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 // Copyright 2019 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //      https://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
 #include "absl/base/config.h"
 #include "absl/strings/internal/cordz_handle.h"
 #include "absl/strings/internal/cordz_info.h"
 
 #ifndef ABSL_STRINGS_INTERNAL_CORDZ_SAMPLE_TOKEN_H_
 #define ABSL_STRINGS_INTERNAL_CORDZ_SAMPLE_TOKEN_H_
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
 // The existence of a CordzSampleToken guarantees that a reader can traverse the
 // global_cordz_infos_head linked-list without needing to hold a mutex. When a
 // CordzSampleToken exists, all CordzInfo objects that would be destroyed are
 // instead appended to a deletion queue. When the CordzSampleToken is destroyed,
 // it will also clean up any of these CordzInfo objects.
 //
 // E.g., ST are CordzSampleToken objects and CH are CordzHandle objects.
 //   ST1 <- CH1 <- CH2 <- ST2 <- CH3 <- global_delete_queue_tail
 //
 // This list tracks that CH1 and CH2 were created after ST1, so the thread
 // holding ST1 might have a reference to CH1, CH2, ST2, and CH3. However, ST2
 // was created later, so the thread holding the ST2 token cannot have a
 // reference to ST1, CH1, or CH2. If ST1 is cleaned up first, that thread will
 // delete ST1, CH1, and CH2. If instead ST2 is cleaned up first, that thread
 // will only delete ST2.
 //
 // If ST1 is cleaned up first, the new list will be:
 //   ST2 <- CH3 <- global_delete_queue_tail
 //
 // If ST2 is cleaned up first, the new list will be:
 //   ST1 <- CH1 <- CH2 <- CH3 <- global_delete_queue_tail
 //
 // All new CordzHandle objects are appended to the list, so if a new thread
 // comes along before either ST1 or ST2 are cleaned up, the new list will be:
 //   ST1 <- CH1 <- CH2 <- ST2 <- CH3 <- ST3 <- global_delete_queue_tail
 //
 // A thread must hold the global_delete_queue_mu mutex whenever it's altering
 // this list.
 //
 // It is safe for thread that holds a CordzSampleToken to read
 // global_cordz_infos at any time since the objects it is able to retrieve will
 // not be deleted while the CordzSampleToken exists.
 class CordzSampleToken : public CordzSnapshot {
  public:
   class Iterator {
    public:
     using iterator_category = std::input_iterator_tag;
     using value_type = const CordzInfo&;
     using difference_type = ptrdiff_t;
     using pointer = const CordzInfo*;
     using reference = value_type;
 
     Iterator() = default;
 
     Iterator& operator++();
     Iterator operator++(int);
     friend bool operator==(const Iterator& lhs, const Iterator& rhs);
     friend bool operator!=(const Iterator& lhs, const Iterator& rhs);
     reference operator*() const;
     pointer operator->() const;
 
    private:
     friend class CordzSampleToken;
     explicit Iterator(const CordzSampleToken* token);
 
     const CordzSampleToken* token_ = nullptr;
     pointer current_ = nullptr;
   };
 
   CordzSampleToken() = default;
   CordzSampleToken(const CordzSampleToken&) = delete;
   CordzSampleToken& operator=(const CordzSampleToken&) = delete;
 
   Iterator begin() { return Iterator(this); }
   Iterator end() { return Iterator(); }
 };
 
 }  // namespace cord_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_CORDZ_SAMPLE_TOKEN_H_

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