A) How do you catch tarballs that have extra files injected that aren't part of your manifest
B) What does the performance of this look like? Certainly for traditional HDDs this is going to kill performance, but even for SSDs I think verifying a bunch of small files is going to be less efficient than verifying the tarball.
B would just be a normal git checkout, which already validates that all the objects are reachable and git tags (and commits for that matter) can be signed, and since the sha1 hash is signed as well it validates that the entire tree of commits has not been tampered with. So as long you trust git to not lie about what it is writing to disk, you have a valid checkout of that tag.
And if you do expect it to lie, why do you expect tar to not lie about what it is unpacking?
The other method would be having Manifest file with checksum of every file inside the tar and compare that in-flight, could be simple "read from tar, compare to hash, write to disk" (with maybe some tmpfiles for the bigger ones)
