-fuzzmessagestest=N : randomly corrupt 1-of-N sent messages

I needed this to test the new "reject" p2p message, but it should be generally
useful for fuzz-testing network message handling code.

src/net.cpp

@@ -1894,3 +1894,38 @@ uint64 CNode::GetTotalBytesSent()
     LOCK(cs_totalBytesSent);
     return nTotalBytesSent;
 }
+
+void CNode::Fuzz(int nChance)
+{
+    if (!fSuccessfullyConnected) return; // Don't fuzz initial handshake
+    if (GetRand(nChance) != 0) return; // Fuzz 1 of every nChance messages
+
+    switch (GetRand(3))
+    {
+    case 0:
+        // xor a random byte with a random value:
+        if (!ssSend.empty()) {
+            CDataStream::size_type pos = GetRand(ssSend.size());
+            ssSend[pos] ^= (unsigned char)(GetRand(256));
+        }
+        break;
+    case 1:
+        // delete a random byte:
+        if (!ssSend.empty()) {
+            CDataStream::size_type pos = GetRand(ssSend.size());
+            ssSend.erase(ssSend.begin()+pos);
+        }
+        break;
+    case 2:
+        // insert a random byte at a random position
+        {
+            CDataStream::size_type pos = GetRand(ssSend.size());
+            char ch = (char)GetRand(256);
+            ssSend.insert(ssSend.begin()+pos, ch);
+        }
+        break;
+    }
+    // Chance of more than one change half the time:
+    // (more changes exponentially less likely):
+    Fuzz(2);
+}

src/net.h

@@ -218,6 +218,9 @@ protected:
     static CCriticalSection cs_setBanned;
     int nMisbehavior;
 
+    // Basic fuzz-testing
+    void Fuzz(int nChance); // modifies ssSend
+
 public:
     uint256 hashContinue;
     CBlockIndex* pindexLastGetBlocksBegin;
@@ -434,12 +437,17 @@ public:
     // TODO: Document the precondition of this function.  Is cs_vSend locked?
     void EndMessage() UNLOCK_FUNCTION(cs_vSend)
     {
-        if (mapArgs.count("-dropmessagestest") && GetRand(atoi(mapArgs["-dropmessagestest"])) == 0)
+        // The -*messagestest options are intentionally not documented in the help message,
+        // since they are only used during development to debug the networking code and are
+        // not intended for end-users.
+        if (mapArgs.count("-dropmessagestest") && GetRand(GetArg("-dropmessagestest", 2)) == 0)
         {
             LogPrint("net", "dropmessages DROPPING SEND MESSAGE\n");
             AbortMessage();
             return;
         }
+        if (mapArgs.count("-fuzzmessagestest"))
+            Fuzz(GetArg("-fuzzmessagestest", 10));
 
         if (ssSend.size() == 0)
             return;