Sourced from multi_json's changelog.

1.20.0

• Drop the UnannotatedEmptyCollection Steep diagnostic override by inline-annotating Options::EMPTY_OPTIONS with #: options and routing MultiJson.current_adapter's ||= fallback through that constant. Also enable rubocop's Layout/LeadingCommentSpace AllowSteepAnnotation / AllowRBSInlineAnnotation so future inline #: casts don't need a per-line disable.
• Hoist the block_given? check in MutexStore#fetch outside @mutex.synchronize so the no-block read path runs the check once per call instead of inside the critical section.
• Short-circuit Adapter.blank? on inputs that start with { or [ so the dominant JSON object and array load paths skip the blank-pattern regex entirely.
• Drop the (...) argument forwarding in MultiJson::Options#load_options, dump_options, resolve_options, and invoke_callable in favor of explicit *args so the signatures document that they forward positional arguments to a callable provider and nothing else.
• Collapse the five MultiJson::Concurrency.synchronize_* wrapper methods into a single Concurrency.synchronize(name, &block) keyed by symbol, with the mutex catalog in a MUTEXES hash. The synchronization surface is now one method instead of five and adding a new mutex is a one-line entry.
• Walk the superclass chain manually in Adapter.walk_default_options instead of allocating an ancestors array on every call. The dump/load hot path no longer pays for an iteration over the (mostly module) ancestor list when looking up an adapter's defaults.
• Add a # frozen_string_literal: true magic comment to every Ruby file in lib/ and test/, and flip the Style/FrozenStringLiteralComment rubocop cop to EnforcedStyle: always so future files inherit the freeze.
• Include the original exception's class name in MultiJson::AdapterError.build's formatted message so a downstream consumer reading just the wrapped error can distinguish a LoadError from a validator ArgumentError without having to inspect error.cause separately.
• Mark the five MultiJson::Concurrency mutex constants as private_constant and add matching synchronize_* wrapper methods so callers don't reach into the module's internals.
• DRY up lib/multi_json/deprecated.rb with a small deprecate_alias / deprecate_method DSL so adding or removing a deprecation is a one-liner instead of a 4-line copy of the warn-then-delegate template.
• Hoist a shared Gson::Decoder and Gson::Encoder to handle the empty-options case in the JRuby Gson adapter so the dominant MultiJson.load(json) / MultiJson.dump(obj) call path no longer allocates a fresh decoder/encoder per call.
• Memoize the per-adapter ParseError lookup in MultiJson.parse_error_class_for so the constant resolution runs at most once per adapter, instead of on every MultiJson.load call.
• Walk the superclass chain in Adapter.default_load_options / default_dump_options instead of copying the parent's defaults into the subclass at inheritance time, so a parent calling defaults :load, ... after a subclass has been defined now propagates to the subclass.
• Hold @eviction_mutex around ConcurrentStore#reset's @cache.clear so a JRuby fetcher in the middle of its evict-then-insert sequence cannot interleave with a concurrent reset, mirroring MutexStore#reset's mutex usage.
• Collect the five process-wide mutexes that protect MultiJson's lazy initializers and adapter swap into a new MultiJson::Concurrency module so the library's concurrency surface is documented in one place.
• Replace the per-adapter loaded lambdas in AdapterSelector::ADAPTERS with constant name strings, walked through Object.const_defined? directly. The lookup table is half as large and no longer holds six closure objects whose only job was to call defined?.
• Wrap AdapterSelector#default_adapter_excluding in DEFAULT_ADAPTER_MUTEX so concurrent callers can't both walk the detection chain and double-fire fallback_adapter's one-time warning.
• Raise a clear MultiJson::AdapterError when a custom adapter passed to MultiJson.load does not define a ParseError constant, instead of letting the bare NameError from the rescue clause propagate.
• Drop the duplicate Adapter::EMPTY_OPTIONS constant in favor of the MultiJson::Options::EMPTY_OPTIONS it was shadowing.
• Defer the fast_jsonparser adapter's dump-delegate resolution until the first dump call instead of locking it in at file load time. The adapter no longer inherits from another adapter, so loading multi_json/adapters/fast_jsonparser before oj no longer locks the dump path to whichever adapter happened to be available at that moment.
• Make the lazy default_load_options and default_dump_options initializers in MultiJson::Options thread-safe so two threads accessing an adapter's defaults for the first time can't both run the ||= initializer.
• Make AdapterSelector#default_adapter's lazy ||= initializer thread-safe so two threads racing past the unset @default_adapter ivar can't both run detection (and double-emit the fallback warning in the no-adapters-installed branch).
• Wrap MultiJson.use's OptionsCache.reset and @adapter swap in a mutex so two threads calling use concurrently can't interleave their cache reset and adapter assignment.
