How do I use 'git filter-branch' to update the SHA of a submodule?

Question

Let's assume that I have two Git repositories, A and B. B is a submodule of A.

For the sake of simplicity, let's also assume that I have a magic function, get_sha_B that, given an SHA commit from A, returns the desired SHA commit of B.

How do I run filter-branch on repo A such that each commit of repo A is rewritten to point to the desired commit of repo B, as returned by get_sha_B?

Thank you!

Answer 1

Figured it out! I'm going to answer my own question here:

I can use git update-index in conjunction with git filter-branch --index-filter.

Here's how to call update-index to modify the commit SHA of a submodule:

git update-index --cacheinfo 160000 88e6a302c42840440f9faac73f27efc6a3e0c1a6 pathto/mysubmodule

As best as I understand it, the 160000 is a magic number in Git used to identify a submodule.