odb: introduce mtime fields for object info requests

There are some use cases where we need to figure out the mtime for
objects. Most importantly, this is the case when we want to prune
unreachable objects. But getting at that data requires users to manually
derive the info either via the loose object's mtime, the packfiles'
mtime or via the ".mtimes" file.

Introduce a new `struct object_info::mtimep` pointer that allows callers
to request an object's mtime. This new field will be used in a
subsequent commit.

Note that the concept of "mtime" is ambiguous: given an object, it may
be stored multiple times in the object database, and each of these
instances may have a different mtime. Disambiguating these mtimes is
nothing that can happen on the generic ODB layer: the caller may search
for the oldest object, the newest object, or even the relation of object
mtimes depending on the specific source they are located in. As such, it
is the responsibility of the caller to disambiguate mtimes.

A consequence of this is that it's most likely incorrect to look up the
mtime via `odb_read_object_info()`, as this interface does not give us
enough information to disambiguate the mtime. Document this accordingly
and tell users to use `odb_for_each_object()` instead.

Even with this gotcha though it's sensible to have this request as part
of the object info, as the mtime is a property of the object storage
format. If we for example had a "black-box" storage backend, we'd still
need to be able to query it for the mtime info in a generic way.

We could introduce a safety mechanism that for example calls `BUG()` in
case we look up the mtime outside of `odb_for_each_object()`. But that
feels somewhat heavy-handed.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions