[Bug #21127] Thread deadlock does not display backtraces (#12721)

Previously, Ruby displayed backtraces for each thread on deadlock. However, it has not been shown since Ruby 3.0.
It should display the backtrace for debugging.

Co-authored-by: Jeremy Evans <code@jeremyevans.net>

1 files changed, 32 insertions, 0 deletions

diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index 19f39705bd..7784e0bdae 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -1553,4 +1553,36 @@ q.pop
     assert_equal(true, t.pending_interrupt?(Exception))
     assert_equal(false, t.pending_interrupt?(ArgumentError))
   end
+
+  def test_deadlock_backtrace
+    bug21127 = '[ruby-core:120930] [Bug #21127]'
+
+    expected_stderr = [
+      /-:12:in 'Thread#join': No live threads left. Deadlock\? \(fatal\)\n/,
+      /2 threads, 2 sleeps current:\w+ main thread:\w+\n/,
+      /\* #<Thread:\w+ sleep_forever>\n/,
+      :*,
+      /^\s*-:6:in 'Object#frame_for_deadlock_test_2'/,
+      :*,
+      /\* #<Thread:\w+ -:10 sleep_forever>\n/,
+      :*,
+      /^\s*-:2:in 'Object#frame_for_deadlock_test_1'/,
+      :*,
+    ]
+
+    assert_in_out_err([], <<-INPUT, [], expected_stderr, bug21127)
+      def frame_for_deadlock_test_1
+        yield
+      end
+
+      def frame_for_deadlock_test_2
+        yield
+      end
+
+      q = Thread::Queue.new
+      t = Thread.new { frame_for_deadlock_test_1 { q.pop } }
+
+      frame_for_deadlock_test_2 { t.join }
+    INPUT
+  end
 end