The `Broadcast`/`ResendWalletTransactions` interface is strange:

- it's the node saying to the wallet "Try to submit your unconfirmed transactions to the mempool again"
- it's called in one place by the node, in `SendMessages()` here: https://github.com/bitcoin/bitcoin/blob/c033c4b5cef89a654e4d9d5c5f9bd823871b068b/src/net_processing.cpp#L3515. `SendMessages()` is called on the message handling thread on a loop.
- the actual scheduling of when the wallet sends messages is handled in the `ResendWalletTransactions()` function itself, using the `nNextResend` and `nLastResend` wallet globals (which could as easily be static variables in `ResendWalletTransactions()`).

Therefore the only use of `Broadcast()` is to periodically prod the wallet. The wallet itself decides whether it's time to resend transactions. This could be done by scheduling regular calls to `ResendWalletTransactions()`, in the same way that we schedule regular wallet flushes here: https://github.com/bitcoin/bitcoin/blob/c033c4b5cef89a654e4d9d5c5f9bd823871b068b/src/wallet/init.cpp#L215.

A couple of minor implementation details:

- the call to `Broadcast()` in `SendMessages()` is protected by an if statement `if (!fReindex && !fImporting && !IsInitialBlockDownload()) {`. Those tests could be moved into `ResendWalletTransactions()` (through the `Chain` interface).
- The `Broadcast` notification passes in the `nTimeBestReceived` time of the best block. That could be fetched by `ResendWalletTransactions()` through the `Chain` interface.
if (pindex->IsValid(BLOCK_VALID_TRANSACTIONS)) {
                
                
                
                
                MarkBlockAsReceived(pblock->GetHash());
            }
        }
        return true;
    }
    if (strCommand == NetMsgType::BLOCKTXN)
    {
        
        if (fImporting || fReindex) {
            LogPrint(BCLog::NET, "Unexpected blocktxn message received from peer %d\n", pfrom->GetId());
            return true;
        }
        BlockTransactions resp;
        vRecv >> resp;
        std::shared_ptr<CBlock> pblock = std::make_shared<CBlock>();
        bool fBlockRead = false;
        {
            LOCK(cs_main);
            std::map<uint256, std::pair<NodeId, std::list<QueuedBlock>::iterator> >::iterator it = mapBlocksInFlight.find(resp.blockhash);
            if (it == mapBlocksInFlight.end() || !it->second.second->partialBlock ||
                    it->second.first != pfrom->GetId()) {
                LogPrint(BCLog::NET, "Peer %d sent us block transactions for block we weren't expecting\n", pfrom->GetId());
                return true;
            }
            PartiallyDownloadedBlock& partialBlock = *it->second.second->partialBlock;