ruby-grape / grape / 33321195155

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Add Grape::PrecompiledJson for bodies that are already JSON (#2869)

The JSON formatters encode whatever the endpoint returned, so a String holding
pre-rendered JSON -- a blob cached in Redis, a json_agg column read straight
from Postgres, output from another serializer -- comes back encoded a second
time: '{"a":1}' becomes "\"{\\\"a\\\":1}\"".

The workarounds for this were accidents of implementation rather than
documented behaviour: :serializable_hash happens to return a String untouched,
and :binary has no formatter class at all, so it falls through to
DEFAULT_LAMBDA_FORMATTER -- but answers application/octet-stream.

PrecompiledJson marks one body as already rendered:

  get '/cached' do
    body Grape::PrecompiledJson.new(Rails.cache.read('payload'))
  end

Formatter::Json and Formatter::SerializableHash return it verbatim; everything
unwrapped encodes exactly as before. The opt-in is per response rather than per
API on purpose. A formatter that passed every String through could not tell
pre-rendered JSON from an endpoint returning 'hello', and would answer with a
bare hello -- invalid JSON, silently, and not something a type check can catch
since a String is precisely what it is looking for. A marker never has to
guess.

An Array is joined without its members being parsed, which is what makes a
cached collection cheap: N rendered blobs are spliced together rather than
round-tripped. This is load-bearing rather than a convenience -- handing an
encoder an array of wrappers renders them as ordinary objects
([{"value":"..."}]), so the join has to happen here. Array#join calls to_s, so
members may themselves be PrecompiledJson.

For the same reason a wrapper is only valid as the whole body; nesting one
inside a Hash is documented as unsupported, since no encoder knows to unwrap
it. A value that is neither String nor Array raises
Grape::Exceptions::InvalidFormatter, which the formatter middleware already
rescues into a 500, rather than letting a body... (continued)

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