Share content-type tables between middleware instances

[ericproulx]

Summary

Grape builds one content-type-aware middleware per API instance, and each one memoized its own derived tables. Both tables are derived from nothing but the content-type registry, so they now live in value-keyed caches on Grape::ContentTypes — Hash keys compare by value, so every API registering the same content types shares one table.

• Grape::ContentTypes::LookupCache / MimeTypesCache follow the PartsCache / PatternCache / CoercerCache idiom already in the codebase. lookup_for moved here from Grape::Middleware::PrecomputedContentTypes.
• Grape::Middleware::Formatter warms mime_types itself. It is the only reader — Error resolves content_type through the lookup, and the versioners read the registry directly — so PrecomputedContentTypes no longer builds that table for every instance that includes it.
• The == DEFAULTS fast path in mime_types_for stays, so Grape::ContentTypes::MIME_TYPES is still returned by identity for a default registry.

Memory

A memory_profiler run over an app mounting 13 APIs held 42 content-type-aware middleware (20 Formatter, 20 Error, 2 Versioner) covering four distinct content-type registrations:

	live objects before → after	held before → after
@content_types_lookup	42 → 4	7,488 B → 1,024 B
@mime_types	41 → 3	6,560 B → 480 B
@content_types	21 → 21	3,360 B (unchanged)

17.4 kB → 4.9 kB, of which the duplicated portion goes from 15.1 kB to 2.7 kB. It scales with the number of mounted APIs — this app held roughly 750 B of duplicate tables per API instance. What is left is the registry Hash itself, which InheritableSetting builds fresh per API instance; deduplicating that belongs in InheritableSetting, not here.

Nothing moves per request: the tables are still warmed at middleware construction and read from an ivar, so dup.call!(env) inherits them exactly as before.

Backward compatibility

Grape::ContentTypes.mime_types_for now returns a frozen Hash shared between callers with equal content types, where it previously returned a fresh mutable one — except for a default registry, where it already returned the frozen MIME_TYPES constant. Each cache also stores a frozen copy of the registry as its key rather than the caller's Hash: the caller's registry stays reachable through middleware.options[:content_types], and mutating a live key would corrupt a cache that is now process-wide. A spec pins that a post-lookup mutation of the registry leaves the cached table alone.

Test plan

• Full RSpec suite passes locally (2657 examples, 0 failures).
• RuboCop clean on the changed files.
• Ran a real mounted app's suite against the patched gem; failures are byte-identical to the same run without this change (live-server integration specs and a grape-swagger parser error).
• CI green.

🤖 Generated with Claude Code

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