Zejun Wu authored Nov 22, 2018 and Ben Gamari committed Nov 22, 2018

In D5211, we use `withMVar` to guard writes to the same segment, this
is unsafe to be duplicated. It can lead to deadlock if it is only run
partially and `putMVar` is not called after `takeMVar`.

Test Plan:
  ./validate

We used to see deadlock when building stackage without this fix, and it
no longer happens.

Reviewers: simonmar, bgamari

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5349