Eliminate TX trickle bypass, sort TX invs for privacy and priority.

Previously Bitcoin would send 1/4 of transactions out to all peers
instantly. This causes high overhead because it makes >80% of
INVs size 1. Doing so harms privacy, because it limits the
amount of source obscurity a transaction can receive.

These randomized broadcasts also disobeyed transaction dependencies
and required use of the orphan pool. Because the orphan pool is
so small this leads to poor propagation for dependent transactions.

When the bypass wasn't in effect, transactions were sent in the
order they were received. This avoided creating orphans but
undermines privacy fairly significantly.

This commit:
Eliminates the bypass. The bypass is replaced by halving the
 average delay for outbound peers.

Sorts candidate transactions for INV by their topological
 depth then by their feerate (then hash); removing the
 information leakage and providing priority service to
 higher fee transactions.

Limits the amount of transactions sent in a single INV to
 7tx/sec (and twice that for outbound); this limits the
 harm of low fee transaction floods, gives faster relay
 service to higher fee transactions. The 7 sounds lower
 than it really is because received advertisements need
 not be sent, and because the aggregate rate is multipled
 by the number of peers.

 Coming from btc@f2d3ba73860e875972738d1da1507124d0971ae5

src/net_processing.cpp

@@ -1940,6 +1940,29 @@ bool ProcessMessages(CNode* pfrom, CConnman& connman, std::atomic<bool>& interru
     return fMoreWork;
 }
 
+class CompareInvMempoolOrder
+{
+    CTxMemPool *mp;
+public:
+    CompareInvMempoolOrder(CTxMemPool *mempool)
+    {
+        mp = mempool;
+    }
+
+    bool operator()(const CInv &a, const CInv &b)
+    {
+        if (a.type != MSG_TX && b.type != MSG_TX) {
+            return false;
+        } else {
+            if (a.type != MSG_TX) {
+                return true;
+            } else if (b.type != MSG_TX) {
+                return false;
+            }
+            return mp->CompareDepthAndScore(a.hash, b.hash);
+        }
+    }
+};
 
 bool SendMessages(CNode* pto, CConnman& connman, std::atomic<bool>& interruptMsgProc)
 {
@@ -2067,38 +2090,31 @@ bool SendMessages(CNode* pto, CConnman& connman, std::atomic<bool>& interruptMsg
             bool fSendTrickle = pto->fWhitelisted;
             if (pto->nNextInvSend < nNow) {
                 fSendTrickle = true;
-                pto->nNextInvSend = PoissonNextSend(nNow, AVG_INVENTORY_BROADCAST_INTERVAL);
+                // Use half the delay for outbound peers, as their is less privacy concern for them.
+                pto->nNextInvSend = PoissonNextSend(nNow, INVENTORY_BROADCAST_INTERVAL >> !pto->fInbound);
             }
             LOCK(pto->cs_inventory);
-            vInv.reserve(pto->vInventoryToSend.size());
+            if (fSendTrickle && pto->vInventoryToSend.size() > 1) {
+                // Topologically and fee-rate sort the inventory we send for privacy and priority reasons.
+                CompareInvMempoolOrder compareInvMempoolOrder(&mempool);
+                std::stable_sort(pto->vInventoryToSend.begin(), pto->vInventoryToSend.end(), compareInvMempoolOrder);
+            }
+            vInv.reserve(std::min<size_t>(INVENTORY_BROADCAST_MAX, pto->vInventoryToSend.size()));
             vInvWait.reserve(pto->vInventoryToSend.size());
             for (const CInv& inv : pto->vInventoryToSend) {
                 if (inv.type == MSG_TX && pto->filterInventoryKnown.contains(inv.hash))
                     continue;
 
