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.
}
                }
            }
            EraseOrphanTx(orphanHash);
            done = true;
        } else if (!fMissingInputs2) {
            int nDos = 0;
            if (stateDummy.IsInvalid(nDos) && nDos > 0) {
                
                Misbehaving(fromPeer, nDos);
                setMisbehaving.insert(fromPeer);
                LogPrint(BCLog::MEMPOOL, "   invalid orphan tx %s\n", orphanHash.ToString());
            }
            
            
            LogPrint(BCLog::MEMPOOL, "   removed orphan tx %s\n", orphanHash.ToString());
            if (!orphanTx.HasWitness() && !stateDummy.CorruptionPossible()) {
                
                
                
                assert(recentRejects);
                recentRejects->insert(orphanHash);
            }
            EraseOrphanTx(orphanHash);
            done = true;
        }
        mempool.check(pcoinsTip.get());
    }
}
bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStream& vRecv, int64_t nTimeReceived, const CChainParams& chainparams, CConnman* connman, const std::atomic<bool>& interruptMsgProc, bool enable_bip61)