Re: [RFC PATCH 13/13] bundle-uri docs: add design notes

> Add a design doc for the bundle-uri protocol extension to go along
> with the packfile-uri extension added in cd8402e0fd8 (Documentation:
> add Packfile URIs design doc, 2020-06-10).
> 
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
>  Documentation/technical/bundle-uri.txt  | 119 ++++++++++++++++++++++++
>  Documentation/technical/protocol-v2.txt |   5 +
>  2 files changed, 124 insertions(+)
>  create mode 100644 Documentation/technical/bundle-uri.txt
> 
> diff --git a/Documentation/technical/bundle-uri.txt b/Documentation/technical/bundle-uri.txt
> new file mode 100644
> index 0000000000..5ae9a15eaf
> --- /dev/null
> +++ b/Documentation/technical/bundle-uri.txt
> @@ -0,0 +1,119 @@
> +Bundle URI Design Notes
> +=======================
> +
> +Protocol
> +--------
> +
> +See `bundle-uri` in the link:protocol-v2.html[protocol-v2]
> +documentation for a discussion of the bundle-uri command, and the
> +expectations of clients and servers.
> +
> +This document is a a more general discussion of how the `bundle-uri`
> +command fits in with the rest of the git ecosystem, its design goals
> +and non-goals, comparison to alternatives etc.
> +
> +Comparison with Packfile URIs
> +-----------------------------
> +
> +There is a similar "Packfile URIs" facility, see the
> +link:packfile-uri.html[packfile-uri] documentation for details.
> +
> +The Packfile URIs facility requires a much closer cooperation between
> +CDN and server than the bundle URI facility.
> +
> +I.e. the server MUST know what objects exist in the packfile URI it's
> +pointing to, as well as its pack checksum. Failure to do so will not
> +only result in a client error (the packfile hash won't match), but
> +even if it got past that would likely result in a corrupt repository
> +with tips pointing to unreachable objects.
> +
> +By comparison the bundle URIs are meant to be a "dumb" solution
> +friendly to e.g. having a weekly cronjob take a snapshot of a git
> +repository, that snapshot being uploaded to a network of FTP mirrors
> +(which may be inconsistent or out of date).
> +
> +The server does not need to know what state the side-channel download
> +is at, because the client will first validate it, and then optionally
> +negotiate with the server using what it discovers there.
> +
> +Using the local `transfer.injectBundleURI` configuration variable (see
> +linkgit:git-config[1]) the `bundle-uri` mechanism doesn't even need
> +the server to support it.