add aggressive caching in the db layer

[dustin]

This should be implemented at master.db.cache and should have a configurable global size limit. It should probably also be able to cache process objects (the results of fromSsdict, fromChdict, etc.)

A few details:

• lots of tables are write-only, so those can be cached easily and newly-written records should go into the cache too (write-through)
• dynamic state should be written through a write-coalescing cache
  • scheduler state
  • build request / buildset results

[dustin]

Amar recommends  http://beaker.groovie.org/

[dustin]

beaker doesn't seem to implement what we need, yet brings a lot of baggage to implement things we don't need (web sessions, pluggable backends, etc.)

Options with compatible licenses:

•  http://code.activestate.com/recipes/252524-length-limited-o1-lru-cache-implementation/ (GPL)
•  http://evan.prodromou.name/Software/Python/LRUCache (AFL)
•  http://code.activestate.com/recipes/498245-lru-and-lfu-cache-decorators/ (PSF; By Raymond Hettinger!)

[catlee]

It would be nice if you could configure buildbot to use an external memcached or redis cache.

[Dustin J. Mitchell]

Implement caching

This is conservative in how it caches database dictionaries, but more aggressive with process objects.

• bug1953: cache Change instances cache BuildRequest? instances cache SourceStamps?; add a fakemaster module to help tests cache db results Add @base.cached for DBConnectorComponents add master.caches, so-far unused AsyncLRUCache changes

Fixes #1953.

Changeset: 9e2408449d0fb9d10807ab03bb0cd4cb38a42800