How do you work around IndexedDb Transaction with async call

I have a Worker which starts on a page opening. The worker opens a transaction, checks to see if a value is present on a record and if not needs to make a fetch call to get the value.

The issue I have is that I do this in a read/write transaction but the fetch is async which commits the transaction before the fetch is returned. If I could be guaranteed only a single page accessing the logic at the same time then I would just do it without a transaction, eg get the data, check it, if required get value and update. But I cant be guaranteed a single page.

So the question is how do I guaranteed that the second page doesn't start the process of getting a new value once the first page has started the process?

async function start(message) {
    var result = await new Promise(async (resolve, reject) => {
        let transaction = db.transaction(["Client"], "readwrite")
        let objectStore = transaction.objectStore("Client");
        let request = objectStore.get(message.client_name);

        request.onerror = (event) => {
            reject(event);
        };

        request.onsuccess = async () => {
            try {
                let data = request.result
                if (!data) {
                    data = {
                        client_name: message.client_name,
                        sequence: 0,
                        api_base_url: message.api_base_url || 'https://api.local'
                    }
                }
                
                if (!data.last_access || dateDiff(Date.parse(data.last_access), new Date(), "m") > 20) {
                    let response = await fetch(data.api_base_url + '/v1/hello');
                    let responseData = await response.json();
                    data.id = responseData.id;
                    data.last_access = responseData.dt;
                }
                objectStore.put(data); // <-- Here is where it errors
                resolve(data.sequence);
            } catch (error) {
                reject(error);
            }
        }
    });

    return result;
}