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 (!AlreadyHave(inv)) {
                
                
                int64_t last_request_time = GetTxRequestTime(inv.hash);
                if (last_request_time <= nNow - GETDATA_TX_INTERVAL) {
                    LogPrint(BCLog::NET, "Requesting %s peer=%d\n", inv.ToString(), pto->GetId());
                    vGetData.push_back(inv);
                    if (vGetData.size() >= MAX_GETDATA_SZ) {
                        connman->PushMessage(pto, msgMaker.Make(NetMsgType::GETDATA, vGetData));
                        vGetData.clear();
                    }
                    UpdateTxRequestTime(inv.hash, nNow);
                    state.m_tx_download.m_tx_in_flight.insert(inv.hash);
                } else {
                    
                    
                    
                    
                    RequestTx(&state, txid, nNow);
                }
            } else {
                
                state.m_tx_download.m_tx_announced.erase(inv.hash);
                state.m_tx_download.m_tx_in_flight.erase(inv.hash);
            }
            tx_process_time.erase(tx_process_time.begin());
        }
        if (!vGetData.empty())
            connman->PushMessage(pto, msgMaker.Make(NetMsgType::GETDATA, vGetData));