Fix `can't modify frozen String` error in `DebugLocks`

Without this, `DebugLocks` middleware raises an error as follwing:

```
Puma caught this error: can't modify frozen String (RuntimeError)
actionpack/lib/action_dispatch/middleware/debug_locks.rb:97:in `block in render_details'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `each'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `map'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `render_details'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:37:in `call'
railties/lib/rails/engine.rb:524:in `call'
```

2 files changed, 42 insertions, 4 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/debug_locks.rb b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
index c61a941010..541fb25b6c 100644
--- a/actionpack/lib/action_dispatch/middleware/debug_locks.rb
+++ b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
@@ -63,19 +63,19 @@ module ActionDispatch
 
         str = threads.map do |thread, info|
           if info[:exclusive]
-            lock_state = "Exclusive"
+            lock_state = "Exclusive".dup
           elsif info[:sharing] > 0
-            lock_state = "Sharing"
+            lock_state = "Sharing".dup
             lock_state << " x#{info[:sharing]}" if info[:sharing] > 1
           else
-            lock_state = "No lock"
+            lock_state = "No lock".dup
           end
 
           if info[:waiting]
             lock_state << " (yielded share)"
           end
 
-          msg = "Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n"
+          msg = "Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n".dup
 
           if info[:sleeper]
             msg << "  Waiting in #{info[:sleeper]}"
diff --git a/actionpack/test/dispatch/debug_locks_test.rb b/actionpack/test/dispatch/debug_locks_test.rb
new file mode 100644
index 0000000000..d69614bd79
--- /dev/null
+++ b/actionpack/test/dispatch/debug_locks_test.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+
+class DebugLocksTest < ActionDispatch::IntegrationTest
+  setup do
+    build_app
+  end
+
+  def test_render_threads_status
+    thread_ready = Concurrent::CountDownLatch.new
+    test_terminated = Concurrent::CountDownLatch.new
+
+    thread = Thread.new do
+      ActiveSupport::Dependencies.interlock.running do
+        thread_ready.count_down
+        test_terminated.wait
+      end
+    end
+
+    thread_ready.wait
+
+    get "/rails/locks"
+
+    test_terminated.count_down
+
+    assert_match(/Thread.*?Sharing/, @response.body)
+  ensure
+    thread.join
+  end
+
+  private
+    def build_app
+      @app = self.class.build_app do |middleware|
+        middleware.use ActionDispatch::DebugLocks
+      end
+    end
+end