[net processing] Only call MaybeDiscourageAndDisconnect from SendMessages

`nMisbehavior` is a tally in `CNodeState` that can be incremented from
anywhere. That almost always happens inside a `ProcessMessages()` call
(because we increment the misbehavior score when receiving a bad
messages from a peer), but not always. See, for example, the call to
`MaybePunishNodeForBlock()` inside `BlockChecked()`, which is an
asynchronous callback from the validation interface, executed on the
scheduler thread.

As long as `MaybeDiscourageAndDisconnect()` is called regularly for the
node, then the misbehavior score exceeding the 100 threshold will
eventually result in the peer being punished. It doesn't really matter
where that `MaybeDiscourageAndDisconnect()` happens, but it makes most
sense in `SendMessages()` which is where we do general peer
housekeeping/maintenance.

Therefore, remove the `MaybeDiscourageAndDisconnect()` call in
`ProcessMessages()` and move the `MaybeDiscourageAndDisconnect()` call
in `SendMessages()` to the top of the function. This moves it out of the
cs_main lock scope, so take that lock directly inside
`MaybeDiscourageAndDisconnect()`.

Historic note: `MaybeDiscourageAndDisconnect()` was previously
`SendRejectsAndCheckIfBanned()`, and before that was just sending
rejects.  All of those things required cs_main, which is why
`MaybeDiscourageAndDisconnect()` was called after the ping logic.

Author: jnewbery

src/net_processing.cpp

@@ -3559,7 +3559,7 @@ void ProcessMessage(
 
 bool PeerLogicValidation::MaybeDiscourageAndDisconnect(CNode& pnode)
 {
-    AssertLockHeld(cs_main);
+    LOCK(cs_main);
     CNodeState &state = *State(pnode.GetId());
 
     if (state.m_should_discourage) {
@@ -3675,9 +3675,6 @@ bool PeerLogicValidation::ProcessMessages(CNode* pfrom, std::atomic<bool>& inter
         LogPrint(BCLog::NET, "%s(%s, %u bytes): Unknown exception caught\n", __func__, SanitizeString(msg_type), nMessageSize);
     }
 
-    LOCK(cs_main);
-    MaybeDiscourageAndDisconnect(*pfrom);
-
     return fMoreWork;
 }
 
@@ -3839,6 +3836,10 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
 {
     const Consensus::Params& consensusParams = Params().GetConsensus();
 
+    // We must call MaybeDiscourageAndDisconnect first, to ensure that we'll
+    // disconnect misbehaving peers even before the version handshake is complete.
+    if (MaybeDiscourageAndDisconnect(*pto)) return true;
+
     // Don't send anything until the version handshake is complete
     if (!pto->fSuccessfullyConnected || pto->fDisconnect)
         return true;
@@ -3878,8 +3879,6 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
     {
         LOCK(cs_main);
 
-        if (MaybeDiscourageAndDisconnect(*pto)) return true;
-
         CNodeState &state = *State(pto->GetId());
 
         // Address refresh broadcast

src/net_processing.h

@@ -31,7 +31,7 @@ class PeerLogicValidation final : public CValidationInterface, public NetEventsI
     ChainstateManager& m_chainman;
     CTxMemPool& m_mempool;
 
-    bool MaybeDiscourageAndDisconnect(CNode& pnode) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    bool MaybeDiscourageAndDisconnect(CNode& pnode);
 
 public:
     PeerLogicValidation(CConnman* connman, BanMan* banman, CScheduler& scheduler, ChainstateManager& chainman, CTxMemPool& pool);