ruby-grape / grape / 26721434661

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Skip per-request instrumentation work when no subscribers are listening

Grape fires up to six ActiveSupport::Notifications.instrument calls per
request (endpoint run / render / run_validators / run_filters and
format_response). Each builds a payload Hash and enters the notification
machinery on every request, even when nothing is subscribed to the event.

Guard each emission behind ActiveSupport::Notifications.notifier.listening?
via a small private instrument_<event> method on the owning class. With no
subscriber the block runs directly, skipping the payload Hash and the
instrument dispatch; the block is forwarded anonymously so nothing extra is
allocated unless a subscriber is present. Behaviour is unchanged when a
subscriber exists - same events, same payloads.

Saves one Hash allocation per emitted event (-6 allocations/request on an
endpoint with before/after filters and params), ~+4.6% median throughput
under YJIT on that endpoint; scales with events emitted per request.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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