The proposal is an update to the `-jsem` mechanism for sharing compute resources between Cabal/stack and GHC, which was previously accepted as proposal #540.
In summary #673 fixes an issue with #540: the specified implementation had a potential for runtime crashes due to dependence on the representation of a runtime shared data structure (the system semaphore).
The technical fix is to provide a protocol layer to abstract away from the representation. This is an implementation detail though; the problem with the proposal is that it overspecified the implementation. The fix is to avoid doing that, and introduce a mechanism for versioning the protocol to allow for future changes.
As shepherd I'm recommending acceptance.