Improve LeftRight (#16524)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16524
- Make it exception safe. When an exception happens during write, the old state is recovered.
- Use RAII instead of try/catch to increment counters in readers. This is more readable, and it also makes it work with reader closures that return void, which previously didn't work because the reader return value was stored on the stack.
- Assert there's no reads or writes happening when it's destructed to avoid destruction race conditions
- Explain the algorithm in detail in comments
- Add test cases
Reviewed By: ezyang
Differential Revision: D13866609
fbshipit-source-id: 01306a282a3f555569caa13d8041486f960d00e2
