This eliminates the primary leak that causes the orphan map to

always grow to its maximum size.

This does not go so far as to attempt to connect orphans made
 connectable by a new block.

Keeping the orphan map less full helps improve the reliability
 of relaying chains of transactions.

src/main.cpp

@@ -2345,6 +2345,7 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
 
     CCheckQueueControl<CScriptCheck> control(fScriptChecks && nScriptCheckThreads ? &scriptcheckqueue : NULL);
 
+    std::vector<uint256> vOrphanErase;
     std::vector<int> prevheights;
     CAmount nFees = 0;
     int nInputs = 0;
@@ -2377,6 +2378,17 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
                 prevheights[j] = view.AccessCoins(tx.vin[j].prevout.hash)->nHeight;
             }
 
+            // Which orphan pool entries must we evict?
+            for (size_t j = 0; j < tx.vin.size(); j++) {
+                auto itByPrev = mapOrphanTransactionsByPrev.find(tx.vin[j].prevout);
+                if (itByPrev == mapOrphanTransactionsByPrev.end()) continue;
+                for (auto mi = itByPrev->second.begin(); mi != itByPrev->second.end(); ++mi) {
+                    const CTransaction& orphanTx = (*mi)->second.tx;
+                    const uint256& orphanHash = orphanTx.GetHash();
+                    vOrphanErase.push_back(orphanHash);
+                }
+            }
+
             if (!SequenceLocks(tx, nLockTimeFlags, &prevheights, *pindex)) {
                 return state.DoS(100, error("%s: contains a non-BIP68-final transaction", __func__),
                                  REJECT_INVALID, "bad-txns-nonfinal");
@@ -2464,6 +2476,15 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
     GetMainSignals().UpdatedTransaction(hashPrevBestCoinBase);
     hashPrevBestCoinBase = block.vtx[0].GetHash();
 
+    // Erase orphan transactions include or precluded by this block
+    if (vOrphanErase.size()) {
+        int nErased = 0;
+        BOOST_FOREACH(uint256 &orphanHash, vOrphanErase) {
+            nErased += EraseOrphanTx(orphanHash);
+        }
+        LogPrint("mempool", "Erased %d orphan tx included or conflicted by block\n", nErased);
+    }
+
     int64_t nTime6 = GetTimeMicros(); nTimeCallbacks += nTime6 - nTime5;
     LogPrint("bench", "    - Callbacks: %.2fms [%.2fs]\n", 0.001 * (nTime6 - nTime5), nTimeCallbacks * 0.000001);