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Fix dangling and reassigned Variants
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This commit addresses multiple issues with `Variant`s that point to an `Object`
which is later released, when it's tried to be accessed again.

Formerly, **while running on the debugger the system would check if the instance id was
still valid** to print warnings or return special values. Some cases weren't being
warned about whatsoever.

Also, a newly allocated `Object` could happen to be allocated at the same memory
address of an old one, making cases of use hard to find and having **`Variant`s pointing
to the old one magically reassigned to the new**.

This commit makes the engine realize all these situations **under debugging**
so you can detect and fix them. Running without a debugger attached will still
behave as it always did.

Also the warning messages have been extended and made clearer.

All that said, in the name of performance there's still one possible case of undefined
behavior: in multithreaded scripts there would be a race condition between a thread freeing
an `Object` and another one trying to operate on it. The latter may not realize the
`Object` has been freed soon enough. But that's a case of bad scripting that was never
supported anyway.
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@RandomShaper
RandomShaper committed Apr 23, 2020
1 parent 07c4dc1 commit d904d05
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Showing 8 changed files with 375 additions and 163 deletions.
44 changes: 44 additions & 0 deletions core/object.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,7 @@
#include "core/class_db.h"
#include "core/core_string_names.h"
#include "core/message_queue.h"
#include "core/object_rc.h"
#include "core/os/os.h"
#include "core/print_string.h"
#include "core/resource.h"
Expand Down Expand Up @@ -968,6 +969,37 @@ void Object::cancel_delete() {
_predelete_ok = true;
}

#ifdef DEBUG_ENABLED
ObjectRC *Object::_use_rc() {

// The RC object is lazily created the first time it's requested;
// that way, there's no need to allocate and release it at all if this Object
// is not being referred by any Variant at all.

// Although when dealing with Objects from multiple threads some locking
// mechanism should be used, this at least makes safe the case of first
// assignment.

ObjectRC *rc = nullptr;
ObjectRC *const creating = reinterpret_cast<ObjectRC *>(1);
if (unlikely(_rc.compare_exchange_strong(rc, creating, std::memory_order_acq_rel))) {
// Not created yet
rc = memnew(ObjectRC(this));
_rc.store(rc, std::memory_order_release);
return rc;
}

// Spin-wait until we know it's created (or just return if it's already created)
for (;;) {
if (likely(rc != creating)) {
rc->increment();
return rc;
}
rc = _rc.load(std::memory_order_acquire);
}
}
#endif

void Object::set_script_and_instance(const RefPtr &p_script, ScriptInstance *p_instance) {

//this function is not meant to be used in any of these ways
Expand Down Expand Up @@ -1944,6 +1976,9 @@ Object::Object() {
instance_binding_count = 0;
memset(_script_instance_bindings, 0, sizeof(void *) * MAX_SCRIPT_INSTANCE_BINDINGS);
script_instance = NULL;
#ifdef DEBUG_ENABLED
_rc.store(nullptr, std::memory_order_release);
#endif
#ifdef TOOLS_ENABLED

_edited = false;
Expand All @@ -1957,6 +1992,15 @@ Object::Object() {

Object::~Object() {

#ifdef DEBUG_ENABLED
ObjectRC *rc = _rc.load(std::memory_order_acquire);
if (rc) {
if (rc->invalidate()) {
memfree(rc);
}
}
#endif

if (script_instance)
memdelete(script_instance);
script_instance = NULL;
Expand Down
14 changes: 13 additions & 1 deletion core/object.h
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Original file line number Diff line number Diff line change
Expand Up @@ -34,11 +34,16 @@
#include "core/hash_map.h"
#include "core/list.h"
#include "core/map.h"
#include "core/object_id.h"
#include "core/os/rw_lock.h"
#include "core/set.h"
#include "core/variant.h"
#include "core/vmap.h"

