merge revision(s) eacedcfe44a0ae22bf54ddb7df193c48d4c857c6: [Backport #19105]

	mutex: Raise a ThreadError when detecting a fiber deadlock (#6680)

	[Bug #19105]

	If no fiber scheduler is registered and the fiber that
	owns the lock and the one that try to acquire it
	both belong to the same thread, we're in a deadlock case.

	Co-authored-by: Jean Boussier <byroot@ruby-lang.org>
	---
	 test/fiber/test_mutex.rb | 22 +++++++++++++++++++++-
	 thread_sync.c            |  4 ++++
	 2 files changed, 25 insertions(+), 1 deletion(-)

1 files changed, 21 insertions, 1 deletions

diff --git a/test/fiber/test_mutex.rb b/test/fiber/test_mutex.rb
index b0655f06a5..449c49f38b 100644
--- a/test/fiber/test_mutex.rb
+++ b/test/fiber/test_mutex.rb
@@ -194,7 +194,7 @@ class TestFiberMutex < Test::Unit::TestCase
   end
 
   def test_mutex_deadlock
-    error_pattern = /No live threads left. Deadlock\?/
+    error_pattern = /lock already owned by another fiber/
 
     assert_in_out_err %W[-I#{__dir__} -], <<-RUBY, ['in synchronize'], error_pattern, success: false
     require 'scheduler'
@@ -217,4 +217,24 @@ class TestFiberMutex < Test::Unit::TestCase
     thread.join
     RUBY
   end
+
+  def test_mutex_fiber_deadlock_no_scheduler
+    thr = Thread.new do
+      loop do
+        sleep 1
+      end
+    end
+
+    mutex = Mutex.new
+    mutex.synchronize do
+      error = assert_raise ThreadError do
+        Fiber.new do
+          mutex.lock
+        end.resume
+      end
+      assert_includes error.message, "deadlock; lock already owned by another fiber belonging to the same thread"
+    end
+  ensure
+    thr&.kill&.join
+  end
 end