Tag peeling peculiarities

I have been working on the pack-refs code [1] and noticed what looks
like a problem with the handling of peeled refs in the packed-refs file
and in the reference cache.  In particular, the peeled versions of tags
outside of refs/tags are *not* stored in packed-refs, but after the
packed-refs file is read it is assumed that such tags cannot be peeled.

It is clear that annotated tags want to live under refs/tags, but there
are some ways to create them in other places (see below).  It is not
clear to me whether the prohibition of tags outside of refs/tags should
be made more airtight or whether the peeling of tags outside of
refs/tags should be fixed.

Example:

~/tmp$ git init foo
Initialized empty Git repository in /home/mhagger/tmp/foo/.git/
~/tmp$ cd foo
~/tmp/foo$ git config user.name 'Lou User'
~/tmp/foo$ git config user.email 'luser@exaple.com'
~/tmp/foo$
~/tmp/foo$ git commit --allow-empty -m "Initial commit"
[master (root-commit) 7e80ddd] Initial commit
~/tmp/foo$ git tag -m footag footag
~/tmp/foo$
~/tmp/foo$ # This is prohibited:
~/tmp/foo$ git update-ref refs/heads/foobranch $(git rev-parse footag)
error: Trying to write non-commit object
d9cdc84dd156ff83799f5226794711fbb2c8273a to branch refs/heads/foobranch
fatal: Cannot update the ref 'refs/heads/foobranch'.
~/tmp/foo$
~/tmp/foo$ # But this is allowed:
~/tmp/foo$ git update-ref refs/remotes/foo/bar $(git rev-parse footag)
~/tmp/foo$
~/tmp/foo$ # So is this:
~/tmp/foo$ git update-ref refs/yak/foobranch $(git rev-parse footag)
~/tmp/foo$
~/tmp/foo$ # Before packing, all tags are available in peel versions:
~/tmp/foo$ git show-ref -d
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/heads/master
d9cdc84dd156ff83799f5226794711fbb2c8273a refs/remotes/foo/bar
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/remotes/foo/bar^{}
d9cdc84dd156ff83799f5226794711fbb2c8273a refs/tags/footag
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/tags/footag^{}
d9cdc84dd156ff83799f5226794711fbb2c8273a refs/yak/foobranch
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/yak/foobranch^{}
~/tmp/foo$
~/tmp/foo$ git pack-refs --all
~/tmp/foo$
~/tmp/foo$ # After packing, tags outside of refs/tags are not peeled any
more:
~/tmp/foo$ git show-ref -d
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/heads/master
d9cdc84dd156ff83799f5226794711fbb2c8273a refs/remotes/foo/bar
d9cdc84dd156ff83799f5226794711fbb2c8273a refs/tags/footag
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/tags/footag^{}
d9cdc84dd156ff83799f5226794711fbb2c8273a refs/yak/foobranch
~/tmp/foo$
~/tmp/foo$ # Peeling the tags via "tag^0" works even after packing:
~/tmp/foo$ git rev-parse refs/yak/foobranch^0
7e80ddd68f0225a0ea221f7cddbacf050be5a265
~/tmp/foo$
~/tmp/foo$ # Here is another way to create a tag outside of refs/tags:
~/tmp/foo$ cd ..
~/tmp$ git clone foo foo-clone
Cloning into 'foo-clone'...
done.
~/tmp$ cd foo-clone
~/tmp/foo-clone$ git config --add remote.origin.fetch
'+refs/tags/*:refs/remotes/origin/tags/*'
~/tmp/foo-clone$ git fetch
From /home/mhagger/tmp/foo
 * [new tag]         footag     -> origin/tags/footag
~/tmp/foo-clone$
~/tmp/foo-clone$ # Again, the tag outside of refs/tags are not peeled
correctly after packing:
~/tmp/foo-clone$ git pack-refs --all
~/tmp/foo-clone$ git show-ref -d
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/heads/master
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/remotes/origin/HEAD
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/remotes/origin/master
d9cdc84dd156ff83799f5226794711fbb2c8273a refs/remotes/origin/tags/footag
d9cdc84dd156ff83799f5226794711fbb2c8273a refs/tags/footag
7e80ddd68f0225a0ea221f7cddbacf050be5a265 refs/tags/footag^{}

Michael

[1] I am trying to fix the problem that peeled refs are lost whenever a
packed reference is deleted.

-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/

On 03/13/2013 03:59 PM, Michael Haggerty wrote:

I have been working on the pack-refs code [1] and noticed what looks
like a problem with the handling of peeled refs in the packed-refs file
and in the reference cache.  In particular, the peeled versions of tags
outside of refs/tags are *not* stored in packed-refs, but after the
packed-refs file is read it is assumed that such tags cannot be peeled.

It is clear that annotated tags want to live under refs/tags, but there
are some ways to create them in other places (see below).  It is not
clear to me whether the prohibition of tags outside of refs/tags should
be made more airtight or whether the peeling of tags outside of
refs/tags should be fixed.

Example:
[...]

I should have mentioned that I already understand the programmatic
*cause* of the behavior that I described in my last email:

* in pack-refs.c:handle_one_ref(), tags that are not in refs/tags are
  explicitly excluded from being peeled.

* in refs.c:read_packed_refs(), if the packed-refs file starts with

      "# pack-refs with: peeled "

  then the REF_KNOWS_PEELED bit is set on *every* reference read from
  the file into the packed refs cache, whether or not it is under
  refs/tags.

* in refs.c:peel_ref(), if a refs cache entry has its REF_KNOWS_PEELED
  bit set but its peeled field is empty, then it is assumed that the
  reference is unpeelable.

What I am *not* clear about is which of these steps is incorrect, and
also whether this problem will have any significant ill effects.

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/