net: limit BIP37 filter lifespan (active between 'filterload' and 'filterclear')

Previously, a default match-everything bloom filter was set for every peer,
i.e. even before receiving a 'filterload' message and after receiving a
'filterclear' message code branches checking for the existence of the filter
by testing the pointer "pfilter" were _always_ executed.

Author: theStack

src/net.h

@@ -809,14 +809,13 @@ class CNode
     RecursiveMutex cs_inventory;
 
     struct TxRelay {
-        TxRelay() { pfilter = MakeUnique<CBloomFilter>(); }
         mutable RecursiveMutex cs_filter;
         // We use fRelayTxes for two purposes -
         // a) it allows us to not relay tx invs before receiving the peer's version message
         // b) the peer may tell us in its version message that we should not relay tx invs
         //    unless it loads a bloom filter.
         bool fRelayTxes GUARDED_BY(cs_filter){false};
-        std::unique_ptr<CBloomFilter> pfilter PT_GUARDED_BY(cs_filter) GUARDED_BY(cs_filter);
+        std::unique_ptr<CBloomFilter> pfilter PT_GUARDED_BY(cs_filter) GUARDED_BY(cs_filter){nullptr};
 
         mutable RecursiveMutex cs_tx_inventory;
         CRollingBloomFilter filterInventoryKnown GUARDED_BY(cs_tx_inventory){50000, 0.000001};

src/net_processing.cpp

@@ -3198,7 +3198,7 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
         }
         LOCK(pfrom->m_tx_relay->cs_filter);
         if (pfrom->GetLocalServices() & NODE_BLOOM) {
-            pfrom->m_tx_relay->pfilter.reset(new CBloomFilter());
+            pfrom->m_tx_relay->pfilter = nullptr;
         }
         pfrom->m_tx_relay->fRelayTxes = true;
         return true;