Re: [PATCH] doc: describe Git bundle format

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 31, 2020, 20:38 UTC
Message-ID: <xmqqk1579xa4.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20200130225818.193825-1-masayasuzuki@google.com>

Masaya Suzuki <masayasuzuki@google.com> writes:

> The bundle format was not documented. Describe the format with ABNF and
> explain the meaning of each part.

Thanks.

Show 25 quoted lines

>
> Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>
> ---
>  Documentation/technical/bundle-format.txt | 40 +++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
>  create mode 100644 Documentation/technical/bundle-format.txt
>
> diff --git a/Documentation/technical/bundle-format.txt b/Documentation/technical/bundle-format.txt
> new file mode 100644
> index 0000000000..dbb80225b5
> --- /dev/null
> +++ b/Documentation/technical/bundle-format.txt
> @@ -0,0 +1,40 @@
> += Git bundle v2 format
> +
> +The Git bundle format is a format that represents both refs and Git objects.
> +
> +== Format
> +
> +We will use ABNF notation to define the Git bundle format. See
> +protocol-common.txt for the details.
> +
> +----
> +bundle    = signature references pack
> +signature = "# v2 git bundle" LF

Good. "signature" is the name used by bundle.c::create_bundle() to call this part.

> +references   = *(prerequisite / ref) LF

This allows prereq and ref can come inter-mixed, but I think we show all prerequisites first before refs.

> +prerequisite = "-" obj-id SP comment LF
> +comment      = *CHAR

Do readers know what CHAR consists of? Anything other than NUL and LF?

> +ref          = obj-id SP refname LF

OK.

"prerequisite" and "ref" are both used in bundle.c::create_bundle(), so calling these parts with these names is consistent with the code. "head" is also a good name for the latter as "git bundle list-heads" is the way the end-users access them from outside.

Show 12 quoted lines

> +
> +pack         = ... ; packfile
> +----
> +
> +== Semantics
> +
> +A Git bundle consists of three parts.
> +
> +*   Prerequisites: Optional list of objects that are not included in the bundle
> +    file. A bundle can reference these prerequisite objects (or it can reference
> +    the objects reachable from the prerequisite objects). The bundle itself
> +    might not contain those objects.

While not incorrect per-se, the above misses the more important points (and defers the description to a later paragraph). It is better to describe what it means to have prereqs upfront.

> +*   References: Mapping of ref names to objects.
> +*   Git objects: Commit, tree, blob, and tags. These are included in the pack
> +    format.
> +

Match the name you used to descibe the parts in the earlier ABNF description, so that the correspondence is clear to the readers. You somehow used "references" to mean both prereqs and heads, but in the above you are describing only "heads" under the label of "references".

Perhaps something like this?

    * "Prerequisites" lists the objects that are NOT included in the
      bundle and the receiver of the bundle MUST already have, in
      order to use the data in the bundle.  The objects stored in
      the bundle may refer to prerequiste objects and anything
      reachable from them and/or expressed as a delta against
      prerequisite objects.

    * "Heads" record the tips of the history graph, iow, what the
      receiver of the bundle CAN "git fetch" from it.

    * "Pack" is the pack data stream "git fetch" would send, if you
      fetch from a repository that has the references recorded in
      the "Heads" above into a repository that has references
      pointing at the objects listed in "Prerequisites" above.

> +If a bundle contains prerequisites, it means the bundle has a thin pack and the
> +bundle alone is not enough for resolving all objects. When you read such
> +bundles, you should have those missing objects beforehand.

With the above rewrite, this paragraph is unneeded.

> +In the bundle format, there can be a comment following a prerequisite obj-id.
> +This is a comment and it has no specific meaning. When you write a bundle, you
> +can put any string here. When you read a bundle, you can ignore this part.

Is it "you can"? At least the last one should be "readers of a bundle MUST ignore the comment", I think.