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Make Ref assignment atomic #46302

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Make Ref assignment atomic #46302

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46 changes: 23 additions & 23 deletions core/object/reference.h
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Original file line number Diff line number Diff line change
Expand Up @@ -55,26 +55,33 @@ class Reference : public Object {

template <class T>
class Ref {
T *reference = nullptr;
std::atomic<T *> reference = nullptr;
// Fail loudly rather than silently introduce locks at every Ref assingment. Passes on most architectures.
static_assert(std::atomic<T *>::is_always_lock_free);

void ref(const Ref &p_from) {
if (p_from.reference == reference) {
// Copy new reference to method-local variable immediately.
T *new_reference_ptr = p_from.reference.load(std::memory_order_relaxed);

// Increment the reference count before exchanging to prevent exchange of a deleted reference.
if (new_reference_ptr && !new_reference_ptr->reference()) {
// Reference count was at zero and cannot be incremented. p_from will be imminently deleted by another thread.
return;
}

unref();
T *old_reference_ptr = reference.exchange(new_reference_ptr, std::memory_order_relaxed);

reference = p_from.reference;
if (reference) {
reference->reference();
// old_reference_ptr now contains the previous value of this->reference
if (old_reference_ptr && old_reference_ptr->unreference()) {
memdelete(old_reference_ptr);
}
}

void ref_pointer(T *p_ref) {
ERR_FAIL_COND(!p_ref);

if (p_ref->init_ref()) {
reference = p_ref;
reference.store(p_ref);
}
}

Expand Down Expand Up @@ -144,20 +151,17 @@ class Ref {
void operator=(const Variant &p_variant) {
Object *object = p_variant.get_validated_object();

if (object == reference) {
return;
}

unref();

if (!object) {
unref();
return;
}

T *r = Object::cast_to<T>(object);
if (r && r->reference()) {
reference = r;
if (!r) {
unref();
return;
}
ref(r);
}

template <class T_Other>
Expand Down Expand Up @@ -205,22 +209,18 @@ class Ref {

T *r = Object::cast_to<T>(object);
if (r && r->reference()) {
reference = r;
reference.store(r, std::memory_order_relaxed);
}
}

inline bool is_valid() const { return reference != nullptr; }
inline bool is_null() const { return reference == nullptr; }

void unref() {
//TODO this should be moved to mutexes, since this engine does not really
// do a lot of referencing on references and stuff
// mutexes will avoid more crashes?

if (reference && reference->unreference()) {
memdelete(reference);
T *old_reference_ptr = reference.exchange(nullptr, std::memory_order_relaxed);
if (old_reference_ptr && old_reference_ptr->unreference()) {
memdelete(old_reference_ptr);
}
reference = nullptr;
}

void instance() {
Expand Down