Windows build is broken in a new way.

When I run validate I end up with sh.exe processes that consume a full CPU forever.   See the process log below.

Note that these are not GHC processes: they are shells!  I have no conception of what they are doing.

Any ideas, or things I can to do gather more evidence?

This is an up to date HEAD, with some small changes to GHC.  I suppose I can try a completely clean HEAD, but I can’t see how my changes could make the shell loop.

Simon