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.
return true;
        }
        LOCK(cs_main);
        const CBlockIndex* pindex = LookupBlockIndex(req.blockhash);
        if (!pindex || !(pindex->nStatus & BLOCK_HAVE_DATA)) {
            LogPrint(BCLog::NET, "Peer %d sent us a getblocktxn for a block we don't have\n", pfrom->GetId());
            return true;
        }
        if (pindex->nHeight < chainActive.Height() - MAX_BLOCKTXN_DEPTH) {
            
            
            
            
            
            
            
            LogPrint(BCLog::NET, "Peer %d sent us a getblocktxn for a block > %i deep\n", pfrom->GetId(), MAX_BLOCKTXN_DEPTH);
            CInv inv;
            inv.type = State(pfrom->GetId())->fWantsCmpctWitness ? MSG_WITNESS_BLOCK : MSG_BLOCK;
            inv.hash = req.blockhash;
            pfrom->vRecvGetData.push_back(inv);
            
            return true;
        }
        CBlock block;
        bool ret = ReadBlockFromDisk(block, pindex, chainparams.GetConsensus());
        assert(ret);
        SendBlockTransactions(block, req, pfrom, connman);
        return true;
    }