Fix can't modify frozen String error in DebugLocks

actionpack/lib/action_dispatch/middleware/debug_locks.rb

@@ -43,7 +43,7 @@ def call(env)
 
     private
       def render_details(req)
-        threads = ActiveSupport::Dependencies.interlock.raw_state do |threads|
+        threads = ActiveSupport::Dependencies.interlock.raw_state do |raw_threads|
           # The Interlock itself comes to a complete halt as long as this block
           # is executing. That gives us a more consistent picture of everything,
           # but creates a pretty strong Observer Effect.
@@ -53,29 +53,29 @@ def render_details(req)
           # strictly diagnostic tool (to be used when something has gone wrong),
           # and not for any sort of general monitoring.
 
-          threads.each.with_index do |(thread, info), idx|
+          raw_threads.each.with_index do |(thread, info), idx|
             info[:index] = idx
             info[:backtrace] = thread.backtrace
           end
 
-          threads
+          raw_threads
         end
 
         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]}"

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