indexer: check pack file connectivity

When passing `--strict` to `git-unpack-objects`, core git will verify
the pack file that is currently being read. In addition to the typical
checksum verification, this will especially cause it to verify object
connectivity of the received pack file. So it checks, for every received
object, if all the objects it references are either part of the local
object database or part of the pack file. In libgit2, we currently have
no such mechanism, which leaves us unable to verify received pack files
prior to writing them into our local object database.

This commit introduce the concept of `expected_oids` to the indexer.
When pack file verification is turned on by a new flag, the indexer will
try to parse each received object first. If the object has any links to
other objects, it will check if those links are already satisfied by
known objects either part of the object database or objects it has
already seen as part of that pack file. If not, it will add them to the
list of `expected_oids`. Furthermore, the indexer will remove the
current object from the `expected_oids` if it is currently being
expected.

Like this, we are able to verify whether all object links are being
satisfied. As soon as we hit the end of the object stream and have
resolved all objects as well as deltified objects, we assert that
`expected_oids` is in fact empty. This should always be the case for a
valid pack file with full connectivity.

0 files changed, 0 insertions, 0 deletions