Reported here: https://github.com/bitcoin/bitcoin/pull/10244#issuecomment-378035305

Travis log here: https://travis-ci.org/bitcoin/bitcoin/jobs/361243719#L2810

Note the JSON exception:

```
test_framework.authproxy.JSONRPCException: The mempool was not loaded yet (-1) 
```

That exception was introduced here: https://github.com/bitcoin/bitcoin/commit/cb1e319fe9e198c9c5cf5236fe9af5a3d748b9e8#diff-a0c8f511d90e83aa9b5857e819ced344R1617

cc @jtimon 
}.ToString());
    int timeout = 0;
    if (!request.params[0].isNull())
        timeout = request.params[0].get_int();
    CUpdatedBlock block;
    {
        WAIT_LOCK(cs_blockchange, lock);
        block = latestblock;
        if(timeout)
            cond_blockchange.wait_for(lock, std::chrono::milliseconds(timeout), [&block]{return latestblock.height != block.height || latestblock.hash != block.hash || !IsRPCRunning(); });
        else
            cond_blockchange.wait(lock, [&block]{return latestblock.height != block.height || latestblock.hash != block.hash || !IsRPCRunning(); });
        block = latestblock;
    }
    UniValue ret(UniValue::VOBJ);
    ret.pushKV("hash", block.hash.GetHex());
    ret.pushKV("height", block.height);
    return ret;
}
static UniValue waitforblock(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 2)
        throw std::runtime_error(
            RPCHelpMan{"waitforblock",
                "\nWaits for a specific new block and returns useful info about it.\n"
                "\nReturns the current block on timeout or exit.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "Block hash to wait for."},
                    {"timeout", RPCArg::Type::NUM,  "0", "Time in milliseconds to wait for a response. 0 indicates no timeout."},
                },