Halla Rempt authored Sep 02, 2016 and Wolthera van Hövell committed Sep 02, 2016

Sneakily, the "buffer" variable was made a class variable. Marking
it mutable allows us to change it from a const function, which hides
the fact that this is global data (doubly hidden by there not being
an m_ prefix), which makes the const function thread unsafe.