Internal protocol

turnip uses an extension of the Git pack protocol for most communication between its servers. The only change is that turnip requests can specify arbitrary named parameters, not just a hostname.

The relevant part of the Git pack protocol’s git-proto-request is represented in ABNF as follows:

git-proto-request = request-command SP pathname NUL [ host-parameter NUL ]
host-parameter = "host=" hostname [ ":" port ]

turnip-proto-request alters it to this:

turnip-proto-request = request-command SP pathname NUL \*( param NUL )
param = param-name "=" param-value
param-name = \*( %x01-3C / %x3E-FF ) ; exclude NUL and =
param-value = \*%x01-FF ; exclude NUL

The only additional parameters implemented today are turnip-stateless-rpc and turnip-advertise-refs, which are used by the smart HTTP server to proxy to the standard pack protocol.

turnip implements one externally-visible extension: a turnip-set-symbolic-ref service that sets a symbolic ref (currently only HEAD is permitted) to a given target. This may be used over the various protocols (git, SSH, smart HTTP), requesting the service in the same way as the existing git-upload-pack and git-receive-pack services:

turnip-set-symbolic-ref-request = set-symbolic-ref-line
set-symbolic-ref-line           = PKT-LINE(refname SP refname)

The server replies with an ACK indicating the symbolic ref name that was changed, or an error message:

turnip-set-symbolic-ref-response = set-symbolic-ref-ack / error-line
set-symbolic-ref-ack             = PKT-LINE("ACK" SP refname)

Internally, Turnip also implements an extension to create repositories: turnip-create-repo. It receives the new repository’s pathname and the same authentication parameters used by the external interface. The authentication details are used to confirm/abort the repository creation on Launchpad.