Document device communication and bulletproofing findings

DEVICE-COMMUNICATION.md: exhaustive reference for every phone<->watch
exchange - triggers, wire datapoints, transports (T1-T4), and the internal
structures each side mutates - plus end-to-end sequences and failure modes.

BULLETPROOFING.md: ranked gap analysis (H1-H3 session lifecycle, M1-M4
stranding/stale-UI traps, L-tier) with code evidence, failure scenarios,
fix directions, and a recommended fix order.

Claude-Session: https://claude.ai/code/session_01PVNBVKp5bcq52X722uMjwT

BULLETPROOFING.md

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+# Bulletproofing the Watch/Phone UX — Findings
+
+An adversarial gap analysis of the iPhone ↔ Apple Watch communication layer, aimed at making the cross-device workout UX bulletproof. Architecture reference: `DEVICE-COMMUNICATION.md` (transports T1–T4, exchanges P1–P9 / W1–W4). Analysis date: 2026-07-09.
+
+Findings are ranked by user impact. Each carries code evidence, the concrete failure scenario, and a fix direction. Statuses: **open** / **fixed** (update in place as items land).
+
+---
+
+## What's already solid (verified)
+
+Re-verified against code, not just prior docs — these need no work and should not be re-litigated:
+
+- **Per-log merge** (`WorkoutMergePlanner`) — unordered T3/T4 delivery and concurrent edits to different exercises commute; phone-authored `deletedLogIDs` tombstones disambiguate absent logs.
+- **Tombstone veto** (`SyncEngine.ingestFromWatch`) — a stale watch push can never resurrect a deleted workout; authoritative state is re-pushed instead.
+- **Latest-wins context + authoritative prune** (`WatchCacheApplier`) — "absent ⇒ no longer active" reliably ends the watch session on phone-side delete/discard, and a nil-decode can never wipe real rows.
+- **Launch-race guard** (`SessionEndPlanner.decide`: `!previouslyActiveIDs.isEmpty`) — a freshly launched session is never killed just because its run document hasn't synced yet (empty-baseline case).
+- **Timer hardening** — countdown/haptic logic is wall-clock anchored on both platforms; sleep and Always-On drift self-correct without double-firing.
+- **Pop-on-prune** (`ActiveWorkoutGateView.popIfNavigatedRunUnavailable`) — deletion/discard of the navigated run, or a phone edit lock, pops the watch back to the gate.
+
+---
+
+## HIGH — the session lifecycle has a single point of failure
+
+### H1 — The `HKWorkoutSession` can only ever be born via the T1 one-shot  ·  open
+
+`WorkoutSessionManager.start(with:)` has exactly one caller: `WatchAppDelegate.handle(_:)` (`WatchAppDelegate.swift:31`), i.e. the phone's `startWatchApp(toHandle:)` handoff. That handoff is best-effort `try?` with **no retry** (`WorkoutLauncher.swift:37-40`). There is no `HKHealthStore.recoverActiveWorkoutSession` anywhere, no `WKExtendedRuntimeSession`, and nothing gates run-flow entry on `sessionManager.isRunning`.
+
+Failure scenarios (all real):
+
+1. **T1 fails silently** (watch briefly unreachable, Bluetooth blip, watch on charger at start) → the entire workout runs sessionless: no HR/energy, no Health save, no wrist-raise resurfacing, and the watch app **fully suspends** wrist-down — rest haptics and done-countdowns don't fire until the user manually reopens the app. The user gets no hint anything is wrong.
+2. **Watch app crash or reboot mid-run** → watchOS expects the app to reclaim the live session via `recoverActiveWorkoutSession`; unimplemented, so the session is orphaned — metrics lost, zombie session may linger.
+3. **User manually opens an active run on the watch** (run arrived via T2; T1 never fired for this device state) → same sessionless degradation.
+
+**Fix direction:** make the watch own its session lifecycle. Self-start a session when entering (or reconciling into) an active run with `isRunning == false`; implement session recovery at launch. T1 then becomes a fast-launch optimization rather than the sole source of sessions. This also absorbs most of H2 and much of the M-tier degradation.
+
+### H2 — "End Current & Start New" swallows the new run's session  ·  open
+
+`endActiveThenStart` (`WorkoutLogsView.swift:271-281`) runs the end-saves in a `Task` but calls `start(with:)` synchronously — so **T1 goes out while the old run's session is still running on the watch**. `handle(_:)` → `start(with:)` hits the `session == nil` idempotency guard (`WorkoutSessionManager.swift:61`) and is **silently dropped**. Moments later the "A completed" push arrives and the coordinator finishes A's session — leaving run B sessionless (all of H1's fallout). The one-shot was consumed while blocked; nothing replays it.
+
+Timing variant: if B's session *does* start (old one already ended) but the transient "A completed, B not yet present" push applies next, `SessionEndPlanner.decide` sees `isRunning`, empty active set, non-empty baseline `{A}` → `.finish(A)` — **ending B's seconds-old session and attributing its metrics to A**. The empty-baseline launch guard doesn't cover a *non-empty stale* baseline.
+
+**Fix direction:** on the watch, when `handle(_:)` arrives while a session is running, stash the configuration and start it immediately after the current session finishes; and/or a short grace window after `session.startActivity(...)` during which `decide` refuses `.finish`/`.discard`. H1's self-start largely absorbs this too.
+
+### H3 — A system-ended session silently drops the Health save  ·  open
+
+`workoutSession(_:didChangeTo:)` for `.ended`/`.stopped` only calls `clear()` (`WorkoutSessionManager.swift:199-202`) — no `endCollection`/`finishWorkout`, no metrics capture, no forward to the phone. `PLAN-watch-session-end.md` lists this as a known residual. Rare (OS reclaim under resource pressure), but it is a "my workout never made it to Health" support report waiting to happen.
