Reproduce JRuby issue #4699

ExceptionHelper.java

import java.util.function.Supplier;

public class ExceptionHelper {
  private final Throwable error;

  public ExceptionHelper(Supplier<Void> errorRaisingSupplier) {
    this.error = errorFromSupplier(errorRaisingSupplier);
  }

  private static Throwable errorFromSupplier(Supplier<Void> errorRaisingSupplier) {
    try {
      errorRaisingSupplier.get();
    } catch (Throwable t) {
      return t;
    }
    throw new RuntimeException("Supplier did not raise an error");
  }

  public void throwCachedError() throws RuntimeException {
    throw new RuntimeException("Wrapper around cached error", this.error);
  }
}

jruby_cause_bug.rb

#!/usr/bin/env jruby

NUM_THREADS = 5

puts RUBY_DESCRIPTION

class CustomError < StandardError; end

class ErrorRaiser
  def initialize
    @java_exception_helper = Java::ExceptionHelper.new { raise CustomError }
  end

  def get
    begin
      begin
        @java_exception_helper.throwCachedError();
      rescue Java::JavaLang::RuntimeException => e
        raise e.cause
      end
    rescue CustomError
      nil
    end
  end
end

error_raiser = ErrorRaiser.new

puts "Starting #{NUM_THREADS} threads"
threads = NUM_THREADS.times.map do
  Thread.new do
    begin
      10_000.times do
        error_raiser.get
      end
      puts "#{Thread.current} finished successfully!\n"
    rescue => e
      puts "#{Thread.current} failed"
      puts "#{e.class}: #{e}\n\t#{e.backtrace.join("\n\t")}\n"
    end
  end
end
puts "Waiting for all threads to finish"

threads.map(&:join)

puts "DONE"

Building and running this results in this:

$ javac ExceptionHelper.java && ./bin/jruby java_cause_bug.rb 
jruby 9.1.13.0-SNAPSHOT (2.3.3) 2017-08-31 23f1bb3 Java HotSpot(TM) 64-Bit Server VM 25.121-b13 on 1.8.0_121-b13 +jit [darwin-x86_64]
Starting 5 threads
Waiting for all threads to finish
#<Thread:0x7d0d15c6> failed
Java::JavaLang::IllegalStateException: Can't overwrite cause with java.lang.RuntimeException: Wrapper around cached error
[ ... snip ... ]
#<Thread:0x4a6b6fc3> failed
Java::JavaLang::IllegalStateException: Can't overwrite cause with java.lang.RuntimeException: Wrapper around cached error
[ ... snip ... ]
#<Thread:0x11823d3c> failed
Java::JavaLang::IllegalStateException: Can't overwrite cause with java.lang.RuntimeException: Wrapper around cached error
[ ... snip ... ]
#<Thread:0x3cc378a6> failed
Java::JavaLang::IllegalStateException: Can't overwrite cause with java.lang.RuntimeException: Wrapper around cached error
[ ... snip ... ]
#<Thread:0x2e879dc9> finished successfully!
DONE