This introduces djblets.protect, a new module for service protection

capabilities, and specifically djblets.protect.locks.CacheLock, which

is a simple distributed lock utilizing the cache backend. This can help

avoid cache stampede issues, and overall reduce the work required by a

service.
Locks have an expiration, and consumers can block waiting on a lock to

be available or return immediately, giving control over how to best

utilize a lock.
Locks are set by performing an atomic add() with a UUID4. If the value

is added, the lock is acquired. If it already exists, the lock has to

either block waiting or return a result. Waiting supports a timeout and

a time between retries.
Locks are released when they expire or (ideally) when release() is

called.
When using a lock as a context manager, both acquiring and releasing the

lock is handled automatically.
The interface is designed to be largely API-compatible with

threading.Lock and similar lock interfaces, but with more flexibility

useful for distributed lock behavior.
A pattern I expect to be common will be to lock a cache key when

calculating state to store and then writing it, which may be expensive

(for instance, talking to a remote service and storing the result).
For this, cache_memoize() and cache_memoize_iter() have been updated

to work with locks. They now take a lock= argument, which accepts a

CacheLock with the parameters controlling the lock behavior. If

provided, the lock will be acquired if the initial fetch doesn't yield a

value. A second fetch is then attempted (in case it had to wait for

another process to finish), and if it still needs to compute data to

cache, it will do so under the protection of the lock, releasing when

complete.
Locks are entirely optional and not enabled by default for any current

caching behavior, but are something we'll likely want to opt into any

time we're working on caching something that's expensive to generate.

Unit tests pass.