Important to use pinia correctly on SSR - State leaking between concurrent sessions when useStore comes after an await etc

[iPrytz]

We have encountered this a few times, and I think really it's important to know about because it's hard to test and and an easy mistake to make. It can easily leak user data if not handled correctly.

We need some feedback on how to to solve passing the pinia instance all the way to the API-functions in the example

Reproduction link

https://github.com/iPrytz/vue3-pinia-ssr-example

Steps to reproduce

1. Create a SSR Vue application
2. Create two stores, exampe userStore and myDataStore

• initialize userStore with some user data from session, example a sessionId
• in myDataStore get (and log) session data from userStore useUserStore().sessionId
• then do a fetch with await
• then get (and log) data from userStore useUserStore().sessionId again

3. When tested (for example with autocannon) with multiple different sessions fetching the SSR app at the same time, data will leak between the sessions. SessionId will be different the second time from useStore.
4. It is solved if const store = useUserStore() and store.sessionId is used both before and after await, but in more complex cases it can still be a missmatch.
   For example, in our store, we fetch specific user data connected to the sessionId, but before we do that we need to fetch and await the users profile settings, then we can fetch the specific user data. From our store we call the API and in the API-handler we then add the sessionId/token from userStore, and now we have a missmatch because we do useUserStore() after the await user profile while we have multipe requests comming in to our node server. So the sessionId/token added to the request is now from another user and we get mixed up data.

I added a repo to reproduce the problem, instructions how to test this is found in the README

Would also love to get some explanation on what is actually hapening here that is causing this behavior.

Thank you to everyone working on this project, you're awesome!! 💯 🙌