT request the
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'thin-pack' capability if it cannot turn a thin pack into a
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self-contained pack.
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Receive-pack, on the other hand, is assumed by default to be able to
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handle thin packs, but can ask the client not to use the feature by
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advertising the 'no-thin' capability. A client MUST NOT send a thin
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pack if the server advertises the 'no-thin' capability.
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The reasons for this asymmetry are historical. The receive-pack
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program did not exist until after the invention of thin packs, so
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historically the reference implementation of receive-pack always
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understood thin packs. Adding 'no-thin' later allowed receive-pack
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to disable the feature in a backwards-compatible manner.
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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 referring 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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agent
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-----
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The server may optionally send a capability of the form `agent=X` to
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notify the client that the server is running version `X`. The client may
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optionally return its own agent string by responding with an `agent=Y`
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capability (but it MUST NOT do so if the server did not mention the
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agent capability). The `X` and `Y` strings may contain any printable
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ASCII characters except space (i.e., the byte range 32 < x < 127), and
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are typically of the form "package/version" (e.g., "git/1.8.3.1"). The
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agent strings are purely informative for statistics and debugging
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purposes, and MUST NOT be used to programmatically assume the presence
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or absence of particular features.
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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 receive-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 a 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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quiet
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-----
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If the receive-pack server advertises the 'quiet' capability, it is
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capable of silencing human-readable progress output which otherwise may
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be shown when processing the received pack. A send-pack client should
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respond with the 'quiet' capability to suppress server-side progress
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reporting if the local progress reporting is also being suppressed
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(e.g., via `push -q`, or if stderr does not go to a tty).
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atomic
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------
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If the server sends the 'atomic' capability it is capable of accepting
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atomic pushes. If the pushing client requests this capability, the server
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will update the refs in one atomic transaction. Either all refs are
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updated or none.
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allow-tip-sha1-in-want
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----------------------
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If the upload-pack server advertises this capability, fetch-pack may
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send "want" lines with SHA-1s that exist at the server but are not
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advertised by upload-pack.
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allow-reachable-sha1-in-want
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----------------------------
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If the upload-pack server advertises this capability, fetch-pack may
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send "want" lines with SHA-1s that exist at the server but are not
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advertised by upload-pack.
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push-cert=<nonce>
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-----------------
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The receive-pack server that advertises this capability is willing
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to accept a signed push certificate, and asks the <nonce> to be
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included in the push certificate. A send-pack client MUST NOT
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send a push-cert packet unless the receive-pack server advertises
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this capability.
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