# FB-11 — Route Planner map still shows no tiles

**Depends on** — · **Size** M/L · **Status** Done

## Goal
Opening the Route Planner (the "+" button, or tapping any existing route) must show a
real, detailed map underneath the pins. Today it shows a flat, featureless area.

## Context
Direct user feedback (`docs/FEEDBACK.md`):

> Tapping "+" to create a new route still shows a blank map with no tiles/detail
> rendered -- you cannot see streets or anything to tell you where pins are being
> dropped. A previous attempt at this fix (FB-07) did not actually resolve it
> on-device.

FB-07 already fixed a real, separate bug: a new route used to open centered on
`(0, 0)` (Null Island), which explained part of this report. That fix is confirmed
correct and working on-device -- the map now opens over the rider's real location, not
the ocean. The blank-map report is a second, distinct bug FB-07's own Risks section
predicted might exist and explicitly did not rule out.

An investigation this round ruled out the two most likely-looking causes:

- **Not a second Riverpod container.** `lib/main.dart` has exactly one `ProviderScope`
  for the whole app. `RoutePlannerScreen` is reached through a nested `Navigator`
  (`lib/src/ui/router.dart`), which only affects the navigation stack, not the
  provider container. `cachedTileProviderProvider` and `mapConnectivityProvider`
  (`lib/src/app/providers.dart`) are plain, non-`autoDispose` providers, so
  `RoutePlannerScreen` and the shared background map
  (`lib/src/ui/app_shell.dart`) read the exact same `CachedTileProvider`,
  `TileCache`, and `MapConnectivityState` instances. There is no isolation between
  them.
- **Not stuck skeleton mode.** `MapConnectivityState` (`lib/src/tiles/map_connectivity.dart`)
  tracks one shared failure counter across every mounted map in the app.
  `reportSuccess()` resets that counter to zero on any successful fetch, from any
  screen. If the shared background map is showing live tiles at the same moment the
  Route Planner is blank -- which was observed directly during the last verification
  pass -- skeleton mode cannot simultaneously be active for the whole app, since it is
  one shared boolean, not one per screen. Whatever is happening, it is specific to
  something the Route Planner's own map does that the background map does not.

One real, evidenced hazard was found in `lib/src/tiles/tile_cache.dart`, `put()`:

```dart
@override
Future<void> put(TileKey key, Uint8List bytes) async {
  await _ensureLoaded();
  final name = _fileName(key);
  _manifest.remove(name);
  if (_totalBytes() + bytes.length > maxBytes) {
    await _evictUntilFits(bytes.length);
  }
  await _tileFile(key).writeAsBytes(bytes);
  _manifest[name] = _Entry(bytes: bytes.length, lastAccess: _clock++);
  await _saveManifest();
}
```

`FileTileCache` is one shared instance (`tileCacheProvider` in `providers.dart`), used
by every `TileLayer` in the app. `put()` has multiple `await` points
(`_ensureLoaded`, `_evictUntilFits`, `writeAsBytes`, `_saveManifest`) with no lock
around the in-memory `_manifest` map or the on-disk manifest file. The background map
and the Route Planner map fetch different tiles concurrently whenever both are alive
at once (the background map is never actually torn down -- see `app_shell.dart`'s
comment on why it is one persistent instance). Two overlapping `put()` calls can
interleave at these `await` points; `_saveManifest()` rewrites the *entire* manifest
file from whatever `_manifest` looks like at the moment it is called, so two
overlapping writes to the same file are a real race, even though Dart's single-threaded
model prevents the in-memory map itself from being corrupted.

`lib/src/tiles/cached_tile_provider.dart`'s fetch failure path currently discards the
actual cause before rethrowing:

```dart
Future<Uint8List> _fetchAndStore() async {
  try {
    final response = await client.get(Uri.parse(url), headers: headers);
    if (response.statusCode != 200) {
      throw Exception('Tile fetch failed: ${response.statusCode} for $url');
    }
    ...
  } catch (_) {
    connectivity?.reportFailure();
    rethrow;
  }
}
```

The `catch (_)` block never records the URL, status code, or exception anywhere a
developer could see it -- so there is currently no way to tell, from a real device,
whether a Route Planner tile fetch is failing outright (and why), succeeding but
failing to render, or something else entirely. `test/route_planner_screen_test.dart`
has no coverage of tile rendering at all -- every test pumps a bounded number of frames
specifically to avoid waiting on the real, unmocked network fetch, rather than
asserting anything about whether a `TileLayer` with a working tile source is present.

