maxmustermann/godot
1
0
Fork 0
Code Issues Pull requests Projects Releases Wiki Activity

Add a SafeList data structure for future audio server usage.

Browse source
This commit is contained in:
Ellen Poe 2021-08-08 07:32:46 -07:00 committed by Ellen Poe
parent 90a35dac48
commit 460e0ce314
1 changed files with 375 additions and 0 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

375
core/templates/safe_list.h Normal file
View file

@ -0,0 +1,375 @@
/*************************************************************************/
/* safe_list.h */
/*************************************************************************/
/* This file is part of: */
/* GODOT ENGINE */
/* https://godotengine.org */
/*************************************************************************/
/* Copyright (c) 2007-2021 Juan Linietsky, Ariel Manzur. */
/* Copyright (c) 2014-2021 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 SAFE_LIST_H
#define SAFE_LIST_H
#include "core/os/memory.h"
#include "core/typedefs.h"
#include <functional>
#if !defined(NO_THREADS)
#include <atomic>
#include <type_traits>
// Design goals for these classes:
// - Accessing this list with an iterator will never result in a use-after free,
// even if the element being accessed has been logically removed from the list on
// another thread.
// - Logical deletion from the list will not result in deallocation at that time,
// instead the node will be deallocated at a later time when it is safe to do so.
// - No blocking synchronization primitives will be used.
// This is used in very specific areas of the engine where it's critical that these guarantees are held.
template <class T, class A = DefaultAllocator>
class SafeList {
struct SafeListNode {
std::atomic<SafeListNode *> next = nullptr;
// If the node is logically deleted, this pointer will typically point
// to the previous list item in time that was also logically deleted.
std::atomic<SafeListNode *> graveyard_next = nullptr;
std::function<void(T)> deletion_fn = [](T t) { return; };
T val;
};
static_assert(std::atomic<T>::is_always_lock_free);
std::atomic<SafeListNode *> head = nullptr;
std::atomic<SafeListNode *> graveyard_head = nullptr;
std::atomic_uint active_iterator_count = 0;
public:
class Iterator {
friend class SafeList;
SafeListNode *cursor;
SafeList *list;
Iterator(SafeListNode *p_cursor, SafeList *p_list) :
cursor(p_cursor), list(p_list) {
list->active_iterator_count++;
}
public:
Iterator(const Iterator &p_other) :
cursor(p_other.cursor), list(p_other.list) {
list->active_iterator_count++;
}
~Iterator() {
list->active_iterator_count--;
}
public:
T &operator*() {
return cursor->val;
}
Iterator &operator++() {
cursor = cursor->next;
return *this;
}
// These two operators are mostly useful for comparisons to nullptr.
bool operator==(const void *p_other) const {
return cursor == p_other;
}
bool operator!=(const void *p_other) const {
return cursor != p_other;
}
// These two allow easy range-based for loops.
bool operator==(const Iterator &p_other) const {
return cursor == p_other.cursor;
}
bool operator!=(const Iterator &p_other) const {
return cursor != p_other.cursor;
}
};
public:
// Calling this will cause an allocation.
void insert(T p_value) {
SafeListNode *new_node = memnew_allocator(SafeListNode, A);
new_node->val = p_value;
SafeListNode *expected_head = nullptr;
do {
expected_head = head.load();
new_node->next.store(expected_head);
} while (!head.compare_exchange_strong(/* expected= */ expected_head, /* new= */ new_node));
}
Iterator find(T p_value) {
for (Iterator it = begin(); it != end(); ++it) {
if (*it == p_value) {
return it;
}
}
return end();
}
void erase(T p_value, std::function<void(T)> p_deletion_fn) {
Iterator tmp = find(p_value);
erase(tmp, p_deletion_fn);
}
void erase(T p_value) {
Iterator tmp = find(p_value);
erase(tmp, [](T t) { return; });
}
void erase(Iterator &p_iterator, std::function<void(T)> p_deletion_fn) {
p_iterator.cursor->deletion_fn = p_deletion_fn;
erase(p_iterator);
}
void erase(Iterator &p_iterator) {
if (find(p_iterator.cursor->val) == nullptr) {
// Not in the list, nothing to do.
return;
}
// First, remove the node from the list.
while (true) {
Iterator prev = begin();
SafeListNode *expected_head = prev.cursor;
for (; prev != end(); ++prev) {
if (prev.cursor && prev.cursor->next == p_iterator.cursor) {
break;
}
}
if (prev != end()) {
// There exists a node before this.
prev.cursor->next.store(p_iterator.cursor->next.load());
// Done.
break;
} else {
if (head.compare_exchange_strong(/* expected= */ expected_head, /* new= */ p_iterator.cursor->next.load())) {
// Successfully reassigned the head pointer before another thread changed it to something else.
break;
}
// Fall through upon failure, try again.
}
}