• Stop relying on Oj::ParseError's ::SyntaxError ancestor when matching exceptions in Oj::ParseError.===. Walk the exception's ancestor chain by class name instead, so a future Oj release that re-parents its error class doesn't silently break our rescue clauses.
• Improve AdapterSelector#load_adapter's error message for unrecognized adapter specs so it names the expected types and shows the offender's inspect output instead of just to_s.
• Validate the value argument in Adapter.defaults so a non-Hash (e.g. defaults :load, "oops") raises ArgumentError at definition time instead of crashing later in the merge path.
• Skip String#scrub in Adapter.blank? when the input is already valid UTF-8 so the common load path no longer allocates a scrubbed copy on every call.
• Move Oj#load's :symbolize_keys translation into a private translate_load_options helper that drops the redundant :symbolize_keys passthrough alongside :symbol_keys, mirroring the cleanup already in JsonGem#load.
• Skip the per-call hash merge in JsonGem#dump when pretty: true is the only option, passing PRETTY_STATE_PROTOTYPE directly.
• Type-check the Yajl, JrJackson, and Gson adapter wrappers under Steep, with stubbed RBS sigs for the underlying libraries living in sig/external_libraries.rbs.
• Unify LOADED_ADAPTER_DETECTORS and REQUIREMENT_MAP in AdapterSelector into a single ADAPTERS source-of-truth so the require path and detection lambda for each adapter live in one place.
• Extract deprecated public API (decode, encode, engine, engine=, default_engine, with_engine, default_options, default_options=, cached_options, reset_cached_options!) into lib/multi_json/deprecated.rb and drop the matching Style/Documentation, Style/ModuleFunction, and Style/OpenStructUse rubocop opt-outs.
• Validate the action argument in Adapter.defaults so a typo (e.g. defaults :encode, ...) raises ArgumentError at definition time instead of silently producing a no-op default.
• Drop the stale ok_json reference from the fast_jsonparser adapter's docstring.
• Remove the MultiJson::REQUIREMENT_MAP legacy alias; the canonical map already lives on MultiJson::AdapterSelector.
• Drop the dead JrJackson dump arity branch (and its SimpleCov filter). JrJackson 0.4.18+ accepts an options hash as the second argument to Json.dump.
• Drop the redundant options.except(:adapter) allocation in JsonGem#dump; Adapter.merged_dump_options already strips :adapter before the cached hash reaches the adapter.
• Forward all merged options through Yajl#load instead of honoring only :symbolize_keys.
• Tighten Adapter.blank? so it scrubs invalid UTF-8 bytes up front instead of swallowing every ArgumentError from the underlying String calls.
• Guard ConcurrentStore eviction against a TOCTOU race so two concurrent JRuby threads cannot both pass the size check and briefly exceed OptionsCache.max_cache_size.
• Synchronize warn_deprecation_once so concurrent fibers and threads cannot race past the membership check and emit the same one-time deprecation warning twice.
• Stop resetting OptionsCache when MultiJson.use raises so a failed use(:nonexistent) no longer discards the cached entries belonging to the still-active previous adapter.
• Stop mutating cached options in JsonGem#load, mirroring the cache-pollution fix already in place for Oj#load.
• Empty the mutant ignore list. The Gson and JrJackson ignores were dead — those adapters ship in the java-platform gem and aren't present when mutant runs on MRI — and Store#reset's mutex wrapper is now directly tested by stubbing Mutex#synchronize.
• Remove the vendored ok_json adapter. The json gem has been a Ruby default gem since 1.9, so an external pure-Ruby fallback is no longer needed on any supported Ruby version. The last-resort fallback when no other JSON library can be loaded is now json_gem. The ConvertibleHashKeys helper module, which only ok_json used, is also removed.
• Surface parse error locations as error.line and error.column on MultiJson::ParseError, extracted from the underlying adapter's message for adapters that include one (Oj, the json gem).
• Make MultiJson::OptionsCache.max_cache_size configurable so applications that generate many distinct option hashes can raise the cache ceiling at runtime.
• Reorganize lib/multi_json.rb into clearer sections and document why both the module_function and singleton-only definition patterns coexist.
• Restructure OptionsCache backend selection so MRI and JRuby execute the same physical require_relative line, restoring JRuby's line coverage threshold to 100%.

... (truncated)

• f262be6 Work around sigstore-ruby JRuby signing failure
• 8cc1f09 Version 1.20.0
• 2fe0169 Drop the UnannotatedEmptyCollection Steep override
• 6e8ed17 Hoist block_given? out of MutexStore#fetch's critical section
• 9fe1255 Short-circuit Adapter.blank? on JSON object and array sigils
• ccc6425 Replace (...) forwarding with explicit *args in Options
• 1760acb Collapse Concurrency wrappers into a single synchronize(name)
• 05669e6 Walk superclass chain manually in Adapter.walk_default_options
• 2e4a9ac Freeze string literals across the lib and test trees
• dc67097 Pin concurrent-ruby in the development Gemfile
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