Implement on-the-fly mempool size limitation.

After each transaction which is added to mempool, we first call
Expire() to remove old transactions, then throwing away the
lowest-feerate transactions.

After throwing away transactions by feerate, we set the minimum
relay fee to the maximum fee transaction-and-dependant-set we
removed, plus the default minimum relay fee.

After the next block is received, the minimum relay fee is allowed
to decrease exponentially. Its halflife defaults to 12 hours, but
is decreased to 6 hours if the mempool is smaller than half its
maximum size, and 3 hours if the mempool is smaller than a quarter
its maximum size.

The minimum -maxmempool size is 40*-limitdescendantsize, as it is
easy for an attacker to play games with the cheapest
-limitdescendantsize transactions. -maxmempool defaults to 300MB.

This disables high-priority transaction relay when the min relay
fee adjustment is >0 (ie when the mempool is full). When the relay
fee adjustment drops below the default minimum relay fee / 2 it is
set to 0 (re-enabling priority-based free relay).

1 files changed, 14 insertions, 2 deletions

diff --git a/src/main.cpp b/src/main.cpp
index f01bb8ec9e..c1df9998a9 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -885,8 +885,11 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
             return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "insufficient fee", false,
                 strprintf("%d < %d", nFees, txMinFee));
 
-        // Require that free transactions have sufficient priority to be mined in the next block.
-        if (GetBoolArg("-relaypriority", true) && nFees < ::minRelayTxFee.GetFee(nSize) && !AllowFree(view.GetPriority(tx, chainActive.Height() + 1))) {
+        CAmount mempoolRejectFee = pool.GetMinFee(GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000).GetFee(nSize);
+        if (mempoolRejectFee > 0 && nFees < mempoolRejectFee) {
+            return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "mempool min fee not met", false, strprintf("%d < %d", nFees, mempoolRejectFee));
+        } else if (GetBoolArg("-relaypriority", true) && nFees < ::minRelayTxFee.GetFee(nSize) && !AllowFree(view.GetPriority(tx, chainActive.Height() + 1))) {
+            // Require that free transactions have sufficient priority to be mined in the next block.
             return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "insufficient priority");
         }
 
@@ -951,6 +954,15 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
 
         // Store transaction in memory
         pool.addUnchecked(hash, entry, setAncestors, !IsInitialBlockDownload());
+
+        // trim mempool and check if tx was trimmed
+        int expired = pool.Expire(GetTime() - GetArg("-mempoolexpiry", DEFAULT_MEMPOOL_EXPIRY) * 60 * 60);
+        if (expired != 0)
+            LogPrint("mempool", "Expired %i transactions from the memory pool\n", expired);
+
+        pool.TrimToSize(GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
+        if (!pool.exists(tx.GetHash()))
+            return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "mempool full");
     }
 
     SyncWithWallets(tx, NULL);