Fix TSan warning in sub-interpreter test (#5729)

include/pybind11/detail/internals.h

@@ -521,8 +521,11 @@ public:
     /// Drop all the references we're currently holding.
     void unref() {
 #ifdef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
-        last_istate_.reset();
-        internals_tls_p_.reset();
+        if (get_num_interpreters_seen() > 1) {
+            last_istate_.reset();
+            internals_tls_p_.reset();
+            return;
+        }
 #endif
         internals_singleton_pp_ = nullptr;
     }

tests/test_embed/test_subinterpreter.cpp

@@ -20,11 +20,21 @@ bool has_state_dict_internals_obj();
 uintptr_t get_details_as_uintptr();
 
 void unsafe_reset_internals_for_single_interpreter() {
-    // unsafe normally, but for subsequent tests, put this back.. we know there are no threads
-    // running and only 1 interpreter
+    // NOTE: This code is NOT SAFE unless the caller guarantees no other threads are alive
+    // NOTE: This code is tied to the precise implementation of the internals holder
+
+    // first, unref the thread local internals
     py::detail::get_internals_pp_manager().unref();
     py::detail::get_local_internals_pp_manager().unref();
+
+    // we know there are no other interpreters, so we can lower this. SUPER DANGEROUS
     py::detail::get_num_interpreters_seen() = 1;
+
+    // now we unref the static global singleton internals
+    py::detail::get_internals_pp_manager().unref();
+    py::detail::get_local_internals_pp_manager().unref();
+
+    // finally, we reload the static global singleton
     py::detail::get_internals();
     py::detail::get_local_internals();
 }