random: convert GetExponentialRand into rand_exp_duration

Author: sipa

src/net.cpp

@@ -2481,9 +2481,9 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
     auto start = GetTime<std::chrono::microseconds>();
 
     // Minimum time before next feeler connection (in microseconds).
-    auto next_feeler = GetExponentialRand(start, FEELER_INTERVAL);
-    auto next_extra_block_relay = GetExponentialRand(start, EXTRA_BLOCK_RELAY_ONLY_PEER_INTERVAL);
-    auto next_extra_network_peer{GetExponentialRand(start, EXTRA_NETWORK_PEER_INTERVAL)};
+    auto next_feeler = start + FastRandomContext().rand_exp_duration(FEELER_INTERVAL);
+    auto next_extra_block_relay = start + FastRandomContext().rand_exp_duration(EXTRA_BLOCK_RELAY_ONLY_PEER_INTERVAL);
+    auto next_extra_network_peer{start + FastRandomContext().rand_exp_duration(EXTRA_NETWORK_PEER_INTERVAL)};
     const bool dnsseed = gArgs.GetBoolArg("-dnsseed", DEFAULT_DNSSEED);
     bool add_fixed_seeds = gArgs.GetBoolArg("-fixedseeds", DEFAULT_FIXEDSEEDS);
     const bool use_seednodes{gArgs.IsArgSet("-seednode")};
@@ -2642,10 +2642,10 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
             // Because we can promote these connections to block-relay-only
             // connections, they do not get their own ConnectionType enum
             // (similar to how we deal with extra outbound peers).
-            next_extra_block_relay = GetExponentialRand(now, EXTRA_BLOCK_RELAY_ONLY_PEER_INTERVAL);
+            next_extra_block_relay = now + FastRandomContext().rand_exp_duration(EXTRA_BLOCK_RELAY_ONLY_PEER_INTERVAL);
             conn_type = ConnectionType::BLOCK_RELAY;
         } else if (now > next_feeler) {
-            next_feeler = GetExponentialRand(now, FEELER_INTERVAL);
+            next_feeler = now + FastRandomContext().rand_exp_duration(FEELER_INTERVAL);
             conn_type = ConnectionType::FEELER;
             fFeeler = true;
         } else if (nOutboundFullRelay == m_max_outbound_full_relay &&
@@ -2658,7 +2658,7 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
             // This is not attempted if the user changed -maxconnections to a value
             // so low that less than MAX_OUTBOUND_FULL_RELAY_CONNECTIONS are made,
             // to prevent interactions with otherwise protected outbound peers.
-            next_extra_network_peer = GetExponentialRand(now, EXTRA_NETWORK_PEER_INTERVAL);
+            next_extra_network_peer = now + FastRandomContext().rand_exp_duration(EXTRA_NETWORK_PEER_INTERVAL);
         } else {
             // skip to next iteration of while loop
             continue;

src/net_processing.cpp

@@ -1244,7 +1244,7 @@ std::chrono::microseconds PeerManagerImpl::NextInvToInbounds(std::chrono::micros
         // If this function were called from multiple threads simultaneously
         // it would possible that both update the next send variable, and return a different result to their caller.
         // This is not possible in practice as only the net processing thread invokes this function.
-        m_next_inv_to_inbounds = GetExponentialRand(now, average_interval);
+        m_next_inv_to_inbounds = now + FastRandomContext().rand_exp_duration(average_interval);
     }
     return m_next_inv_to_inbounds;
 }
@@ -5654,13 +5654,13 @@ void PeerManagerImpl::MaybeSendAddr(CNode& node, Peer& peer, std::chrono::micros
             CAddress local_addr{*local_service, peer.m_our_services, Now<NodeSeconds>()};
             PushAddress(peer, local_addr);
         }
-        peer.m_next_local_addr_send = GetExponentialRand(current_time, AVG_LOCAL_ADDRESS_BROADCAST_INTERVAL);
+        peer.m_next_local_addr_send = current_time + FastRandomContext().rand_exp_duration(AVG_LOCAL_ADDRESS_BROADCAST_INTERVAL);
     }
 
     // We sent an `addr` message to this peer recently. Nothing more to do.
     if (current_time <= peer.m_next_addr_send) return;
 
