Ben Gamari authored Aug 29, 2017 and Ben Gamari committed Aug 29, 2017

As pointed out in #13945, some filesystems only allow allow exclusive
locks if the fd being locked was opened for write access. This causes
ghc-pkg to fail as it first attempts to open and exclusively lock its
lockfile in read-only mode to accomodate package databases for which we
lack write permissions (e.g.  global package databases).

Instead, we now try read-write mode first, falling back to read-only
mode if this fails.

Reviewers: austin

Subscribers: rwbarton, thomie

GHC Trac Issues: #13945

Differential Revision: https://phabricator.haskell.org/D3897