A 'dogpile' lock, typically used as a component of a larger caching solution.

Allows a single thread to generate an expensive resource while other threads
use the "old" value, until the "new" value is ready.

Dogpile is basically the locking code extracted from the Beaker package,
for simple and generic usage.

WWW: https://bitbucket.org/zzzeek/dogpile.core