Remove NOTFOUND transactions from in-flight data structures

This prevents a bug where the in-flight queue for our peers will not be
drained, resulting in not downloading any new transactions from our peers.

Thanks to ajtowns for reporting this bug.

src/net_processing.cpp

@@ -3120,8 +3120,27 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
     }
 
     if (strCommand == NetMsgType::NOTFOUND) {
-        // We do not care about the NOTFOUND message, but logging an Unknown Command
-        // message would be undesirable as we transmit it ourselves.
+        // Remove the NOTFOUND transactions from the peer
+        LOCK(cs_main);
+        CNodeState *state = State(pfrom->GetId());
+        std::vector<CInv> vInv;
+        vRecv >> vInv;
+        if (vInv.size() <= MAX_PEER_TX_IN_FLIGHT + MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
+            for (CInv &inv : vInv) {
+                if (inv.type == MSG_TX || inv.type == MSG_WITNESS_TX) {
+                    // If we receive a NOTFOUND message for a txid we requested, erase
+                    // it from our data structures for this peer.
+                    auto in_flight_it = state->m_tx_download.m_tx_in_flight.find(inv.hash);
+                    if (in_flight_it == state->m_tx_download.m_tx_in_flight.end()) {
+                        // Skip any further work if this is a spurious NOTFOUND
+                        // message.
+                        continue;
+                    }
+                    state->m_tx_download.m_tx_in_flight.erase(in_flight_it);
+                    state->m_tx_download.m_tx_announced.erase(inv.hash);
+                }
+            }
+        }
         return true;
     }