THRIFT-6158: Classify malformed request arguments as protocol errors

[kpumuk]

Malformed request arguments can raise a ProtocolException while a generated Ruby processor is decoding its argument structure. The processor currently handles that exception together with handler failures and returns INTERNAL_ERROR, making malformed protocol data appear to clients as a server-side failure.

This change marks protocol exceptions at the argument-reading boundary and returns an application exception with the PROTOCOL_ERROR type, preserving the original message, method name, and sequence ID. Protocol exceptions raised later by application handlers remain classified as INTERNAL_ERROR.

Benchmarks

The successful read_args path was measured on Ruby 4.0.6/aarch64-linux with one CPU pinned, a 500,000-call warmup, and 15 trials of 5,000,000 calls:

taskset -c 0 bundle exec ruby -e 'require "thrift"; class BenchProcessor; include Thrift::Processor; end; class BenchArgs; def read(_protocol); end; end; class BenchProtocol; def read_message_end; end; end; processor = BenchProcessor.new(Object.new, Logger.new(IO::NULL)); protocol = BenchProtocol.new; 500_000.times { processor.read_args(protocol, BenchArgs) }; samples = 15.times.map do; started = Process.clock_gettime(Process::CLOCK_MONOTONIC); 5_000_000.times { processor.read_args(protocol, BenchArgs) }; Process.clock_gettime(Process::CLOCK_MONOTONIC) - started; end; sorted = samples.sort; p samples; p sorted.fetch(7)'

Current master median: 0.786545 seconds
Proposed change median: 0.780384 seconds
Difference: -0.8%

This synthetic benchmark uses no-op argument and protocol objects to emphasize processor dispatch overhead; it excludes protocol decoding, transport, and network work. The difference is within run-to-run noise and shows no measurable regression.

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[Copilot]

Pull request overview

This PR updates the Ruby processor error handling so malformed request arguments (i.e., protocol decode failures while reading the generated args struct) are reported back to clients as Thrift::ApplicationException::PROTOCOL_ERROR instead of being lumped into INTERNAL_ERROR, while keeping protocol exceptions raised later by application handlers classified as internal errors.

Changes:

• Introduces a private Thrift::Processor::ArgumentProtocolException wrapper raised only at the read_args boundary.
• Updates Thrift::Processor#process to catch argument-decoding protocol failures and return an ApplicationException with type PROTOCOL_ERROR.
• Adds specs covering both malformed-argument classification and handler-raised protocol exceptions remaining INTERNAL_ERROR.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/rb/lib/thrift/processor.rb	Wraps ProtocolException from read_args and maps those to ApplicationException::PROTOCOL_ERROR in process.
lib/rb/spec/processor_spec.rb	Adds coverage to ensure malformed request args produce PROTOCOL_ERROR, while handler-raised protocol exceptions remain INTERNAL_ERROR.

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