ruby-grape / grape / 35321998280

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Collapse the lazy configuration value tree into one path-based Value (#2950)

A remountable API reads `configuration` while its own class body runs,
long before anybody mounts it and supplies one, so the read has to be
recorded and replayed once per mount.

To remember what had been read, `Grape::Util::Lazy` eagerly mirrored the
whole `with:` Hash as a tree of wrapper objects, and `[]` mutated the
child it returned, stamping it with the path it was reached by before
handing back that shared node. Reading was writing, and it was only safe
because every read re-stamped the path.

The tree carried its own weight in oddities: `@value_hash` held an Array
in `ValueArray`, `@value` was dead in both enumerables (`super` set it,
both override `evaluate`), and the whole structure existed so that
`evaluate` could answer a deep `HashWithIndifferentAccess` - which is
what `Hash#with_indifferent_access` already answers, identically, down to
the classes at every depth.

A read is now one immutable `Lazy::Value` holding the configuration and
the path of keys walked into it. `[]` answers a fresh Value one key
deeper, `evaluate` walks the path, and `evaluate_from` walks that same
path through the configuration a mount supplied. `ValueHash`,
`ValueArray` and `ValueEnumerable` are deleted, and
`EndpointConfiguration` carries the single conversion.

This fixes nested configuration reads. `configuration[:db][:host]`
raised `NoMethodError` while the class body ran, because a key missing
from a base instance's empty configuration yielded a plain `Value`,
which had no `[]`. Reading a key nested in a Hash or an Array was
impossible in a remountable API.

Building a configuration is 1.5x faster, and `evaluate` - which runs per
request through `configuration` inside an endpoint - is 13x faster
(1.79 us to 136 ns).

The old specs asserted which wrapper class `[]` answered, so they are
replaced by specs on what a read answers. Two properties the tree
provided incidentally are now ... (continued)

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