bsky-social-app/plans/current-review.md

Review: async OnSessionChange on session-sketch

Review performed by Fable against the current session-sketch working tree. All 117 focused session and persistence tests passed as a baseline.

Context

What changed relative to main:

• On main, makeSessionHooks.dispatch is synchronous and ignores onSessionChange's result. SessionStore.dispatch persists fire-and-forget with void persisted.write('session', ...).
• On this branch, dispatch is async and awaits onSessionChange (session-core.ts:161-191). That awaits store.dispatch(...), which awaits persisted.writeSession(...) (index.tsx:116-141), and may then perform a second dispatch to rebuild against the committed generation (index.tsx:296-327). Failures land in a per-bundle WeakMap side channel (session-core.ts:71,88-96,178-190) consumed only by refreshSession (index.tsx:655-660).
• PasswordSession awaits its hooks inside the #sessionPromise chain. Its refresh() implementation awaits onUpdated(newSession) before assigning #sessionData = newSession. The app-level persistence pipeline therefore now sits inside the session promise that every fetchHandler call awaits.
• Commit 732cf8cdd narrowed the lock: instead of holding a credential lock around the whole onSessionChange body, only writeSession()'s read-merge-write-broadcast holds the lock (persisted/index.web.ts:111-135, with a synchronous callback). Native uses its FIFO queue (persisted/index.ts:120-131).

Actual bugs

B1 - resumeSession post-persistence continuations lack identity and abort rechecks

Severity: Medium

resumeSession (index.tsx:509-591) checks signal.aborted and the account entry before dispatching switched-to-account. After await store.dispatch(...) at line 542, however, it performs follow-up dispatches without rechecking either:

• index.tsx:560-566: synced-accounts with the potentially stale committedSession.
• index.tsx:568-579: replaced-current-bundle with a newly built, armed bundle.

By comparison, onSessionChange guards equivalent branches with store.getState().currentBundleState.bundle === bundle (index.tsx:304,319-320), and the cross-tab listener uses a shouldActivate guard (index.tsx:764-790). resumeSession has neither.

Concrete interleaving:

1. resumeSession(X) passes its guards and dispatches switched-to-account(X). The reducer commits bundle_X synchronously, write W_X is enqueued, and the function suspends awaiting W_X.
2. The user logs out. logoutCurrentAccount runs synchronously, calls cancelPendingTask() and aborts the resume signal, clears X's tokens, and switches currentBundleState to the public bundle. The UI is logged out. W_logout is queued behind W_X.
3. W_X resolves. Its merge ran before W_logout, so it did not see the tombstone. If another tab meanwhile rotated X, or the resume lost a generation race, committedAccount.refreshJwt !== account.refreshJwt, so the replacement branch runs.
4. replaced-current-bundle installs a live, armed X bundle while preserving the reducer's current DID, which is now undefined. It also restores X's credentials in the reducer account entry.

The result is hasSession === false while useAppviewClient() and usePdsClient() expose X-authenticated clients: a logged-out UI can make authenticated requests. It heals only if the zombie bundle later emits another update and reconciliation encounters the tombstone.

The sibling synced-accounts branch has a milder version: a stale committed snapshot can clobber an account that logged in during the await. Storage remains correct, but same-tab writes do not loop back through onUpdate, so the reducer may not immediately heal.

The window is one storage write, but resumeSession is also driven by cross-tab notifications. This is new branch behavior because main had no asynchronous post-dispatch continuation here.

Minimal fix: After the awaited dispatch, bail out if either:

signal.aborted ||
(store.getState().currentBundleState.bundle as unknown) !== (bundle as unknown)

If a replacement bundle has already been built before bailing, dispose it. This mirrors the discipline already used in onSessionChange.

Test: Gate writeSession on a manually resolved promise, interleave resumeSession with logoutCurrentAccount, then assert that no stale synced-accounts or replaced-current-bundle continuation lands and any unused rebuilt bundle is disposed.

