Summary:
Fixes #9236. My testing indicates that this does *not* lead to problems with
broken pipes and such, but further testing is required. It found
a bug in haddock; I've submitted a pull request upstream.

Reviewers: ekmett, austin

Reviewed By: ekmett, austin

Subscribers: rwbarton, thomie, carter, simonmar

Differential Revision: https://phabricator.haskell.org/D327

GHC Trac Issues: #9236