The temporal model had a hole exactly where files were concerned: every
entity write is an immutable generation with before-images, but VFS content
BYTES lived under an eager refCount GC left over from the pre-8.0 design —
unlink could physically destroy bytes that in-window history still
referenced, and overwrite never released the old hash at all (an unbounded
silent leak whose accidental byproduct was the only thing "preserving"
history). Reading the past could therefore return a stale field, a dangling
hash, or nothing, depending on luck.
Fix: blob reclamation becomes a HISTORY decision instead of a LIVENESS
decision. Each blob's metadata now carries historyRefCount alongside the
live refCount:
- The commit seam counts one history reference per persisted before-image
record carrying a content hash (commitTransaction staging and the
group-commit flush), recorded BEFORE the record-set persists and carried
in the generation delta (blobHashes — always present on new deltas, so
compaction only falls back to reading records for pre-contract
generations). An aborted transaction compensates best-effort.
- unlink/rmdir/overwrite drop ONLY the live reference (BlobStorage.delete →
release; overwrite finally releases the superseded hash — cancelling the
dedup increment on same-content rewrites and closing the leak), and only
AFTER the canonical mutation commits, so a failed delete can never leave a
live file whose bytes compaction might reclaim.
- History compaction is the ONE reclamation point: after deleting a
generation's record-set it releases that set's references and physically
reclaims any hash at zero live AND zero history references. Pins are
exempt automatically. Crash ordering is over-count-only in every path
(record before persist, release after delete), so a crash can leak until
the scrub recounts but can never reclaim bytes a retained generation
needs. scrubBlobHistoryRefCounts() restores exactness; existing stores get
a one-time marker-gated backfill on open, failing into leak-safe mode
(reclamation disabled) rather than guessing.
On top of the protected history, the temporal API the generational model
always implied:
- vfs.readFile(path, { asOf }) — the exact bytes as of a generation or Date,
materialized from the history (pinned view released so compaction is
never blocked by a read).
- vfs.history(path) — FileVersion[] ascending ({ generation, timestamp,
hash, size, mimeType? }), the newest entry being the live state.
- Overwrites now refresh the file entity's data/embedding text — semantic
search and the data field previously served the FIRST version's text
forever (the stale-field defect a consumer's incident recovery depended
on by luck).
Integration suite (temporal-vfs.test.ts): per-version exact reads +
history listing, leak-fix + history protection on overwrite, rm keeps bytes
readable, compaction reclaims past-window bytes and preserves in-window
(including the cross-file dedup case where an old file's history and a
newer file's removal share one hash), data freshness, and scrub exactness.
Subscribe once and receive one post-commit event per affected record for
EVERY canonical write, regardless of origin: direct calls, batch methods,
transact(), imports, and Virtual Filesystem writes all funnel through the
same commit points the feed is emitted from. This is the authoritative
in-process signal for live UIs, cache invalidation, and realtime sync layers
that forward it over their own transports.
Architecture — the post-commit dual of the ifRev precommit hook: mutation
methods hand lightweight event descriptors to the commit seam
(persistSingleOp / transact's plan), which stamps the committed
{generation, timestamp}, enriches entity deletes with the record's LAST
committed state from the commit's own before-images (free — Model B reads
them anyway; removeMany's id-only deletes gain full payloads this way), and
emits only after the commit succeeds — a losing CAS or rejected batch never
announces anything. Dispatch is a microtask FIFO after the mutex releases:
commit-ordered, a slow listener never delays a write, a throwing listener is
logged and isolated, and with no subscribers the write path constructs no
events at all. Single-point emission was chosen over per-method hooks
because the aggregation-hook pattern demonstrably drifted (relation ops and
removeMany were silently missing from it).
Coverage: add/update/remove (+ cascade unrelate per deleted relationship),
relate (both edges when bidirectional)/unrelate/updateRelation, per-item
events for addMany/updateMany/relateMany/removeMany, per-item events sharing
one generation for transact(), and transitively imports + VFS. clear() and
restore() — wholesale raw-state operations outside the per-record commit
path — emit a single store-level event meaning "refetch everything".
brain.close() drops all listeners.
