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       1                 :            : // Copyright (c) 2009-2010 Satoshi Nakamoto
       2                 :            : // Copyright (c) 2009-2022 The Bitcoin Core developers
       3                 :            : // Distributed under the MIT software license, see the accompanying
       4                 :            : // file COPYING or http://www.opensource.org/licenses/mit-license.php.
       5                 :            : 
       6                 :            : #ifndef BITCOIN_UTIL_EPOCHGUARD_H
       7                 :            : #define BITCOIN_UTIL_EPOCHGUARD_H
       8                 :            : 
       9                 :            : #include <threadsafety.h>
      10                 :            : #include <util/macros.h>
      11                 :            : 
      12                 :            : #include <cassert>
      13                 :            : 
      14                 :            : /** Epoch: RAII-style guard for using epoch-based graph traversal algorithms.
      15                 :            :  *     When walking ancestors or descendants, we generally want to avoid
      16                 :            :  * visiting the same transactions twice. Some traversal algorithms use
      17                 :            :  * std::set (or setEntries) to deduplicate the transaction we visit.
      18                 :            :  * However, use of std::set is algorithmically undesirable because it both
      19                 :            :  * adds an asymptotic factor of O(log n) to traversals cost and triggers O(n)
      20                 :            :  * more dynamic memory allocations.
      21                 :            :  *     In many algorithms we can replace std::set with an internal mempool
      22                 :            :  * counter to track the time (or, "epoch") that we began a traversal, and
      23                 :            :  * check + update a per-transaction epoch for each transaction we look at to
      24                 :            :  * determine if that transaction has not yet been visited during the current
      25                 :            :  * traversal's epoch.
      26                 :            :  *     Algorithms using std::set can be replaced on a one by one basis.
      27                 :            :  * Both techniques are not fundamentally incompatible across the codebase.
      28                 :            :  * Generally speaking, however, the remaining use of std::set for mempool
      29                 :            :  * traversal should be viewed as a TODO for replacement with an epoch based
      30                 :            :  * traversal, rather than a preference for std::set over epochs in that
      31                 :            :  * algorithm.
      32                 :            :  */
      33                 :            : 
      34                 :            : class LOCKABLE Epoch
      35                 :            : {
      36                 :            : private:
      37                 :       6857 :     uint64_t m_raw_epoch = 0;
      38                 :       6857 :     bool m_guarded = false;
      39                 :            : 
      40                 :            : public:
      41                 :      13714 :     Epoch() = default;
      42                 :            :     Epoch(const Epoch&) = delete;
      43                 :            :     Epoch& operator=(const Epoch&) = delete;
      44                 :            :     Epoch(Epoch&&) = delete;
      45                 :            :     Epoch& operator=(Epoch&&) = delete;
      46                 :            :     ~Epoch() = default;
      47                 :            : 
      48                 :            :     bool guarded() const { return m_guarded; }
      49                 :            : 
      50                 :            :     class Marker
      51                 :            :     {
      52                 :            :     private:
      53                 :     407144 :         uint64_t m_marker = 0;
      54                 :            : 
      55                 :            :         // only allow modification via Epoch member functions
      56                 :            :         friend class Epoch;
      57                 :            :         Marker& operator=(const Marker&) = delete;
      58                 :            : 
      59                 :            :     public:
      60                 :     814288 :         Marker() = default;
      61                 :            :         Marker(const Marker&) = default;
      62                 :            :         Marker(Marker&&) = delete;
      63                 :            :         Marker& operator=(Marker&&) = delete;
      64                 :            :         ~Marker() = default;
      65                 :            :     };
      66                 :            : 
      67                 :            :     class SCOPED_LOCKABLE Guard
      68                 :            :     {
      69                 :            :     private:
      70                 :            :         Epoch& m_epoch;
      71                 :            : 
      72                 :            :     public:
      73                 :       1622 :         explicit Guard(Epoch& epoch) EXCLUSIVE_LOCK_FUNCTION(epoch) : m_epoch(epoch)
      74                 :            :         {
      75         [ +  - ]:       1622 :             assert(!m_epoch.m_guarded);
      76                 :       1622 :             ++m_epoch.m_raw_epoch;
      77                 :       1622 :             m_epoch.m_guarded = true;
      78                 :       1622 :         }
      79                 :       1622 :         ~Guard() UNLOCK_FUNCTION()
      80                 :            :         {
      81         [ +  - ]:       1622 :             assert(m_epoch.m_guarded);
      82                 :       1622 :             ++m_epoch.m_raw_epoch; // ensure clear separation between epochs
      83                 :       1622 :             m_epoch.m_guarded = false;
      84                 :       1622 :         }
      85                 :            :     };
      86                 :            : 
      87                 :     462956 :     bool visited(Marker& marker) const EXCLUSIVE_LOCKS_REQUIRED(*this)
      88                 :            :     {
      89         [ +  - ]:     462956 :         assert(m_guarded);
      90         [ +  + ]:     462956 :         if (marker.m_marker < m_raw_epoch) {
      91                 :            :             // marker is from a previous epoch, so this is its first visit
      92                 :     153052 :             marker.m_marker = m_raw_epoch;
      93                 :     153052 :             return false;
      94                 :            :         } else {
      95                 :     309904 :             return true;
      96                 :            :         }
      97                 :     462956 :     }
      98                 :            : };
      99                 :            : 
     100                 :            : #define WITH_FRESH_EPOCH(epoch) const Epoch::Guard UNIQUE_NAME(epoch_guard_)(epoch)
     101                 :            : 
     102                 :            : #endif // BITCOIN_UTIL_EPOCHGUARD_H