JRuby returns nil backtrace for living newborn threads, whereas MRI returns []

[ivoanjo]

Environment

• JRuby: jruby 9.1.15.0 (2.3.3) 2017-12-07 929fde8 Java HotSpot(TM) 64-Bit Server VM 25.151-b12 on 1.8.0_151-b12 +jit [linux-x86_64]
• Kernel: Linux u186024434db159d25c92 4.10.0-38-generic #42~16.04.1-Ubuntu SMP Tue Oct 10 16:32:20 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
• Distro: Ubuntu 16.04.3 LTS

Expected Behavior

Example code:

puts RUBY_DESCRIPTION

puts Thread.new { sleep }.backtrace.inspect

Output on MRI 2.0 to 2.5 (this seems to have changed from 1.9, where it returned nil):

ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux]
[]

(This is of course a race and may sometimes yield non-empty results)

Actual Behavior

Output on JRuby:

jruby 9.1.15.0 (2.3.3) 2017-12-07 929fde8 Java HotSpot(TM) 64-Bit Server VM 25.151-b12 on 1.8.0_151-b12 +jit [linux-x86_64]
nil

[headius]

So the mismatch is that we return nil when there's no trace and MRI returns an empty array, yes?

[headius]

@ivoanjo I know this behavior is a little fiddly, but maybe you can come up with a spec?

[ivoanjo]

So the mismatch is that we return nil when there's no trace and MRI returns an empty array, yes?

Yeah, that's the behavior I'm seeing.

@ivoanjo I know this behavior is a little fiddly, but maybe you can come up with a spec?

May take a few days due to x-mas shopping, but will do 👍

[headius]

Looks like I broke something by making all cases return []:

     [exec]   1) Failure:
     [exec] TestThread#test_backtrace [/home/travis/build/jruby/jruby/test/mri/ruby/test_thread.rb:763]:
     [exec] <nil> expected but was
     [exec] <[]>.

Investigating.