B2 - Persistence-failure divergence can downgrade credentials and force logout instead of self-healing

Severity: Medium-Low

On main, a failed session write left storage stale, but the next dispatch rewrote the complete session unconditionally. Storage therefore healed on the next refresh. On this branch, the jti-chained merge (session-merge.ts:171-200) cannot distinguish another tab advancing the chain from this tab losing its own previous chain-link write.

Concrete sequence:

1. At t0, automatic refresh rotates generation 0 to generation 1. The reducer commits generation 1, but writeSession fails because of quota, private-mode behavior, or another storage error. The failure is recorded in the bundle WeakMap, but no automatic-refresh caller consumes it. Storage remains at generation 0 while the live session is generation 1.
2. At t1, the next automatic refresh rotates generation 1 to generation 2. Its mutation has generation 1 as the base. Authoritative storage still contains generation 0, so applySessionUpdate drops the mutation.
3. The committed account contains generation 0 credentials. Because those differ from the refreshed generation 2 credentials, onSessionChange rebuilds the live bundle from generation 0, actively discarding valid generation 2 credentials.
4. If generation 0 remains valid in the PDS refresh grace chain, a later refresh may self-heal at the cost of extra round trips. Outside that grace period, generation 0 expires and the app logs out, whereas main would have retained generation 2 in memory and healed storage during the next full write.

This is not a lock race; it is a false positive in the deliberate conditional merge. The trigger is narrow: a storage-write failure while the app remains alive, followed by enough idle time for the old generation to leave the server grace window.

Recommendation: Accept and document this as a tradeoff unless a reliable distinction can be made. Add a distinctive log when a refresh mutation is dropped and its result jti differs from the stored one, since that identifies credentials being discarded.

Tests:

• Unit-test applySessionUpdate for a missing intermediate chain-link.
• Provider-test the committed-generation mismatch path that rebuilds onto the older generation, so the downgrade remains explicit and visible.

B3 - Cross-tab listener's void resumeSession(...) has new unhandled-rejection paths

Severity: Low

At index.tsx:740, the cross-tab listener calls:

void resumeSession(syncedAccount)

The pattern existed on main, where the resume factory was its main rejection source. This branch adds persistence rejection because resumeSession now awaits store.dispatch, which awaits writeSession. A storage failure can therefore become an unhandled promise rejection.

Minimal fix: Attach .catch(...) and log the failure.

B4 - Login-shaped methods reject after committing reducer state

Severity: Low

login, createAccount, and partialRefreshSession synchronously commit reducer state and then await persistence:

• index.tsx:353-370: login.
• index.tsx:394-411: account creation.
• index.tsx:608-620: partial session refresh.

If writeSession rejects, the method rejects after the UI has already switched to the authenticated state. This can produce an error from a login form behind or over an already signed-in app. The account:create:success or account:loggedIn metric is also skipped even though the in-memory session exists.

On main, persistence was fire-and-forget. Logout and removal currently make the opposite policy choice by explicitly swallowing persistence rejections with .catch(() => {}), so behavior is asymmetric.

This requires genuinely broken storage, such as quota exhaustion or a browser security error.

Recommendation: Choose and document one policy:

1. swallow and log, matching logout/removal; or
2. deliberately reject, with callers treating "session established but durability failed" as success with a warning rather than a failed login.

Intended but consequential behavior

T1 - Requests wait for persistence during refresh

PasswordSession.fetchHandler awaits #sessionPromise on entry. That promise now resolves only after:

network refresh
  -> optional getSession backfill
  -> onSessionChange
  -> reducer
  -> writeSession
  -> possible committed-generation rebuild

Normally this adds only a small storage delay. In the worst case, wedged AsyncStorage or a large shared root blob stalls all in-flight and new requests, whereas main only blocked on refresh network work.