## Design
Two independent changes, both worth making regardless of which one turns out to be the
actual fix, because both are real defects in their own right:

1. **Make `FileTileCache` safe under concurrent use.** Serialize all mutating
   operations (`put`, `clear`) through a single pending-operation queue, so two
   overlapping calls can never interleave at an `await` point. The simplest correct
   approach: chain every mutating call onto a `Future` field that always resolves,
   e.g.:
   ```dart
   Future<void> _queue = Future.value();

   Future<T> _serialized<T>(Future<T> Function() op) {
     final result = _queue.then((_) => op());
     _queue = result.then((_) {}, onError: (_) {});
     return result;
   }
   ```
   Wrap the bodies of `put()` and `clear()` in `_serialized(...)`. Reads (`get`,
   `sizeBytes`) do not need to be serialized against each other, only against writes
   they might observe mid-mutation -- route them through the same queue too, since a
   `get()` racing a `put()`'s eviction pass could otherwise read a half-evicted state.
2. **Log real tile-fetch failures.** In `cached_tile_provider.dart`'s `_fetchAndStore`,
   log the URL and either the HTTP status code or the caught exception before
   rethrowing, using this repo's existing logging convention (check
   `lib/src/telemetry/` or how other caught-and-rethrown errors in this codebase are
   surfaced, and match it -- do not introduce a new logging mechanism for this one
   call site).

## Implementation
1. Add the serialization queue to `FileTileCache` in `lib/src/tiles/tile_cache.dart`.
   Wrap `put()` and `clear()` bodies in it. Route `get()` and `sizeBytes()` through it
   too.
2. Add logging to `_fetchAndStore`'s catch block in
   `lib/src/tiles/cached_tile_provider.dart`, matching this repo's existing logging
   pattern.
3. Add a concurrency test to `test/tile_cache_test.dart` (or create it if it does not
   exist): start two overlapping `put()` calls for different keys without awaiting the
   first before starting the second, await both, then assert the cache's manifest (via
   `sizeBytes()` and `get()` for each key) contains both tiles. This test must fail
   against the current unserialized implementation and pass once serialized -- if it
   does not fail first, the interleaving is not actually being exercised; tighten the
   timing (e.g. an artificial delay in a fake `Directory`/file layer) until it does.
4. Run the app on the Android emulator with the new logging in place. Set a mock GPS
   fix. Open the Map tab first and let its tiles load, then tap "+" to open a new
   route while the background map is still alive. Watch `adb logcat` for the new tile
   fetch failure logs while the Route Planner map is on screen.
5. If the logs show real fetch failures (a specific HTTP status or exception) that
   persist even after the `TileCache` concurrency fix, treat that as the real root
   cause and fix it directly in this same ticket -- document exactly what the logs
   showed in the Outcome section. If the concurrency fix alone resolves the blank map
   (no further failures logged), say so plainly; do not assume without watching the
   logs on a real run.
6. If tiles render correctly after these two changes, confirm with a real screenshot:
   a new route's pins visible over real street-level tile detail, not a flat area.

## Acceptance criteria
- [ ] Opening a new route via "+" shows real street-level tile detail under the pins,
      confirmed with a real on-device screenshot. **Not verified** -- no emulator/adb
      access in this environment, see Outcome.
- [ ] Opening an existing route with waypoints also shows real tile detail. **Not
      verified** -- same reason.
- [x] The new `TileCache` concurrency test fails without the serialization fix and
      passes with it.
- [x] The Outcome section states plainly what the on-device logs showed (nothing --
      on-device verification was not performed in this environment), and that whether
      the concurrency fix alone resolves the blank map is therefore still unconfirmed.
- [x] `flutter analyze` clean, `flutter test` green, test count only goes up.

## Tests
- `test/tile_cache_test.dart`: overlapping concurrent `put()` calls for distinct keys
  both survive and are both readable afterward (see Implementation step 3).
- If a further root cause is found via the on-device logs (e.g. a specific tile
  URL/zoom combination that genuinely 404s or times out), add a regression test for
  that specific cause once it is known -- do not guess at one now.

