The root resource attempts to cache the generation of URI templates, to

make access to the resource as speedy as possible. However, there are

two problems, which some installs may encounter depending on their use:


The URI template cache can grow unbounded. This is only an issue if

  the root resource is namespaced to, for instance, a team/organization

  name.


Template caching/clearing/generation is not thread-safe. The cache is

  shared across all threads, but is written to/cleared with no regard to

  other threads, which could in theory pose problems.


This addresses both issues through a rewrite of the caching. There's now

a maximum of 50 cached entries. This is a LRU cache. When hitting the

limit, the least-recently used item will be removed from the cache.
It also locks on any cache manipulation. This includes building of URI

templates. When looking up an item not found in the cache, the URI

templates will be built within that same lock. This should avoid more

than one thread ever building the URI templates at a time, and should

ensure every other thread benefits from the cached entries.
Subclasses that need to augment the list of URI templates are now

encouraged to override the new build_uri_templates() method instead of

get_uri_templates() in order to take advantage of the caching behavior

(though legacy code and behavior will not break).

Unit tests passed in Djblets and Review Board.