Merge bitcoin/bitcoin#24021: Rename and move PoissonNextSend functions

9b8dcb25b57ad31b77c9f37d9a1f5b07dc6378b4 [net processing] Rename PoissonNextSendInbound to NextInvToInbounds (John Newbery)
ea99f5d01e56ab0192d211da1034ffb299876937 [net processing] Move PoissonNextSendInbound to PeerManager (John Newbery)
bb060746df22c956b8f44e5b8cd1ae4ed73faddc scripted-diff: replace PoissonNextSend with GetExponentialRand (John Newbery)
03cfa1b6035dbcf6a414f9bc432bd9e612801ebb [refactor] Use uint64_t and std namespace in PoissonNextSend (John Newbery)
9e64d69bf74c8a381fb59841519cc3736bce14d4 [move] Move PoissonNextSend to src/random and update comment (John Newbery)

Pull request description:

  `PoissonNextSend` and `PoissonNextSendInbound` are used in the p2p code to obfuscate various regularly occurring processes, in order to make it harder for others to get timing-based information deterministically.

  The naming of these functions has been confusing to several people (including myself, see also #23347) because the resulting random timestamps don't follow a Poisson distribution but an exponential distribution (related to events in a Poisson process, hence the name). This PR
  - moves `PoissonNextSend()` out of `net` to `random` and renames it to `GetExponentialRand()`
  - moves `PoissonNextSendInbound()` out of `CConnman` to `PeerManager` and renames it to `NextInvToInbounds()`
  - adds documentation for these functions

  This is work by jnewbery - due to him being less active currently, I opened the PR and will address feedback.

ACKs for top commit:
  jnewbery:
    ACK 9b8dcb25b5
  hebasto:
    ACK 9b8dcb25b57ad31b77c9f37d9a1f5b07dc6378b4, I have reviewed the code and it looks OK, I agree it can be merged.
  theStack:
    ACK 9b8dcb25b57ad31b77c9f37d9a1f5b07dc6378b4 📊

Tree-SHA512: 85c366c994e7147f9981fe863fb9838502643fa61ffd32d55a43feef96a38b79a5daa2c4d38ce01074897cc95fa40c76779816edad53f5265b81b05c3a1f4f50

1 files changed, 28 insertions, 5 deletions

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 273cb4fccb..02542a6c87 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -450,6 +450,8 @@ private:
      */
     std::map<NodeId, PeerRef> m_peer_map GUARDED_BY(m_peer_mutex);
 
+    std::atomic<std::chrono::microseconds> m_next_inv_to_inbounds{0us};
+
     /** Number of nodes with fSyncStarted. */
     int nSyncStarted GUARDED_BY(cs_main) = 0;
 
@@ -524,6 +526,15 @@ private:
     Mutex m_recent_confirmed_transactions_mutex;
     CRollingBloomFilter m_recent_confirmed_transactions GUARDED_BY(m_recent_confirmed_transactions_mutex){48'000, 0.000'001};
 
+    /**
+     * For sending `inv`s to inbound peers, we use a single (exponentially
+     * distributed) timer for all peers. If we used a separate timer for each
+     * peer, a spy node could make multiple inbound connections to us to
+     * accurately determine when we received the transaction (and potentially
+     * determine the transaction's origin). */
+    std::chrono::microseconds NextInvToInbounds(std::chrono::microseconds now,
+                                                std::chrono::seconds average_interval);
+
     /** Have we requested this block from a peer */
     bool IsBlockRequested(const uint256& hash) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
@@ -825,6 +836,18 @@ static void UpdatePreferredDownload(const CNode& node, CNodeState* state) EXCLUS
     nPreferredDownload += state->fPreferredDownload;
 }
 
+std::chrono::microseconds PeerManagerImpl::NextInvToInbounds(std::chrono::microseconds now,
+                                                             std::chrono::seconds average_interval)
+{
+    if (m_next_inv_to_inbounds.load() < now) {
+        // 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);
+    }
+    return m_next_inv_to_inbounds;
+}
+
 bool PeerManagerImpl::IsBlockRequested(const uint256& hash)
 {
     return mapBlocksInFlight.find(hash) != mapBlocksInFlight.end();
@@ -4434,13 +4457,13 @@ void PeerManagerImpl::MaybeSendAddr(CNode& node, Peer& peer, std::chrono::micros
             FastRandomContext insecure_rand;
             PushAddress(peer, *local_addr, insecure_rand);
         }
-        peer.m_next_local_addr_send = PoissonNextSend(current_time, AVG_LOCAL_ADDRESS_BROADCAST_INTERVAL);
+        peer.m_next_local_addr_send = GetExponentialRand(current_time, 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 = PoissonNextSend(current_time, AVG_ADDRESS_BROADCAST_INTERVAL);
+    peer.m_next_addr_send = GetExponentialRand(current_time, 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
@@ -4512,7 +4535,7 @@ void PeerManagerImpl::MaybeSendFeefilter(CNode& pto, std::chrono::microseconds c
             m_connman.PushMessage(&pto, CNetMsgMaker(pto.GetCommonVersion()).Make(NetMsgType::FEEFILTER, filterToSend));
             pto.m_tx_relay->lastSentFeeFilter = filterToSend;
         }
-        pto.m_tx_relay->m_next_send_feefilter = PoissonNextSend(current_time, AVG_FEEFILTER_BROADCAST_INTERVAL);
+        pto.m_tx_relay->m_next_send_feefilter = GetExponentialRand(current_time, 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.
@@ -4792,9 +4815,9 @@ bool PeerManagerImpl::SendMessages(CNode* pto)
                 if (pto->m_tx_relay->nNextInvSend < current_time) {
                     fSendTrickle = true;
                     if (pto->IsInboundConn()) {
-                        pto->m_tx_relay->nNextInvSend = m_connman.PoissonNextSendInbound(current_time, INBOUND_INVENTORY_BROADCAST_INTERVAL);
+                        pto->m_tx_relay->nNextInvSend = NextInvToInbounds(current_time, INBOUND_INVENTORY_BROADCAST_INTERVAL);
                     } else {
-                        pto->m_tx_relay->nNextInvSend = PoissonNextSend(current_time, OUTBOUND_INVENTORY_BROADCAST_INTERVAL);
+                        pto->m_tx_relay->nNextInvSend = GetExponentialRand(current_time, OUTBOUND_INVENTORY_BROADCAST_INTERVAL);
                     }
                 }