+
+**Fix direction:** on a system-driven `.ended`, attempt `finishAndSave()` (or at minimum salvage the builder's collected data) before clearing, and forward whatever metrics exist.
+
+---
+
+## MEDIUM — stranding and stale-UI traps
+
+### M1 — Edit locks have no expiry and outlive their editor  ·  open
+
+`editingWorkoutID` / `editingSplitID` ride in the latest-wins context and are cleared only by `onDisappear` (`ExerciseView.swift:78`, `SplitDetailView.swift:55`). Force-quit the phone with an editor open — or just leave it open in a pocket — and the **last-pushed context says "editing" indefinitely**; the watch parks the run ("Editing on iPhone…") until the phone app next runs `pushAll`. Compounding it, `SplitDetailView` is a mostly-*read* screen: merely **viewing** a split in Settings parks any active watch run sourced from it (`SplitDetailView.swift:54`).
+
+**Fix direction:** clear the locks on phone `scenePhase` → background/inactive; scope the split lock to the actual edit sheets rather than the detail screen.
+
+### M2 — Remote completion doesn't pop the watch's open exercise screen  ·  open
+
+`popIfNavigatedRunUnavailable` pops on prune and on lock — but a run flipping to `.completed`/`.skipped` remotely **stays in the cache** (recently-completed runs are still pushed for ~24 h), so no pop fires. `repairFromDurable`'s safety net is gated on `status == .inProgress`, going silent exactly when needed. And the watch's `ExerciseProgressView` lacks the phone's `.skipped` handling, so a user sitting on a stale open exercise can keep ticking and write it back to a **non-terminal status** — resurrecting a skip through the per-log merge, since their edit is "newer".
+
+**Fix direction:** pop (or overlay a terminal state) when the navigated run leaves the active set, not only when it leaves the cache; port the phone's `.skipped` terminal handling to the watch's progress view.
+
+### M3 — Follower mirror cover freezes in auto-advance splits  ·  open
+
+The live-mirror cover (`LiveRunCoverView`, both platforms) is not a passive display — it runs the same local phase engine as a real driver, seeded from the last frame's anchors, and keeps advancing if the driver goes silent. Single-exercise runs self-heal (Finish auto-Done dismisses). But the cover is wired with `onAdvance: nil`, so in a flow-mode (auto-advance) split, when its local chain reaches the terminal between-exercise rest it completes the exercise (durable data stays correct) then **freezes at a 0:00 countdown** — the exercise hand-off exists only in `RunFlowView`, which the cover doesn't use. Related: if the driver dies without `sendLiveEnded`, the follower has **no staleness timeout** at all.
+
+**Fix direction:** give the cover a terminal-rest hand-off (advance or dismiss), and add a staleness timeout that dismisses the cover when no frame arrives past `phaseEnd` + grace.
+
+### M4 — `updateApplicationContext` size ceiling; failure is log-only  ·  open
+
+`pushAll` sends *all* splits plus all active and ≤25 recent-completed runs with their full logs. A heavy user (many splits × many exercises; long runs with `setEntries`) can plausibly exceed WatchConnectivity's ~65 KB context limit. On throw, the error is logged (`PhoneConnectivityBridge.swift:121-124`) and **the watch silently never hears about anything again** — functionally the same total-freeze as the schema-mismatch trap, but reachable in production.
+
+**Fix direction:** on payload-too-large, degrade in tiers (drop completed runs, then trim embedded logs to what the watch renders) and re-push; surface the condition in Diagnostics.
+
+---
+
+## LOW
+
+### L1 — Nondeterministic metrics attribution with parallel runs  ·  open
+
+`SessionEndPlanner.decide` picks the completed survivor by iterating `previouslyActiveIDs` — a `Set`, so **iteration order is undefined** (`WorkoutSessionCoordinator.swift:49-51`). With two parallel runs finishing together, the `HKWorkout`'s metrics attach to an arbitrary one. Deterministic tie-break (e.g. most-recent `end`) would fix it.
+
+### L2 — `requestSync` failures vanish  ·  open
+
+`sendMessage(WCPayload.requestSyncMessage(), replyHandler: nil, errorHandler: nil)` (`WatchConnectivityBridge.swift`) — a failed pull is dropped with no retry. Low because P8 (activation/reachability edges) and the durable T2 slot cover the common cases.
+
+### L3 — Decode-failure freeze is silent on the watch  ·  open
+
+The schema-mismatch trap (see `DEVICE-COMMUNICATION.md` §6) is dev-only today, but the watch could surface "can't read phone data — update both apps" instead of freezing silently; it already tracks `lastSyncDate` and logs the decode failure.
+
+### L4 — Possible log-progress rollback from a late follower write  ·  open · needs device testing
+
+Both the driver and a follower cover persist auto-advance progress via whole-log newest-`updatedAt`-wins merge (not a monotonic max on progress). A follower resuming late from suspend could in theory regress a log if its delayed write's timestamp outranks the driver's more-advanced state before the durable echo catches it up. Unconfirmed on device.
+
+---
+
+## Recommended order
+
+The common theme of every HIGH: **the watch doesn't own its session lifecycle — it's a passive recipient of a fire-and-forget launch.** Fix in this order:
+
+1. **H1** — watch self-starts + recovers its session (one coherent change in `WorkoutSessionManager` / coordinator / run-flow entry; collapses most of H2, part of M-tier).
+2. **H2** — pending-config replay and/or post-start grace window in `SessionEndPlanner`.
+3. **H3** — salvage the Health save on system-ended sessions.
+4. **M1** — lock expiry on scene background; narrow the split lock.
+5. **M4** — tiered payload degradation + Diagnostics surfacing.
+6. Then M2, M3, and the L-tier as opportunity allows.