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.
bool fPeerWantsWitness = State(pfrom->GetId())->fWantsCmpctWitness;
                int nSendFlags = fPeerWantsWitness ? 0 : SERIALIZE_TRANSACTION_NO_WITNESS;
                if (CanDirectFetch(consensusParams) && pindex->nHeight >= chainActive.Height() - MAX_CMPCTBLOCK_DEPTH) {
                    if ((fPeerWantsWitness || !fWitnessesPresentInARecentCompactBlock) && a_recent_compact_block && a_recent_compact_block->header.GetHash() == pindex->GetBlockHash()) {
                        connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, *a_recent_compact_block));
                    } else {
                        CBlockHeaderAndShortTxIDs cmpctblock(*pblock, fPeerWantsWitness);
                        connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, cmpctblock));
                    }
                } else {
                    connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::BLOCK, *pblock));
                }
            }
        }
        
        if (inv.hash == pfrom->hashContinue)
        {
            
            
            
            std::vector<CInv> vInv;
            vInv.push_back(CInv(MSG_BLOCK, chainActive.Tip()->GetBlockHash()));
            connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::INV, vInv));
            pfrom->hashContinue.SetNull();
        }
    }
}
void static ProcessGetData(CNode* pfrom, const CChainParams& chainparams, CConnman* connman, const std::atomic<bool>& interruptMsgProc) LOCKS_EXCLUDED(cs_main)