LGTM, though I only superficially understand this. Does this need a more thorough review from others? And does this warrant an immediate release, or is it acceptable to wait till the next release?

Cheers! Thanks for taking a look! Hmm we can wait a day or two see if @bgamari has any comments, but other than that it fixes the last bits of errors in the GHC testsuite when the new I/O manager is enabled by default.

I don't think it needs an immediate release, we're aiming to enable the I/O manager by default in GHC 9.4, so as long as the code is committed I can bump up the submodule in GHC to kick the CI along.