merge revision(s) 65ae2bb2e045aa8b668d3c30515f5a6cb3eb68ad: [Backport #18902]

	Thread#value: handle threads killed by a fork

	[Bug #18902]

	When a thread is killed because we forked, the `value` if left
	to `Qundef`. Returning it woudl crash the VM.
	---
	 test/ruby/test_thread.rb | 14 ++++++++++++++
	 thread.c                 |  4 ++++
	 2 files changed, 18 insertions(+)

1 files changed, 14 insertions, 0 deletions

diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index ddfeb8b7a8..41cee124be 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -1244,6 +1244,20 @@ q.pop
     assert_predicate(status, :success?, bug9751)
   end if Process.respond_to?(:fork)
 
+  def test_fork_value
+    bug18902 = "[Bug #18902]"
+    th = Thread.start { sleep 2 }
+    begin
+      pid = fork do
+        th.value
+      end
+      _, status = Process.wait2(pid)
+      assert_predicate(status, :success?, bug18902)
+    ensure
+      th.kill
+    end
+  end if Process.respond_to?(:fork)
+
   def test_fork_while_locked
     m = Thread::Mutex.new
     thrs = []