Update comments

Author: XuehaiPan

include/pybind11/detail/internals.h

@@ -627,19 +627,23 @@ class internals_pp_manager {
     /// Get the pointer-to-pointer for the main interpreter, allocating it if it does not already
     /// exist.  May acquire the GIL. Will never return nullptr.
     std::unique_ptr<InternalsType> *get_pp_for_main_interpreter() {
-        // This function **assumes** that the current thread is running in the main interpreter.
         if (!seen_main_interpreter_) {
+            // The first call to this function **MUST** be from the main interpreter.
+            // Here we **ASSUME** that the current thread is running in the main interpreter.
+            // The caller is responsible for ensuring this.
             std::call_once(seen_main_interpreter_flag_, [&] {
                 gil_scoped_acquire_simple gil;
                 internals_singleton_pp_ = get_or_create_pp_in_state_dict();
                 seen_main_interpreter_ = true;
             });
         }
+        // This is shared between all threads and all interpreters.
         return internals_singleton_pp_;
     }
 
     /// Drop all the references we're currently holding.
     void unref() {
+        // See comment in get_pp() above.
 #ifdef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
         if (get_num_interpreters_seen() > 1) {
             last_istate_tls() = nullptr;
@@ -651,6 +655,7 @@ class internals_pp_manager {
     }
 
     void destroy() {
+        // See comment in get_pp() above.
 #ifdef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
         if (get_num_interpreters_seen() > 1) {
             auto *tstate = get_thread_state_unchecked();
@@ -711,8 +716,8 @@ class internals_pp_manager {
 
     char const *holder_id_ = nullptr;
     on_fetch_function *on_fetch_ = nullptr;
+    // Pointer to the singleton internals for the main interpreter
     std::unique_ptr<InternalsType> *internals_singleton_pp_;
-
     std::once_flag seen_main_interpreter_flag_;
     std::atomic_bool seen_main_interpreter_{false};
 };

include/pybind11/gil_safe_call_once.h

@@ -130,7 +130,7 @@ class gil_safe_call_once_and_store {
             // CPython API calls in the `fn()` call below may release and reacquire the GIL.
             gil_scoped_release gil_rel; // Needed to establish lock ordering.
             detail::with_internals([&](detail::internals &internals) {
-                const void *key = reinterpret_cast<const void *>(this);
+                const void *const key = reinterpret_cast<const void *>(this);
                 auto &storage_map = internals.call_once_storage_map;
                 // There can be multiple threads going through here.
                 detail::call_once_storage<T> *value = nullptr;
@@ -168,7 +168,7 @@ class gil_safe_call_once_and_store {
         T *result = last_storage_ptr_;
         if (!is_last_storage_valid()) {
             detail::with_internals([&](detail::internals &internals) {
-                const void *key = reinterpret_cast<const void *>(this);
+                const void *const key = reinterpret_cast<const void *>(this);
                 auto &storage_map = internals.call_once_storage_map;
                 auto *value = static_cast<detail::call_once_storage<T> *>(storage_map.at(key));
                 result = last_storage_ptr_ = reinterpret_cast<T *>(value->storage);