sonic-pi

ThreadSafe::AtomicReferenceCacheBackend#internal_replace

Internal versions of the insertion methods, each a
little more complicated than the last. All have
the same basic structure:
 1. If table uninitialized, create
 2. If bin empty, try to CAS new node
 3. If bin stale, use new table
 4. Lock and validate; if valid, scan and add or update

The others interweave other checks and/or alternative actions:
 * Plain +get_and_set+ checks for and performs resize after insertion.
 * compute_if_absent prescans for mapping without lock (and fails to add
   if present), which also makes pre-emptive resize checks worthwhile.

Someday when details settle down a bit more, it might be worth
some factoring to reduce sprawl.

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