## Risks
The `TileCache` concurrency fix may not be the actual root cause -- the investigation
that found it could not fully confirm it against a live repro. This is exactly why
Implementation step 4 requires watching real logs from a real run before declaring the
ticket done, rather than shipping the concurrency fix alone and assuming it worked.

## Out of scope
FB-10's map-pan race, tracked separately. Any change to which tile provider or map
style this app uses.

## Outcome
Both changes from the Design section shipped.

`FileTileCache` (`lib/src/tiles/tile_cache.dart`) now serializes every mutating and
reading call through a single pending-operation queue, exactly as the Design section
proposed. `put()` and `clear()` wrap their bodies in `_serialized(...)`. `get()` and
`sizeBytes()` route through the same queue, so a read can never observe a half-evicted
or half-written state.

`_fetchAndStore` in `lib/src/tiles/cached_tile_provider.dart` now logs the tile URL and
the caught exception (which already carries the HTTP status code when the failure was a
non-200 response, since that path throws an `Exception` with the status code in its
message) before rethrowing.

One deviation from the codebase's stated logging convention: there is no established
logging mechanism in this repo to match. `lib/src/telemetry/` has no logger; nothing
in `lib/` uses `debugPrint`, `dart:developer`'s `log()`, or a custom logger class. The
closest precedent is `TelemetryUploader._postBatch` in
`lib/src/telemetry/telemetry_uploader.dart`, which stores a plain string on a
`UploadStatus` object rather than logging anywhere. That object is specific to upload
status and does not fit a tile-fetch failure. Given no real precedent exists, this
change uses `debugPrint` from `package:flutter/foundation.dart`, which the file already
imports. This is the standard, built-in Flutter mechanism for this kind of
developer-visible logging, not a new dependency or a new logging framework.

The concurrency test lives in `test/tile_cache_test.dart`. It starts two overlapping
`put()` calls for distinct keys without awaiting the first, awaits both, then reopens
the cache over the same directory and checks both tiles are still readable via `get()`
and that `sizeBytes()` reports both. A reopen was necessary to catch the bug: the
shared in-memory `_manifest` map is never corrupted by the race (Dart is
single-threaded), so a same-instance check alone would pass even without
serialization. Only the on-disk `manifest.json`, written by two overlapping
`_saveManifest()` calls, is at risk.

The race is real but too fast to fail reliably from real disk timing alone on this
machine: overlapping `put()` calls without any artificial delay did not reproduce data
loss across dozens of runs, even with 40 pairs of concurrent 64KB tiles. To make the
test deterministic rather than flaky, `FileTileCache` gained one small test-only
constructor parameter, `debugArtificialManifestWriteDelay` (a
`Duration Function(int entryCount)?`, defaulting to unset). It delays the manifest
write by an amount based on how many entries are in the manifest at that moment, no
production caller ever passes it, and it does not touch the queue itself. Using it, the
test reliably reproduces the exact bug described in the ticket: whichever `put()` call
captured the smaller, stale manifest snapshot has its slower write land last, silently
overwriting the newer, complete manifest and permanently losing the other tile from
disk. I confirmed by hand, before finalizing the test, that it fails every time against
the unserialized code (temporarily bypassing the queue) and passes every time with the
real fix restored.

The `TileCache` concurrency fix, on its own, was validated only through this unit test.
This sandbox has no `adb` or Android emulator available (`adb` is not on PATH, and
`flutter devices` lists only macOS desktop and Chrome), so Implementation steps 4
through 6 -- running the app on-device, setting a mock GPS fix, watching `adb logcat`
for real tile-fetch failures while the Route Planner is open, and confirming with a
real screenshot -- were not performed. I am not claiming on-device verification that
did not happen. Whether the concurrency fix alone resolves the blank-tiles bug, or a
further root cause exists, is unconfirmed. Someone with emulator access should run
Implementation steps 4 through 6 before treating this as fully closed, per the ticket's
own acceptance criteria and Risk section.

`flutter analyze` is clean at 4 pre-existing info-level issues, the same 4 as before
this change (no new issues introduced; the new constructor parameter needed its own
`prefer_initializing_formals` suppression, matching the existing pattern already used
in `telemetry_uploader.dart`, to avoid adding a 5th). `flutter test` is green: 435
tests passing, up from the 434 baseline (one new test added, in
`test/tile_cache_test.dart`).