Merge bitcoin#22144: Randomize message processing peer order

79c02c8 Randomize message processing peer order (Pieter Wuille)

Pull request description:

  Right now, the message handling loop iterates the list of nodes always in the same order: the order they were connected in (see the `vNodes` vector). For some parts of the net processing logic, this order matters. Transaction requests are assigned explicitly to peers since bitcoin#19988, but many other parts of processing work on a "first-served-by-loop-first" basis, such as block downloading. If peers can predict this ordering, it may be exploited to cause delays.

  As there isn't anything particularly optimal about the current ordering, just make it unpredictable by randomizing.

  Reported by Crypt-iQ.

ACKs for top commit:
  jnewbery:
    ACK 79c02c8
  Crypt-iQ:
    ACK 79c02c8
  sdaftuar:
    utACK 79c02c8
  achow101:
    Code Review ACK 79c02c8
  jamesob:
    crACK bitcoin@79c02c8
  jonatack:
    ACK 79c02c8
  vasild:
    ACK 79c02c8
  theStack:
    ACK 79c02c8

Tree-SHA512: 9a87c4dcad47c2d61b76c4f37f59674876b78f33f45943089bf159902a23e12de7a5feae1a73b17cbc3f2e37c980ecf0f7fd86af9e6fa3a68099537a3c82c106

src/net.cpp

@@ -2965,6 +2965,7 @@ void CConnman::ThreadMessageHandler()
 {
     int64_t nLastSendMessagesTimeMasternodes = 0;
 
+    FastRandomContext rng;
     while (!flagInterruptMsgProc)
     {
         std::vector<CNode*> vNodesCopy = CopyNodeVector();
@@ -2976,6 +2977,10 @@ void CConnman::ThreadMessageHandler()
             fSkipSendMessagesForMasternodes = false;
             nLastSendMessagesTimeMasternodes = GetTimeMillis();
         }
+        // Randomize the order in which we process messages from/to our peers.
+        // This prevents attacks in which an attacker exploits having multiple
+        // consecutive connections in the vNodes list.
+        Shuffle(vNodesCopy.begin(), vNodesCopy.end(), rng);
 
         for (CNode* pnode : vNodesCopy)
         {