DaemonThreadFactory creating new Java thread factory each time it creates a new thread

Background Info

• Ruby implementation: JRuby
• concurrent-ruby version: >= 1.1.10, earlier versions may be affected as well
• concurrent-ruby-ext installed: no
• concurrent-ruby-edge used: no

Issue

Concurrent::DaemonThreadFactory#newThread calls defaultThreadFactory() each time it generates a new thread, which creates a new Java ThreadFactory object each time. This is clearly at odds with intended ThreadFactory usage and is (potentially) problematic in a few ways:

• It leads to thread names of the form pool-X-thread-Y, where Y is always 1 while X is different for each thread. This is confusing as it suggests that the Concurrent Ruby thread pool is not actually pooling and reusing threads.
• This does not seem to be happening in the Concurrent Ruby code, but if a reference to each ThreadFactory instance is retained somewhere, the result will be a memory leak.
• Threads created by a ThreadFactory instance normally belong to the same Java thread group, which has permissions implications and may cause errors in code that expects all the threads in a Concurrent Ruby thread pool to belong to the same Java thread group.

Proposed Solution

Call defaultThreadFactory() once in Concurrent::DaemonThreadFactory#initialize, store the resulting factory in an instance variable, and reuse it as needed in Concurrent::DaemonThreadFactory#newThread.

Happy to create a PR with the fix if this seems like an acceptable solution.

Search for Concurrent::DaemonThreadFactory and read its initialize and newThread methods, along with the use of defaultThreadFactory(). Reuse one Java ThreadFactory for the instance as proposed, then verify that threads created by the factory use incrementing thread numbers and share the intended thread factory and group.