tor-browser

cordz_handle.h (4079B)

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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_