Merge bitcoin/bitcoin#21327: net_processing: ignore transactions while in IBD

6aed8b7e9b [test] tx processing before and after ibd (glozow)
b9e105b664 [net_processing] ignore all transactions during ibd (glozow)

Pull request description:

  This is basically a mini, IBD-only version of #21224

  Incoming transactions aren't really relevant until we're caught up. That's why we send a giant feefilter and don't send tx getdatas, but we also shouldn't process them if peers send them anyway. Simply ignore them.

ACKs for top commit:
  jnewbery:
    reACK 6aed8b7e9b
  laanwj:
    Code review ACK 6aed8b7e9b

Tree-SHA512: 8e1616bf355f9d0b180bdbc5461f24c757dc5d7bc7bf651470f3b0bffcca5d5e68287106255b5cede2d96b42bce448a0f8c0649de35a530c5e079f7c89c70a35

src/net_processing.cpp

@@ -3206,6 +3206,11 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
             return;
         }
 
+        // Stop processing the transaction early if we are still in IBD since we don't
+        // have enough information to validate it yet. Sending unsolicited transactions
+        // is not considered a protocol violation, so don't punish the peer.
+        if (m_chainman.ActiveChainstate().IsInitialBlockDownload()) return;
+
         CTransactionRef ptx;
         vRecv >> ptx;
         const CTransaction& tx = *ptx;