Resource state inconsistency during mutate calls in SolidJS

Resource State and Mutation Synchronization

In SolidJS, a manual call to mutate does not override or cancel a pending asynchronous resolution from a resource's fetcher. If a fetcher is already in flight, its eventual resolution will overwrite any manual mutations performed in the interim, provided that the fetcher's request is still considered the "latest" sequence.

The Interaction Between Mutate and Pending Requests

The createResource primitive uses an internal sequence counter to prevent "stale" data from overwriting newer data (race conditions). However, mutate is designed for optimistic updates—it updates the current value of the resource signal immediately but does not increment the internal request sequence or invalidate the pending promise.

The likely sequence of events is as follows:

1. A source signal changes, triggering the fetcher (Request A). The resource enters a loading state.
2. mutate(newValue) is called. The UI updates to newValue immediately.
3. Request A resolves. Because it is the most recent request triggered by the source, the reactivity engine updates the resource state to the result of Request A, effectively erasing the mutation.

Verification Steps

To verify this behavior in your specific environment (assuming SolidJS v1.x), you can use a delayed fetcher:

const [data, { mutate }] = createResource(async () => {
  await new Promise(res => setTimeout(res, 2000));
  return "Server Value";
});

// Trigger fetch, then mutate after 1 second
setTimeout(() => mutate("Optimistic Value"), 1000);

If the UI displays "Optimistic Value" for one second and then switches to "Server Value," the pending resolution has overwritten the mutation.

Recommended Mitigation

If you need a mutation to persist regardless of a pending request, you must ensure the source signal is updated or the fetcher is logically bypassed. For complex state synchronization, consider using a createStore to manage the "authoritative" state and using the resource solely for the initial fetch.

Diagnostic Detail Needed: Are you calling mutate as part of a user-driven action (like a form submission) that also triggers a change in the resource's source signal?