Colin Walters authored Nov 17, 2016 and Philip Withnall committed Jan 31, 2019

This code was a persistent source of `-fsanitize=thread` errors
when I was trying to use it on OSTree.

The problem is that while I think this code is functionally correct,
we hold a mutex during the writes, but not the reads, and TSAN (IMO
correctly) flags that.

Reading this, I don't see a reason we need a mutex at all.  At the
cost of some small code duplication between posix/win32, we can just
pass the data we need down into each implementation.  This ends up
being notably cleaner I think than the awkward "lock/unlock to
serialize" dance.

(Minor review changes made by Philip Withnall <withnall@endlessm.com>.)

GNOME/glib#1224