[PATCH 0/4] tag: don't misreport type of tagged objects in errors

This series fixes a very long-standing issue where we'll get confused when we parse a tag whose "type" lies about the type of the target object.

It goes on top of Jeff King's just-submitted [1], and the two compliment one another. See [2] for my feedback about what was left over, which this fixes.

Currently we'll parse tags and note what the "type" claims to be. Say a pointer to a "blob" object that claims to be a "commit" in the envelope.

Then when we we'd try to parse that supposed "commit' for real we'd emit a message like:

	error: object <oid> is a blob, not a commit

Which is reversed, i.e. we'd remember the first "blob" we saw, and then get confused about seeing a "commit" when we did the actual parsing.

This is now fixed in almost all cases by having the one caller of parse_tag() which actually knows the type tell it "yes, I'm sure this is a commit".

We'll then be able to see that we have a non-parsed object as scaffolding, but that it's really a commit, and emit the correct:

	error: object <oid> is a commit not a blob

Which goes along with other errors where the tag object itself yells about being unhappy with the object reference.

I submitted a version of these patches back in early 2021[3], this is significantly slimmed down since then.

At the time Jeff King noted that this approach inherently can't cover all possible scenarios. I.e. sometimes our parsing of the envelope isn't followed up by the "real" parse.

Even in those cases we can "get it right as 4/4 here demonstrates.

But there are going to be cases left where we get it wrong, but they're all cases where we get it wrong now. It's probably not worth fixing the long tail of those issues, but now we'll emit a sensible error on the common case of "log" etc.

1. https://lore.kernel.org/git/Y3a3qcqNG8W3ueeb@coredump.intra.peff.net/
2. https://lore.kernel.org/git/221118.86cz9lgjxu.gmgdl@evledraar.gmail.com/
3. https://lore.kernel.org/git/YGTGgFI19fS7Uv6I@coredump.intra.peff.net/

Ævar Arnfjörð Bjarmason (4):
  object-file.c: free the "t.tag" in check_tag()
  object tests: add test for unexpected objects in tags
  tag: don't misreport type of tagged objects in errors
  tag: don't emit potentially incorrect "object is a X, not a Y"

 blob.c                                 |  11 +-
 blob.h                                 |   3 +
 commit.c                               |  11 +-
 commit.h                               |   2 +
 object-file.c                          |   1 +
 object.c                               |  20 +++-
 object.h                               |   2 +
 t/t3800-mktag.sh                       |   1 +
 t/t5302-pack-index.sh                  |   2 +
 t/t6102-rev-list-unexpected-objects.sh | 146 +++++++++++++++++++++++++
 tag.c                                  |  22 +++-
 tag.h                                  |   2 +
 tree.c                                 |  11 +-
 tree.h                                 |   2 +
 14 files changed, 222 insertions(+), 14 deletions(-)