I am familiar with how Git creates SHA1 hashes for files (blobs), but not how they are created for tag objects. I assume they are, if I create an annotated tag, but what is the recipe? And how might I replicate it outside of Git (e.g., in Perl or Python)?
The pattern is basically:
sha1("tag " + datasize + "\0" + data)
Where data is the output of git cat-file. One can produce this by piping that output to git-hash-object like so:
git cat-file tag v0.30 | git hash-object -t tag --stdin
And the equivalent a perl one-liner is:
git cat-file tag v0.30 | perl -MDigest::SHA1 -E '$/=undef;$_=<>;say Digest::SHA1->new->add("tag ".length()."\0".$_)->hex digest'
It seems that one can do this same thing with any of the types objects simply by replacing "tag " with the proper object name: "blob ", "tree ", or "commit ".