Environment

See Issue #3729 for full info, but I don't think anything in the environment is notable, rather the test is what's notable.

Expected Behavior

Attempting to work around a thread safety bug in rspec-mocks, I wrote a spec which tries to count the number of times a callback was called.

it 'notifies the callback of each result' do
  count = 0                         # this is a Fixnum
  mutex = Mutex.new
  @thing.search('mime-type:message/rfc822') do
    # synchronise on a common object so that we can be sure the count is updated atomically.
    # I know I could have used the atomic gem, I'm just trying to get something to work right now.
    mutex.synchronize { count += 1 }   # a Fixnum + another Fixnum = a Fixnum
  end
  expect(count).to be(24)
end

This Thing#search method internally performs work in a thread pool, so the callback gets called from multiple threads, possibly all simultaneously. I think my code is thread-safe, so I expect this to pass, and about 4 out of 5 tries it does.

Actual Behavior

About 1 in 5 tries, I get this error:

2. Thing#search when a callback is provided notifies the callback of each result
   Failure/Error: Unable to find matching line from backtrace

 TypeError:
   could not coerce Fixnum to interface java.util.Set

Now, it hasn't given me a stack trace this time, and I'm not even dealing with a set anywhere in my code, so I have no idea how this is even possible, but if it only happens sometimes, then I guess it's a thread safety bug in JRuby itself.