prevent peer flooding request queue for an inv

mapAlreadyAskedFor does not keep track of which peer has a request queued for a
particular tx. As a result, a peer can blind a node to a tx indefinitely by
sending many invs for the same tx, and then never replying to getdatas for it.
Each inv received will be placed 2 minutes farther back in mapAlreadyAskedFor,
so a short message containing 10 invs would render that tx unavailable for 20
minutes.

This is fixed by disallowing a peer from having more than one entry for a
particular inv in mapAlreadyAskedFor at a time.

src/net.cpp

@@ -2489,6 +2489,10 @@ void CNode::AskFor(const CInv& inv)
 {
     if (mapAskFor.size() > MAPASKFOR_MAX_SZ)
         return;
+    // a peer may not occupy multiple positions in an inv's request queue
+    if (!setAskFor.insert(inv.hash).second)
+        return;
+
     // We're using mapAskFor as a priority queue,
     // the key is the earliest time the request can be sent
     int64_t nRequestTime;

src/net.h

@@ -624,6 +624,7 @@ class CNode
     std::vector<CInv> vInventoryTierTwoToSend;
     RecursiveMutex cs_inventory;
     std::multimap<int64_t, CInv> mapAskFor;
+    std::set<uint256> setAskFor;
     std::vector<uint256> vBlockRequested;
     int64_t nNextInvSend;
     // Used for BIP35 mempool sending, also protected by cs_inventory

src/net_processing.cpp

@@ -2371,6 +2371,7 @@ bool SendMessages(CNode* pto, CConnman& connman, std::atomic<bool>& interruptMsg
                     vGetData.clear();
                 }
             }
+            pto->setAskFor.erase(inv.hash);
             pto->mapAskFor.erase(pto->mapAskFor.begin());
         }
         if (!vGetData.empty())