-                // trickle out tx inv to protect privacy
-                if (inv.type == MSG_TX && !fSendTrickle) {
-                    // 1/4 of tx invs blast to all immediately
-                    static uint256 hashSalt;
-                    if (hashSalt.IsNull())
-                        hashSalt = GetRandHash();
-                    uint256 hashRand = inv.hash ^ hashSalt;
-                    hashRand = Hash(BEGIN(hashRand), END(hashRand));
-                    bool fTrickleWait = ((hashRand & 3) != 0);
-
-                    if (fTrickleWait) {
-                        vInvWait.push_back(inv);
-                        continue;
-                    }
+                // No reason to drain out at many times the network's capacity,
+                // especially since we have many peers and some will draw much shorter delays.
+                if (vInv.size() >= INVENTORY_BROADCAST_MAX || (inv.type == MSG_TX && !fSendTrickle)) {
+                    vInvWait.push_back(inv);
+                    continue;
                 }
 
                 pto->filterInventoryKnown.insert(inv.hash);
 
                 vInv.push_back(inv);
-                if (vInv.size() >= 1000) {
-                    connman.PushMessage(pto, msgMaker.Make(NetMsgType::INV, vInv));
-                    vInv.clear();
-                }
             }
             pto->vInventoryToSend = vInvWait;
         }

src/net_processing.h

@@ -21,6 +21,13 @@ static const unsigned int DEFAULT_BLOCK_SPAM_FILTER_MAX_SIZE = 100;
 /** Default for -blockspamfiltermaxavg, maximum average size of an index occurrence in the block spam filter */
 static const unsigned int DEFAULT_BLOCK_SPAM_FILTER_MAX_AVG = 10;
 
+/** Average delay between trickled inventory transmissions in seconds.
+ *  Blocks and whitelisted receivers bypass this, outbound peers get half this delay. */
+static const unsigned int INVENTORY_BROADCAST_INTERVAL = 5;
+/** Maximum number of inventory items to send per transmission.
+ *  Limits the impact of low-fee transaction floods. */
+static const unsigned int INVENTORY_BROADCAST_MAX = 7 * INVENTORY_BROADCAST_INTERVAL;
+
 /** Register with a network node to receive its signals */
 void RegisterNodeSignals(CNodeSignals& nodeSignals);
 /** Unregister a network node */

src/txmempool.cpp

@@ -815,6 +815,21 @@ void CTxMemPool::checkNullifiers() const
     }
 }
 
+bool CTxMemPool::CompareDepthAndScore(const uint256& hasha, const uint256& hashb)
+{
+    LOCK(cs);
+    indexed_transaction_set::const_iterator i = mapTx.find(hasha);
+    if (i == mapTx.end()) return false;
+    indexed_transaction_set::const_iterator j = mapTx.find(hashb);
+    if (j == mapTx.end()) return true;
+    uint64_t counta = i->GetCountWithAncestors();
+    uint64_t countb = j->GetCountWithAncestors();
+    if (counta == countb) {
+        return CompareTxMemPoolEntryByScore()(*i, *j);
+    }
+    return counta < countb;
+}
+
 void CTxMemPool::queryHashes(std::vector<uint256>& vtxid)
 {
     vtxid.clear();

src/txmempool.h

@@ -531,6 +531,7 @@ class CTxMemPool
     void removeForBlock(const std::vector<CTransactionRef>& vtx, unsigned int nBlockHeight, std::list<CTransactionRef>& conflicts, bool fCurrentEstimate = true);
     void clear();
     void _clear();  // lock-free
+    bool CompareDepthAndScore(const uint256& hasha, const uint256& hashb);
     void queryHashes(std::vector<uint256>& vtxid);
     void getTransactions(std::set<uint256>& setTxid);
     bool isSpent(const COutPoint& outpoint);

src/validation.h

@@ -107,9 +107,6 @@ static const unsigned int MAX_REJECT_MESSAGE_LENGTH = 111;
 static const unsigned int AVG_LOCAL_ADDRESS_BROADCAST_INTERVAL = 24 * 24 * 60;
 /** Average delay between peer address broadcasts in seconds. */
 static const unsigned int AVG_ADDRESS_BROADCAST_INTERVAL = 30;
-/** Average delay between trickled inventory broadcasts in seconds.
- *  Blocks, whitelisted receivers, and a random 25% of transactions bypass this. */
-static const unsigned int AVG_INVENTORY_BROADCAST_INTERVAL = 5;
 /** Default multiplier used in the computation for shielded txes min fee */
 static const unsigned int DEFAULT_SHIELDEDTXFEE_K = 100;