138 lines
3.8 KiB
C++
138 lines
3.8 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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namespace folly {
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/**
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* DestructorCheck is a helper class that helps to detect if a tracked object
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* was deleted.
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* This is useful for objects that request callbacks from other components.
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*
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* Classes needing this functionality should:
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* - derive from DestructorCheck
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*
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* Callback context can be extended with an instance of DestructorCheck::Safety
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* object initialized with a reference to the object dereferenced from the
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* callback. Once the callback is invoked, it can use this safety object to
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* check if the object was not deallocated yet before dereferencing it.
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*
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* DestructorCheck does not perform any locking. It is intended to be used
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* only from a single thread.
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*
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* Example:
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*
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* class AsyncFoo : public DestructorCheck {
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* public:
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* ~AsyncFoo();
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* // awesome async code with circuitous deletion paths
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* void async1();
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* void async2();
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* };
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*
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* righteousFunc(AsyncFoo& f) {
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* DestructorCheck::Safety safety(f);
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*
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* f.async1(); // might have deleted f, oh noes
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* if (!safety.destroyed()) {
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* // phew, still there
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* f.async2();
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* }
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* }
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*/
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class DestructorCheck {
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public:
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virtual ~DestructorCheck() {
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rootGuard_.setAllDestroyed();
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}
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class Safety;
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class ForwardLink {
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// These methods are mostly private because an outside caller could violate
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// the integrity of the linked list.
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private:
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void setAllDestroyed() {
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for (auto guard = next_; guard; guard = guard->next_) {
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guard->setDestroyed();
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}
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}
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// This is used to maintain the double-linked list. An intrusive list does
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// not require any heap allocations, like a standard container would. This
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// isolation of next_ in its own class means that the DestructorCheck can
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// easily hold a next_ pointer without needing to hold a prev_ pointer.
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// DestructorCheck never needs a prev_ pointer because it is the head node
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// and this is a special list where the head never moves and never has a
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// previous node.
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Safety* next_{nullptr};
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friend class DestructorCheck;
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friend class Safety;
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};
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// See above example for usage
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class Safety : public ForwardLink {
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public:
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explicit Safety(DestructorCheck& destructorCheck) {
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// Insert this node at the head of the list.
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prev_ = &destructorCheck.rootGuard_;
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next_ = prev_->next_;
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if (next_ != nullptr) {
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next_->prev_ = this;
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}
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prev_->next_ = this;
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}
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~Safety() {
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if (!destroyed()) {
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// Remove this node from the list.
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prev_->next_ = next_;
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if (next_ != nullptr) {
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next_->prev_ = prev_;
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}
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}
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}
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Safety(const Safety&) = delete;
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Safety(Safety&& goner) = delete;
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Safety& operator=(const Safety&) = delete;
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Safety& operator=(Safety&&) = delete;
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bool destroyed() const {
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return prev_ == nullptr;
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}
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private:
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void setDestroyed() {
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prev_ = nullptr;
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}
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// This field is used to maintain the double-linked list. If the root has
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// been destroyed then the field is set to the nullptr sentinel value.
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ForwardLink* prev_;
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friend class ForwardLink;
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};
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private:
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ForwardLink rootGuard_;
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};
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} // namespace folly
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