#ifdef DEBUG_ENABLED
#include <atomic> // For ObjectRC*
#endif

#define VARIANT_ARG_LIST const Variant &p_arg1 = Variant(), const Variant &p_arg2 = Variant(), const Variant &p_arg3 = Variant(), const Variant &p_arg4 = Variant(), const Variant &p_arg5 = Variant()
#define VARIANT_ARG_PASS p_arg1, p_arg2, p_arg3, p_arg4, p_arg5
#define VARIANT_ARG_DECLARE const Variant &p_arg1, const Variant &p_arg2, const Variant &p_arg3, const Variant &p_arg4, const Variant &p_arg5
Expand Down Expand Up @@ -397,7 +402,7 @@ public: \
private:

class ScriptInstance;
typedef uint64_t ObjectID;
class ObjectRC;

class Object {
public:
Expand Down Expand Up @@ -477,6 +482,9 @@ class Object {
int _predelete_ok;
Set<Object *> change_receptors;
ObjectID _instance_id;
#ifdef DEBUG_ENABLED
std::atomic<ObjectRC *> _rc;
#endif
bool _predelete();
void _postinitialize();
bool _can_translate;
Expand Down Expand Up @@ -587,6 +595,10 @@ class Object {
return &ptr;
}

#ifdef DEBUG_ENABLED
ObjectRC *_use_rc();
#endif

bool _is_gpl_reversed() const { return false; }

_FORCE_INLINE_ ObjectID get_instance_id() const { return _instance_id; }
Expand Down
38 changes: 38 additions & 0 deletions core/object_id.h
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Original file line number Diff line number Diff line change
@@ -0,0 +1,38 @@
/*************************************************************************/
/* object_rc.h */
/*************************************************************************/
/* This file is part of: */
/* GODOT ENGINE */
/* https://godotengine.org */
/*************************************************************************/
/* Copyright (c) 2007-2020 Juan Linietsky, Ariel Manzur. */
/* Copyright (c) 2014-2020 Godot Engine contributors (cf. AUTHORS.md). */
/* */
/* Permission is hereby granted, free of charge, to any person obtaining */
/* a copy of this software and associated documentation files (the */
/* "Software"), to deal in the Software without restriction, including */
/* without limitation the rights to use, copy, modify, merge, publish, */
/* distribute, sublicense, and/or sell copies of the Software, and to */
/* permit persons to whom the Software is furnished to do so, subject to */
/* the following conditions: */
/* */
/* The above copyright notice and this permission notice shall be */
/* included in all copies or substantial portions of the Software. */
/* */
/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, */
/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF */
/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY */
/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, */
/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE */
/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. */
/*************************************************************************/

#ifndef OBJECTID_H
#define OBJECTID_H

#include "core/int_types.h"

typedef uint64_t ObjectID;

#endif
75 changes: 75 additions & 0 deletions core/object_rc.h
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Original file line number Diff line number Diff line change
@@ -0,0 +1,75 @@
/*************************************************************************/
/* object_rc.h */
/*************************************************************************/
/* This file is part of: */
/* GODOT ENGINE */
/* https://godotengine.org */
/*************************************************************************/
/* Copyright (c) 2007-2020 Juan Linietsky, Ariel Manzur. */
/* Copyright (c) 2014-2020 Godot Engine contributors (cf. AUTHORS.md). */
/* */
/* Permission is hereby granted, free of charge, to any person obtaining */
/* a copy of this software and associated documentation files (the */
/* "Software"), to deal in the Software without restriction, including */
/* without limitation the rights to use, copy, modify, merge, publish, */
/* distribute, sublicense, and/or sell copies of the Software, and to */
/* permit persons to whom the Software is furnished to do so, subject to */
/* the following conditions: */
/* */
/* The above copyright notice and this permission notice shall be */
/* included in all copies or substantial portions of the Software. */
/* */
/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, */
/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF */
/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY */
/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, */
/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE */
/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. */
/*************************************************************************/

#ifndef OBJECTRC_H
#define OBJECTRC_H

#ifdef DEBUG_ENABLED

#include "core/os/memory.h"
#include "core/typedefs.h"

#include <atomic>

class Object;

// Used to track Variants pointing to a non-Reference Object
class ObjectRC {
std::atomic<Object *> _ptr;
std::atomic<uint32_t> _users;

public:
_FORCE_INLINE_ void increment() {
_users.fetch_add(1, std::memory_order_relaxed);
}

_FORCE_INLINE_ bool decrement() {
return _users.fetch_sub(1, std::memory_order_relaxed) == 1;
}

_FORCE_INLINE_ bool invalidate() {
_ptr.store(nullptr, std::memory_order_release);
return decrement();
}

_FORCE_INLINE_ Object *get_ptr() {
return _ptr.load(std::memory_order_acquire);
}

_FORCE_INLINE_ ObjectRC(Object *p_object) {
// 1 (the Object) + 1 (the first user)
_users.store(2, std::memory_order_relaxed);
_ptr.store(p_object, std::memory_order_release);
}
};

#endif

#endif
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