// Then queue it for deletion by putting it in the node graveyard.
// Don't touch `next` because an iterator might still be pointing at this node.
SafeListNode *expected_head = nullptr;
do {
expected_head = graveyard_head.load();
p_iterator.cursor->graveyard_next.store(expected_head);
} while (!graveyard_head.compare_exchange_strong(/* expected= */ expected_head, /* new= */ p_iterator.cursor));
}
Iterator begin() {
return Iterator(head.load(), this);
}
Iterator end() {
return Iterator(nullptr, this);
}
// Calling this will cause zero to many deallocations.
void maybe_cleanup() {
SafeListNode *cursor = nullptr;
SafeListNode *new_graveyard_head = nullptr;
do {
// The access order here is theoretically important.
cursor = graveyard_head.load();
if (active_iterator_count.load() != 0) {
// It's not safe to clean up with an active iterator, because that iterator
// could be pointing to an element that we want to delete.
return;
}
// Any iterator created after this point will never point to a deleted node.
// Swap it out with the current graveyard head.
} while (!graveyard_head.compare_exchange_strong(/* expected= */ cursor, /* new= */ new_graveyard_head));
// Our graveyard list is now unreachable by any active iterators,
// detached from the main graveyard head and ready for deletion.
while (cursor) {
SafeListNode *tmp = cursor;
cursor = cursor->graveyard_next;
tmp->deletion_fn(tmp->val);
memdelete_allocator<SafeListNode, A>(tmp);
}
}
};
#else // NO_THREADS
// Effectively the same structure without the atomics. It's probably possible to simplify it but the semantics shouldn't differ greatly.
template <class T, class A = DefaultAllocator>
class SafeList {
struct SafeListNode {
SafeListNode *next = nullptr;
// If the node is logically deleted, this pointer will typically point to the previous list item in time that was also logically deleted.
SafeListNode *graveyard_next = nullptr;
std::function<void(T)> deletion_fn = [](T t) { return; };
T val;
};
SafeListNode *head = nullptr;
SafeListNode *graveyard_head = nullptr;
unsigned int active_iterator_count = 0;
public:
class Iterator {
friend class SafeList;
SafeListNode *cursor;
SafeList *list;
public:
Iterator(SafeListNode *p_cursor, SafeList *p_list) :
cursor(p_cursor), list(p_list) {
list->active_iterator_count++;
}
~Iterator() {
list->active_iterator_count--;
}
T &operator*() {
return cursor->val;
}
Iterator &operator++() {
cursor = cursor->next;
return *this;
}
// These two operators are mostly useful for comparisons to nullptr.
bool operator==(const void *p_other) const {
return cursor == p_other;
}
bool operator!=(const void *p_other) const {
return cursor != p_other;
}
// These two allow easy range-based for loops.
bool operator==(const Iterator &p_other) const {
return cursor == p_other.cursor;
}
bool operator!=(const Iterator &p_other) const {
return cursor != p_other.cursor;
}
};
public:
// Calling this will cause an allocation.
void insert(T p_value) {
SafeListNode *new_node = memnew_allocator(SafeListNode, A);
new_node->val = p_value;
new_node->next = head;
head = new_node;
}
Iterator find(T p_value) {
for (Iterator it = begin(); it != end(); ++it) {
if (*it == p_value) {
return it;
}
}
return end();
}
void erase(T p_value, std::function<void(T)> p_deletion_fn) {
erase(find(p_value), p_deletion_fn);
}
void erase(T p_value) {
erase(find(p_value), [](T t) { return; });
}
void erase(Iterator p_iterator, std::function<void(T)> p_deletion_fn) {
p_iterator.cursor->deletion_fn = p_deletion_fn;
erase(p_iterator);
}
void erase(Iterator p_iterator) {
Iterator prev = begin();
for (; prev != end(); ++prev) {
if (prev.cursor && prev.cursor->next == p_iterator.cursor) {
break;
}
}
if (prev == end()) {
// Not in the list, nothing to do.
return;
}
// First, remove the node from the list.
prev.cursor->next = p_iterator.cursor->next;
// Then queue it for deletion by putting it in the node graveyard. Don't touch `next` because an iterator might still be pointing at this node.
p_iterator.cursor->graveyard_next = graveyard_head;
graveyard_head = p_iterator.cursor;
}
Iterator begin() {
return Iterator(head, this);
}
Iterator end() {
return Iterator(nullptr, this);
}
// Calling this will cause zero to many deallocations.
void maybe_cleanup() {
SafeListNode *cursor = graveyard_head;
if (active_iterator_count != 0) {
// It's not safe to clean up with an active iterator, because that iterator could be pointing to an element that we want to delete.
return;
}
graveyard_head = nullptr;
// Our graveyard list is now unreachable by any active iterators, detached from the main graveyard head and ready for deletion.
while (cursor) {
SafeListNode *tmp = cursor;
cursor = cursor->next;
tmp->deletion_fn(tmp->val);
memdelete_allocator<SafeListNode, A>(tmp);
}
}
};
#endif
#endif // SAFE_LIST_H