fix(svelte-query): synchronize observer subscription lifecycle during restoration

[VedAnt-1004]

Description

The Problem

In @tanstack/svelte-query, the observer subscription lifecycle in createBaseQuery.svelte.ts had a subtle race condition during cache restoration and hydration boundaries.

Previously, the subscription setup relied on an $effect with an early return:

$effect(() => {
  const unsubscribe = isRestoring.current
    ? () => undefined
    : observer.subscribe(() => update(createResult()))
  observer.updateResult()
  return unsubscribe
})

Because $effect in Svelte 5 runs post-commit and dependency tracking did not automatically tear down and re-run when isRestoring.current transitioned from true to false, intermediate states were missed when restoration finished. To patch over this behavior, an extra watcher workaround had been introduced:

// The only reason this is necessary is because of `isRestoring`...
watchChanges(() => [resolvedOptions, observer], 'pre', () => {
  update(createResult())
})

This approach had two drawbacks:

1. It left an unnecessary pre-flush watcher running on every options/observer change solely to catch a restoration state flip.
2. Any query state updates that resolved between the initial component render and the post-DOM effect flush could desynchronize or miss the intermediate restored state before the fetch completed.

---

What Changed & How It's Fixed

1. Reactive Subscription Lifecycle in $effect:
   Instead of returning a no-op cleanup, the effect directly checks isRestoring.current and tracks it reactively. When isRestoring flips to false, Svelte automatically re-runs the effect, registers the subscription, and returns a clean unmount closure:

$effect(() => {
  if (isRestoring.current) {
    return
  }

  const unsubscribe = observer.subscribe(() => {
    update(createResult())
  })

  // Catch state settled between render and effect commit
  update(createResult())
  observer.updateResult()

  return () => {
    unsubscribe()
  }
})

2. Immediate State Catch-up:
   Calling update(createResult()) right as the subscription attaches ensures any state updates, prefetched entries, or restored cache data are surfaced to the component immediately upon subscription.

3. Removed the Workaround Block:
   Completely removed the secondary watchChanges(() => [resolvedOptions, observer], ...) workaround, keeping observer synchronization logic in one place.

4. Guaranteed Teardown:
   Cleanly returns () => { unsubscribe() } so listeners are always dismantled whenever isRestoring toggles, dependencies update, or the component unmounts.

---

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

---

Testing

• Regression Test Added:
  • Added tests/createQuery/IsRestoringDynamic.svelte and a companion test case in tests/createQuery/createQuery.svelte.test.ts to test dynamic transitions of isRestoring from true to false.
  • Confirmed the query remains idle and pending while restoration is active, attaches the subscription as soon as isRestoring flips to false, and transitions smoothly from fetching to success without missing intermediate data.
• Test Suite Results: Ran pnpm --filter @tanstack/svelte-query run test:lib run — all 216 tests passed across 23 test suites.
• Type Checking: Ran pnpm --filter @tanstack/svelte-query run test:types (svelte-check) — 0 errors.
• Linting: Ran pnpm --filter @tanstack/svelte-query run test:eslint — 0 errors.
• Changeset: Added a patch changeset for @tanstack/svelte-query.

---

Checklist

• I have read the CONTRIBUTING.md guide.
• I have added unit tests covering the fix.
• All local tests, typechecks, and linters pass.
• A changeset has been added (pnpm changeset).

Summary by CodeRabbit

• Bug Fixes

  • Fixed Svelte Query restoration handling so queries subscribe and fetch correctly after restoration completes.
  • Prevented premature fetching while restoration is still in progress.
  • Ensured query status, fetch status, and data update correctly after restoration transitions.

• Tests

  • Added coverage for dynamic restoration-state transitions and post-restoration fetching.

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📥 Commits

Reviewing files that changed from the base of the PR and between db42d36 and 3c80118.

📒 Files selected for processing (1)

• packages/svelte-query/src/createBaseQuery.svelte.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• packages/svelte-query/src/createBaseQuery.svelte.ts

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---

📝 Walkthrough

Walkthrough

createBaseQuery now refreshes observer results inside Svelte’s untrack, preventing resolvedOptions from becoming a subscription effect dependency. New tests verify fetching after dynamic restoration ends.

Changes

Restoration subscription lifecycle

Layer / File(s)	Summary
Observer lifecycle update packages/svelte-query/src/createBaseQuery.svelte.ts, .changeset/shiny-walls-battle.md	The observer uses an untracked result refresh for subscription callbacks and post-subscription updates. Existing restoration handling remains unchanged. A patch changeset records the fix.
Dynamic restoration coverage packages/svelte-query/tests/createQuery/IsRestoringDynamic.svelte, packages/svelte-query/tests/createQuery/createQuery.svelte.test.ts	The test component exposes query state with a mutable restoration ref. The test verifies no fetch during restoration and one successful fetch after restoration ends.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the observer subscription lifecycle fix during restoration. It is concise, specific, and matches the main changes.
Description check	✅ Passed	The description clearly explains the problem, implementation, regression tests, validation results, and changeset. It does not use the template's exact headings and omits a separate Release Impact sec…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Create a new PR

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Actionable comments posted: 1

---

• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
In `@packages/svelte-query/src/createBaseQuery.svelte.ts`:
- Line 96: Update the subscription synchronization around createResult and
update to use Svelte’s untrack, including the observer callback and the initial
post-subscription refresh, so resolvedOptions is not captured as an effect
dependency. Introduce a shared refreshResult helper and pass it to
observer.subscribe while preserving the existing result-update behavior.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository: TanStack/query/.coderabbit.yaml

Review profile: CHILL

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Run ID: 5ad434d5-ff32-4c1a-a2f9-623c5025ddac

📥 Commits

Reviewing files that changed from the base of the PR and between 2a49597 and db42d36.

📒 Files selected for processing (4)

• .changeset/shiny-walls-battle.md
• packages/svelte-query/src/createBaseQuery.svelte.ts
• packages/svelte-query/tests/createQuery/IsRestoringDynamic.svelte
• packages/svelte-query/tests/createQuery/createQuery.svelte.test.ts

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