Line data    Source code

       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        4860 :     uint64_t m_raw_epoch = 0;
      38        4860 :     bool m_guarded = false;
      39             : 
      40             : public:
      41        9720 :     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        5047 :         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       10094 :         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           0 :         explicit Guard(Epoch& epoch) EXCLUSIVE_LOCK_FUNCTION(epoch) : m_epoch(epoch)
      74             :         {
      75           0 :             assert(!m_epoch.m_guarded);
      76           0 :             ++m_epoch.m_raw_epoch;
      77           0 :             m_epoch.m_guarded = true;
      78           0 :         }
      79           0 :         ~Guard() UNLOCK_FUNCTION()
      80             :         {
      81           0 :             assert(m_epoch.m_guarded);
      82           0 :             ++m_epoch.m_raw_epoch; // ensure clear separation between epochs
      83           0 :             m_epoch.m_guarded = false;
      84           0 :         }
      85             :     };
      86             : 
      87           0 :     bool visited(Marker& marker) const EXCLUSIVE_LOCKS_REQUIRED(*this)
      88             :     {
      89           0 :         assert(m_guarded);
      90           0 :         if (marker.m_marker < m_raw_epoch) {
      91             :             // marker is from a previous epoch, so this is its first visit
      92           0 :             marker.m_marker = m_raw_epoch;
      93           0 :             return false;
      94             :         } else {
      95           0 :             return true;
      96             :         }
      97           0 :     }
      98             : };
      99             : 
     100             : #define WITH_FRESH_EPOCH(epoch) const Epoch::Guard UNIQUE_NAME(epoch_guard_)(epoch)
     101             : 
     102             : #endif // BITCOIN_UTIL_EPOCHGUARD_H