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Re: A few questions regarding git annotated tags

Hey Jeff!

On Sun, Jan 6, 2019 at 7:50 AM Jeff King <peff@xxxxxxxx> wrote:
> On Sat, Jan 05, 2019 at 04:50:30PM +0100, Michal Novotny wrote:
> > I could potentially make it so that I tag subtrees instead of commits
> > and then derive the needed information from these subtree tags. This
> > could be useful if I have multiple rpm packages in different subtrees
> > of the same repo. I could then tag the subtree where the rpm package
> > is placed.
> >
> > This could bring some simplification into the code but as far as I
> > know, you cannot easily checkout a tree tag, which is something a
> > packager should be able to do easily: to checkout a state of repo when
> > a certain subpackage was tagged. This is the first question. Can you
> > e.g. do:
> >
> > git tag somename HEAD:
> >
> > and then do something similar to
> >
> > git checkout somename
> >
> > which would restore the repository or at least the respective subtree
> > of it into the state when "somename" tag was created?
> No, there's no easy way to check out a bare tree (and in fact, HEAD is
> forbidden to point to a non-commit).
> You could hack around that by making a new commit that wraps the tree,
> like:
>   commit=$(echo 'wrap foo package' | git commit-tree HEAD:foo)
>   git tag foo-1.2.3 $commit
> There are also useful things to do with the tag of the bare tree. E.g.,
> export it via git-archive, diff it, "git checkout -p" changes from it,
> etc. But actually creating a working tree state from it is awkward:
>   # move to being on an "unborn" branch foo
>   git checkout --orphan foo
>   # load the desired tree state; "-u" will update the working tree
>   # files
>   git read-tree -m -u foo-1.2.3
>   # if we were to commit now, it would become the root commit of the
>   # "foo" branch, with no parents. That would make it pretty useful for
>   # things like merging between tags.
>   git commit -m 'kind of weird'
> So it seems kind of awkward and useless.  I'm not sure I totally
> understand your problem space, but if you can have actual commits with a
> logical progression (i.e., where the parent links actually mean
> something), I think Git's tools will be more useful.
> > Right now, I am putting a package name directly into tag name so I
> > know what tags belong to what package based on that. And I am using
> > normal annotated tags. This works quite well, I would say, but at one
> > point I need to use shared state to move the discovered package name
> > from one part of the code to another so that the other part can work
> > with the correct subset of the available annotated tags. I wouldn't
> > need to do that if I could derive the correct tag subset based just on
> > the path to the subtree where a package is placed.
> I'm not sure I understand this bit. Even if you tag a subtree, like:
>   git tag foo-1.2.3 HEAD:foo
> then that tree doesn't "know" that it was originally at the path "foo".
> You'd have to tag the root tree, and then know to look in the "foo"
> subtree from there. At which point you might as well tag the commit that
> contains that root tree. Whether it happens to touch the "foo" path or
> not, it represents a particular state.
> > Alternative approach to creating the tree tags would be to store the
> > path information into annotated tag message, which I could do. But is
> > there a relatively simple way to filter tags based on their message
> > content? Can I put the information into some other part of tag than
> > name or the message so that it can be easily filtered?
> I don't think there's an easy way to show only tags matching a pattern.
> You could do something like:
>   git tag -m 'path: foo' foo
>   git for-each-ref --format='%(refname:strip=2) %(subject)' refs/tags/ |
>   grep 'path: foo' |
>   awk '{print $1}'
> to grep their subjects (or body, if you want to make the grep stage a
> little more clever). Obviously that is not really a structured lookup,
> but if you control the tag contents, it might be OK.
> In commit messages there's a concept of machine-readable trailers, like
> "Signed-off-by", etc, and even some tools for displaying those. But
> there's not currently any support for parsing them out of tag objects.
> I sort of answered your questions literally, but TBH I'm still not
> entirely sure what you're trying to accomplish. So hopefully it was
> useful, but feel free to follow up with more questions. ;)

No, I think you summed it up pretty well for me.

I would like to ask one more question, which is now unrelated.

But I will probably ask in a new thread.

Thank you!

> -Peff