7.x stored every object branch-scoped under branches/<branch>/<basePath>; 8.0
stores the IDENTICAL entity structure at the storage ROOT plus the generational
_system/_generations layer. GenerationStore.open tolerates a missing manifest
(opens at gen 0), so a naive 8.0 open of a 7.x directory reported ZERO entities
and SILENTLY LOST ALL DATA. The fix is blocked in the normal init order — cor 3.0's
storage-factory legacy guard fires inside setupStorage(), five steps before the
old migration check — so a new phase runs BEFORE setupStorage().
legacyLayoutMigrationPhase() (init, between loadPlugins and setupStorage):
- Fast-path: returns immediately when there's no top-level branches/ dir (every
native 8.0 brain + fresh dir pays only one fs.existsSync on the init hot path).
- Runs on a temporary BUILT-IN FileSystemStorage (never the plugin adapter, whose
guard would throw). Memory/cloud/pre-built-adapter stores are a strict no-op.
- Detect via _system/migration-layout.json marker + branches/<head>/entities/.
- autoMigrate:false on a legacy layout THROWS explicit guidance (no silent loss).
- Acquire the writer lock, then COLLAPSE branches/<head>/entities/* → entities/*
via the .gz-transparent raw primitives (read→write→delete, idempotent/resume-safe;
NOT fs.rename — logical paths + memory-adapter incompatibility).
- Rebuild persisted counts (rebuildCounts → totalNounCount/counts.json;
rebuildTypeCounts/rebuildSubtypeCounts → per-type stats) — the three indexes are
rebuilt by the normal init's rebuildIndexesIfNeeded afterward.
- Finalize: stamp the flat-v8 marker (re-open no-op), drain the head branch
(non-head branches = 7.x version history 8.0's MVCC does not import; left as-is).
Tests (tests/integration/migration-7x-to-8x.test.ts): the data-loss LOCK (a
built-in FileSystemStorage on a reshaped 7.x dir reports 0 nouns), a byte-equal
ROUND TRIP (find/related/counts equal the pre-reshape reference), IDEMPOTENCY,
the autoMigrate:false GUARD throw, and the native-flat NO-OP. RELEASES upgrade
checklist rewritten (the old note documented the opposite, lossy behavior).
1477 unit + migration 5 + db-mvcc 25 green; no init-path regression.
Follow-ups (hardening, not correctness for the common case): backupTo config
plumbing (currently a loud warning), a crash-mid-collapse resume test, and a
boundary-safe fake-plugin ordering test.
