Re: A generalization of git notes from blobs to trees - git metadata?

From: Johan Herland <johan@herland.net>
Date: Feb 7, 2010, 01:36 UTC
Message-ID: <201002070236.12711.johan@herland.net>
In-Reply-To: <2cfc40321002060532g4d22dd4dx403bf312708e1424@mail.gmail.com>

On Saturday 06 February 2010, Jon Seymour wrote:

> git notes is a nice innovation - well done to all those involved.

Thanks.

> Has consideration ever been given to generalizing the concept to allow
> note (or more correctly -  metadata) trees with arbitrary sha1s?

Not sure what you mean here. The note infrastructure allows _any_ SHA1 (not necessarily the SHA1 of an existing Git object) to be bound to a note object.

Furthermore, although we currently assume that all note objects are blobs, someone (who?) has already suggested (as mentioned in the notes TODO list) that a note object could also be a _tree_ object that can be unpacked/read to reveal further "sub-notes". Hence, in addition to having multiple notes refs (e.g. refs/notes/commits:deadbeef, refs/notes/bugs:deadbeef, etc.) to categorize notes, you could also classify notes _after_ having traversed the notes tree (e.g. refs/notes/bugs:deadbeef/fixes, refs/notes/bugs:deadbeef/causes). Note that support for this has not yet been written, and AFAIK it is also uncertain how such a change would affect the different use cases for notes (e.g. how to display them in 'git log')

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> For example, suppose you had reason to cache the distribution that
> resulted from the build of a particular commit, then it'd be nice to
> be able to do this using a notes like mechanism.
> 
>     git metadata import foo-1.1.0 dist ~/foo/dist
> 
> would create a git tree from the contents of ~/foo/dist and then bind
> it to meta item called dist associated with the sha1 corresponding to
> foo-1.1.0

You can do this already today by simply using 'git tag':
	# Prepare an index with the contents of ~/foo/dist
	git tag foo-1.1.0-dist $(git write-tree)

I don't see why you'd need to add a new metadata command.

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> To retrieve the contents of the previous build, you'd do something like
> 
>    get metadata export foo-1.1.0 dist /tmp/foo-1.1.0
> 
> This would find the metadata tree associated with foo-1.1.0, extract
> the dist subtree from that tree and write it to disk at /tmp/foo-1.1.0

Or, if you use a tag instead:
	git --work-tree=/tmp/foo-1.1.0 checkout foo-1.1.0-dist

Show 6 quoted lines

> I've used build outputs as an example here, but really it needn't be
> limited to that. I can see this facility would be useful for any kind
> of annotation or derived result that is more complex than a single
> text blob. Metadata trees in combination with a name spacing
> technique, could be used to store arbitrary metadata created by an
> arbitrary set of tools to arbitrary SHA1 objects.

I still don't see why this provides anything that isn't already supported by either using 'git tag', or by implementing support for notes-as-trees in the notes feature.

...Johan

-- 
Johan Herland, <johan@herland.net>
www.herland.net