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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.
 
 #ifndef ABSL_STRINGS_INTERNAL_CORDZ_HANDLE_H_
 #define ABSL_STRINGS_INTERNAL_CORDZ_HANDLE_H_
 
 #include <atomic>
 #include <vector>
 
 #include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
 // This base class allows multiple types of object (CordzInfo and
 // CordzSampleToken) to exist simultaneously on the delete queue (pointed to by
 // global_dq_tail and traversed using dq_prev_ and dq_next_). The
 // delete queue guarantees that once a profiler creates a CordzSampleToken and
 // has gained visibility into a CordzInfo object, that CordzInfo object will not
 // be deleted prematurely. This allows the profiler to inspect all CordzInfo
 // objects that are alive without needing to hold a global lock.
 class ABSL_DLL CordzHandle {
  public:
   CordzHandle() : CordzHandle(false) {}
 
   bool is_snapshot() const { return is_snapshot_; }
 
   // Returns true if this instance is safe to be deleted because it is either a
   // snapshot, which is always safe to delete, or not included in the global
   // delete queue and thus not included in any snapshot.
   // Callers are responsible for making sure this instance can not be newly
   // discovered by other threads. For example, CordzInfo instances first de-list
   // themselves from the global CordzInfo list before determining if they are
   // safe to be deleted directly.
   // If SafeToDelete returns false, callers MUST use the Delete() method to
   // safely queue CordzHandle instances for deletion.
   bool SafeToDelete() const;
 
   // Deletes the provided instance, or puts it on the delete queue to be deleted
   // once there are no more sample tokens (snapshot) instances potentially
   // referencing the instance. `handle` should not be null.
   static void Delete(CordzHandle* handle);
 
   // Returns the current entries in the delete queue in LIFO order.
   static std::vector<const CordzHandle*> DiagnosticsGetDeleteQueue();
 
   // Returns true if the provided handle is nullptr or guarded by this handle.
   // Since the CordzSnapshot token is itself a CordzHandle, this method will
   // allow tests to check if that token is keeping an arbitrary CordzHandle
   // alive.
   bool DiagnosticsHandleIsSafeToInspect(const CordzHandle* handle) const;
 
   // Returns the current entries in the delete queue, in LIFO order, that are
   // protected by this. CordzHandle objects are only placed on the delete queue
   // after CordzHandle::Delete is called with them as an argument. Only
   // CordzHandle objects that are not also CordzSnapshot objects will be
   // included in the return vector. For each of the handles in the return
   // vector, the earliest that their memory can be freed is when this
   // CordzSnapshot object is deleted.
   std::vector<const CordzHandle*> DiagnosticsGetSafeToInspectDeletedHandles();
 
  protected:
   explicit CordzHandle(bool is_snapshot);
   virtual ~CordzHandle();
 
  private:
   const bool is_snapshot_;
 
   // dq_prev_ and dq_next_ require the global queue mutex to be held.
   // Unfortunately we can't use thread annotations such that the thread safety
   // analysis understands that queue_ and global_queue_ are one and the same.
   CordzHandle* dq_prev_  = nullptr;
   CordzHandle* dq_next_ = nullptr;
 };
 
 class CordzSnapshot : public CordzHandle {
  public:
   CordzSnapshot() : CordzHandle(true) {}
 };
 
 }  // namespace cord_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_CORDZ_HANDLE_H_

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