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.
ReadStatus status = partialBlock.FillBlock(*pblock, resp.txn);
            if (status == READ_STATUS_INVALID) {
                MarkBlockAsReceived(resp.blockhash); 
                Misbehaving(pfrom->GetId(), 100, strprintf("Peer %d sent us invalid compact block/non-matching block transactions\n", pfrom->GetId()));
                return true;
            } else if (status == READ_STATUS_FAILED) {
                
                std::vector<CInv> invs;
                invs.push_back(CInv(MSG_BLOCK | GetFetchFlags(pfrom), resp.blockhash));
                connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::GETDATA, invs));
            } else {
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                MarkBlockAsReceived(resp.blockhash); 
                fBlockRead = true;