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Pull #2686

github

Add Grape::Middleware::PrecomputedContentTypes and short-circuit merge_headers

Grape::Middleware::Base#call does dup.call!(env) per request, so the
+||=+ memoization on content_types / mime_types / content_types_indifferent_access
lived on the per-request dup — it was computed fresh every request.

This change introduces +Grape::Middleware::PrecomputedContentTypes+, an
opt-in mixin that warms the three caches on the parent instance at
initialization time. Per-request dups then inherit the cached values
via +dup+'s ivar copy instead of recomputing +with_indifferent_access+
(~1 µs each).

Grape::Middleware::Error, Formatter, and Versioner::Base include the
module. Plain Grape::Middleware::Base subclasses continue to compute
content_types lazily on first access — no behavior change for custom
middlewares. The module advertises intent at the class declaration,
making the warmup a named capability rather than an initializer side
effect.

Also short-circuits Grape::Middleware::Base#merge_headers when no
headers were set, avoiding both the lazy +Grape::Util::Header.new+
allocation and an empty +merge!+ against the response.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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