The lock design itself remains sound: the web lock callback is synchronous, so no Web Lock is held across suspension and no nested-lock deadlock was found. This is latency coupling to storage health, not a locking defect.

T2 - refreshSession can reject after a successful server refresh

Two cases:

• Persistence failure: Tokens rotated server-side and entered reducer state, but takeSessionChangeError rethrows the persistence failure. Callers report failure for a live in-memory session. This is defensible because durability failed, but it is new and currently untested.
• A cross-tab generation wins during refresh: Reconciliation rebuilds the bundle, then the bundle-identity check throws The session changed while it was being refreshed even when the account is the same and healthy. The identity check is pre-existing, but committed-generation rebuilds make same-DID bundle replacement more common.

Add a refreshSession test with a rejecting writeSession to pin the intended contract.

Verified non-issues

• WeakMap error-channel races: Hook dispatches for one bundle are serialized by PasswordSession.#sessionPromise. Two concurrent explicit refreshes cannot realistically consume each other's errors. Per-bundle keys prevent cross-bundle contamination.
• Hook failures bricking the session: dispatch catches synchronous and asynchronous onSessionChange failures, preventing #sessionPromise from becoming permanently rejected. Existing tests cover this.
• Concurrent refresh behavior: Automatic refresh deduplication still works. Concurrent explicit refreshes can perform two serialized rotations, but that is pre-existing and harmless apart from extra work.
• Expiry rescue: Bundle/DID guards, the bounded failed-generation set, and reducer identity guards prevent stale bundles from logging out or mutating the active account. Rescue dispatches do not persist and therefore cannot self-deadlock.
• Locking: writeSession's web lock callback contains no await; no reentrancy, frozen-tab lock retention, or lock-ordering defect was found. Native queue order is likewise FIFO. Logout-versus-refresh, rotation-versus-rotation, and tombstone-versus-late-update interleavings converge through the jti merge and reducer guards.
• Disposal and switching: Post-commit bundle disposal, reducer identity checks, and fetch kill switches prevent replaced bundles from consuming later refresh generations. Killed in-flight retries fail loudly rather than silently authenticating.
• baseRefreshJwt capture: Update events read the pre-commit live getter, while expired events use the payload. Both identify the correct base generation, including chained rotations.
• Cross-tab listener closure: The listener resubscribes as state changes, and shouldActivate rereads live store state. The worst stale result found was an unnecessary rebuild that activation then rejects.

Pre-existing issues, not regressions from this branch

• A definitively dead PasswordSession.refresh() can reject with a raw xrpcSafe response object rather than an Error.
• emitSessionDropped runs before the logout dispatch. A throwing listener can skip reducer logout; the hook catches and logs it, but does not redrive logout. The ordering matches main.
• Cross-tab metadata clobbering and the no-Web-Locks lost-update fallback are already documented in plans/versioned-localstorage-session-tangents.md.
• createTemporaryClientsAndResume may rotate a token before logout without persistence hooks; this matches main.

Recommended order

1. Fix B1 by rechecking abort and bundle identity after the awaited resume commit.
2. Fix B3 by catching and logging the cross-tab listener's resumeSession rejection.
3. Decide and document the B4 persistence-failure policy for login-shaped methods.
4. Add tests for:
   • resume versus logout while persistence is pending;
   • refreshSession with rejecting writeSession;
   • a dropped refresh mutation followed by committed-generation regression.
5. Accept and document B2 unless a safe distinction between cross-tab advancement and a missing local chain-link emerges; add a distinctive dropped-generation log.

Verdict

The core async design is sound. Serialization through PasswordSession.#sessionPromise, the per-bundle error channel, arm/kill lifecycle, reducer identity guards, and internally owned persistence lock compose correctly in the reviewed interleavings.

The one genuine state-corruption hole is B1: resumeSession performs unguarded asynchronous continuations after persistence. B2 is a knowable conditional-merge tradeoff; B3 and B4 are lower-severity error-policy issues.