Make the pthread key a singleton
Make the pthread key a singleton

Summary: Make the pthread key a singleton

Reviewed By: yfeldblum

Differential Revision: D8663589

fbshipit-source-id: 1e0c328104bd8c3d3c689dc4955a185336e74276

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly