MirrorSyncClient promised that reads consult both backends "so the
cutover order is irrelevant", but every read called self.primary
unguarded. A Firebase outage therefore produced no data at all and the
GitHub mirror was never consulted -- the union read silently degraded to
nothing, precisely when the fallback existed to help.
This is not hypothetical: screen-locker's workout sync hit it on
2026-08-10, where a DNS failure reading the primary took down the whole
locker rather than falling back.
Reads (list_directory, get_file_text, get_string_map) now degrade to the
mirror when the primary raises, and only fail when NEITHER backend can
answer -- returning an empty result would be indistinguishable from "no
data", which callers act on. Writes keep the old asymmetry on purpose: a
write the primary did not accept has not happened, so it must still fail
the tick.
A mirror that cannot bulk-read (GitHub) contributes nothing but is not a
failure, so it cannot by itself make a revision-map read raise. Both
language ports agree on that.
Same fix in the Dart port, which had all three read paths wrong too and
runs on the phone. Two older tests asserted the buggy read-fail-closed
behaviour and are renamed to describe what they actually exercise: the
both-sides-failed case.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JS5V44jkHZiH1wC7BH5h5n
Extracts the three-method remote surface GitHub was reached through into a
`RemoteStore` interface, then implements it against Firebase Realtime
Database over REST, so apps can move backend with a constructor change.
Why REST and not the SDKs: official FlutterFire has no Linux desktop
support, and `wake_alarm`'s PC side is a dependency-free systemd job. The
RTDB REST endpoints are plain HTTPS and behave identically on Linux,
Android and headless. Firebase is a transport, never the datastore -- each
app keeps its local log as the source of truth, so offline-first survives.
Why RTDB and not Firestore: on the Spark plan RTDB bills only storage and
bandwidth with no per-operation quota, so a runaway loop cannot exhaust a
daily budget and silently stop syncing mid-day.
Two savings make the no-cost tier a guarantee rather than a hope. Measured
against the GitHub-backed history this replaces: 4894 of 5542 commits
(88.3%) were byte-identical no-ops, and every tick re-downloaded every
peer's whole log. Both are now driven by one sha256 revision -- an
unchanged log is not re-pushed, and an unchanged peer is not re-fetched.
That turns egress from "ticks x log size" (~700 MB/month, and growing with
the log) into "edits" (~40 MB/month against a 10 GiB cap).
Revisions are one node per device, written only by its owner, rather than
one shared map -- a whole-map write would erase every peer's entry, after
which those peers would look permanently unchanged and never be fetched
again. Per-device keys make that impossible rather than merely avoided.
`.` is illegal in an RTDB key and the REST `.json` is a format suffix, so
paths get a reversible `~XX` escape (`log.json` -> `log~2Ejson`). Lossy
extension-stripping would break `todo`, whose listings return filenames.
`MirrorStore` / `MirrorSyncClient` dual-writes during the cutover. Reads
consult *both* backends: with primary-only reads a migrated device would
never see an un-migrated peer's writes while still mirroring its own back
-- silent one-directional convergence with no error raised. Writes are
asymmetric on purpose: a primary failure fails the tick, a mirror failure
is logged and survived.
Auth is Firebase email/password over identitytoolkit REST, with mandatory
refresh-token rotation. No service-account key anywhere: it would bypass
the security rules and is trivially extractable from an APK.
Verified live against a real database, not only mocks: sign-in and refresh,
key escaping round-tripping byte-for-byte, an unchanged tick writing
nothing, two devices converging, a revoked session throwing rather than
returning null, and a log written by the Python client being read by the
Dart one.
No app is migrated by this commit; every app still runs on GitHub.
Dart: 194 tests. Python: 203 tests at 100% branch coverage. No lint
suppressions added.