-    peer.m_next_addr_send = GetExponentialRand(current_time, AVG_ADDRESS_BROADCAST_INTERVAL);
+    peer.m_next_addr_send = current_time + FastRandomContext().rand_exp_duration(AVG_ADDRESS_BROADCAST_INTERVAL);
 
     if (!Assume(peer.m_addrs_to_send.size() <= MAX_ADDR_TO_SEND)) {
         // Should be impossible since we always check size before adding to
@@ -5747,7 +5747,7 @@ void PeerManagerImpl::MaybeSendFeefilter(CNode& pto, Peer& peer, std::chrono::mi
             MakeAndPushMessage(pto, NetMsgType::FEEFILTER, filterToSend);
             peer.m_fee_filter_sent = filterToSend;
         }
-        peer.m_next_send_feefilter = GetExponentialRand(current_time, AVG_FEEFILTER_BROADCAST_INTERVAL);
+        peer.m_next_send_feefilter = current_time + FastRandomContext().rand_exp_duration(AVG_FEEFILTER_BROADCAST_INTERVAL);
     }
     // If the fee filter has changed substantially and it's still more than MAX_FEEFILTER_CHANGE_DELAY
     // until scheduled broadcast, then move the broadcast to within MAX_FEEFILTER_CHANGE_DELAY.
@@ -6059,7 +6059,7 @@ bool PeerManagerImpl::SendMessages(CNode* pto)
                     if (pto->IsInboundConn()) {
                         tx_relay->m_next_inv_send_time = NextInvToInbounds(current_time, INBOUND_INVENTORY_BROADCAST_INTERVAL);
                     } else {
-                        tx_relay->m_next_inv_send_time = GetExponentialRand(current_time, OUTBOUND_INVENTORY_BROADCAST_INTERVAL);
+                        tx_relay->m_next_inv_send_time = current_time + FastRandomContext().rand_exp_duration(OUTBOUND_INVENTORY_BROADCAST_INTERVAL);
                     }
                 }
 

src/random.cpp

@@ -773,8 +773,7 @@ void RandomInit()
     ReportHardwareRand();
 }
 
-std::chrono::microseconds GetExponentialRand(std::chrono::microseconds now, std::chrono::seconds average_interval)
+double MakeExponentiallyDistributed(uint64_t uniform) noexcept
 {
-    double unscaled = -std::log1p(FastRandomContext().randbits<48>() * -0.0000000000000035527136788 /* -1/2^48 */);
-    return now + std::chrono::duration_cast<std::chrono::microseconds>(unscaled * average_interval + 0.5us);
+    return -std::log1p((uniform >> 16) * -0.0000000000000035527136788 /* -1/2^48 */);
 }

src/random.h

@@ -81,17 +81,6 @@
  */
 void GetRandBytes(Span<unsigned char> bytes) noexcept;
 
-/**
- * Return a timestamp in the future sampled from an exponential distribution
- * (https://en.wikipedia.org/wiki/Exponential_distribution). This distribution
- * is memoryless and should be used for repeated network events (e.g. sending a
- * certain type of message) to minimize leaking information to observers.
- *
- * The probability of an event occurring before time x is 1 - e^-(x/a) where a
- * is the average interval between events.
- * */
-std::chrono::microseconds GetExponentialRand(std::chrono::microseconds now, std::chrono::seconds average_interval);
-
 uint256 GetRandHash() noexcept;
 
 /**
@@ -139,6 +128,9 @@ concept StdChronoDuration = requires {
         std::type_identity<T>());
 };
 
+/** Given a uniformly random uint64_t, return an exponentially distributed double with mean 1. */
+double MakeExponentiallyDistributed(uint64_t uniform) noexcept;
+
 /** Mixin class that provides helper randomness functions.
  *
  * Intended to be used through CRTP: https://en.cppreference.com/w/cpp/language/crtp.
@@ -327,6 +319,23 @@ class RandomMixin
         return Dur{Impl().randrange(range.count())};
     }
 
+    /**
+     * Return a duration sampled from an exponential distribution
+     * (https://en.wikipedia.org/wiki/Exponential_distribution). Successive events
+     * whose intervals are distributed according to this form a memoryless Poisson
+     * process. This should be used for repeated network events (e.g. sending a
+     * certain type of message) to minimize leaking information to observers.
+     *
+     * The probability of an event occurring before time x is 1 - e^-(x/a) where a
+     * is the average interval between events.
+     * */
+    std::chrono::microseconds rand_exp_duration(std::chrono::microseconds mean) noexcept
+    {
+        using namespace std::chrono_literals;
+        auto unscaled = MakeExponentiallyDistributed(Impl().rand64());
+        return std::chrono::duration_cast<std::chrono::microseconds>(unscaled * mean + 0.5us);
+    }
+
     // Compatibility with the UniformRandomBitGenerator concept
     typedef uint64_t result_type;
     static constexpr uint64_t min() noexcept { return 0; }