Thanks Russell for the report, Marc for the initial patch, Carl for the final review, and everyone who contributed to the design discussion.
This is the result of Christopher Medrela's 2013 Summer of Code project. Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian Apolloner, and Alex Gaynor for review notes along the way. Also: Fixes #8579, fixes #3055, fixes #19844.