New module src/events/changeFeed.ts (BrainyChangeEvent + ChangeFeed,
exported from the package root); guide docs/guides/reacting-to-changes.md.
Integration suite pins the contract: per-op payload fidelity, delete
last-state payloads, batch per-item emission, one-generation transact
batches, VFS-origin events, CAS-loser silence, ordering, listener isolation,
unsubscribe, and store-level events.
Sweeping the vestigial hnsw sharding machinery surfaced two real defects on
the counts-recovery path (the code that rebuilds totalNounCount/totalVerbCount
when counts.json is lost or corrupted — container restarts, partial copies):
- initializeCountsFromDisk counted files in entities/*/hnsw/ — directories the
8.0 write path never populates — so an established store recovered to ZERO
counts on real data: wrong getNounCount()/stats, a "New installation"-style
boot log, and a mis-sized rebuild-strategy decision at open. The scan now
walks the canonical entities/<kind>/<shard>/<id>/ tree (one entity per id
directory), with the same sampled type-distribution estimate as before.
- The sampler called getNounMetadata — whose ensureInitialized() re-enters
init() — from INSIDE init(), a latent deadlock reachable the moment the scan
found anything to sample. The sampled metadata files are now read directly
with fs (gz-transparent), no guarded accessors inside init.
The dead machinery itself (verified zero callers by reachability analysis
before deletion): the nine 7.x hnsw-layout entity/edge CRUD methods, the
sharding-depth prober + its depth-migration engine (unreachable — the probe
always returned null on 8.0 stores), an orphaned streaming verb paginator,
their path/scan helpers, and the now-unused type aliases and fields. The init
block keeps the truthful new-vs-established boot log introduced in 8.0.13,
now decided directly from the canonical layout. Net -1,138 lines; no public
API touched.
Adds a regression test: delete counts.json from a populated store, reopen,
counts recover exactly from the canonical tree.
The fast-follow to the ifRev CAS fix: the sweep for the same check-then-act
class found two more instances, both now closed with the same discipline —
the decision runs at the serialization point that guards the apply.
add({ifAbsent}) / add({upsert}): the absence check ran before the commit
mutex, so N concurrent same-id creates could all pass it and all write — the
second silently overwriting the first, violating ifAbsent's "return the
existing id WITHOUT writing" contract and upsert's merge-never-clobber
contract. The insert leg now carries a must-be-absent commit precondition
(the same conditional-commit primitive ifRev uses), verified under the commit
mutex against the authoritative before-image. A losing caller takes its
documented resolution instead of overwriting: ifAbsent returns the existing
id with zero writes; upsert merges into the now-existing entity via update()
(shared param mapping in upsertMergeParams so the planning-time branch and
the conflict retry can never drift), with a bounded retry if a concurrent
delete lands between the conflict and the merge. The planning-time checks
remain as fast-fails. transact()'s batch-level ifAbsent/upsert keep
planning-time semantics (documented, converges to a valid entity).
BlobStorage: write()'s dedup decision (exists → increment refCount, absent →
create at 1) and delete()'s decrement-then-remove were unserialized
read-modify-writes over blob-meta:<hash>. Concurrent writes of identical
content could lose references, so a later delete removed bytes another file
still referenced (data loss), or leaked unreferenced blobs. All
reference-count-bearing mutations now serialize through a per-hash
InMemoryMutex — distinct content never contends; incrementRefCount/
decrementRefCount are documented lock-assumed internals.
Both fixes are complete by construction in-process: storage enforces
single-writer-per-directory, so the process is the whole concurrency domain.
Tests (tests/integration/ifabsent-upsert-blob-concurrency.test.ts): 8-way
ifAbsent storm advances the store by exactly one generation with _rev 1;
8-way upsert storm on an absent id yields one create + seven merges
(_rev === 8); sequential ifAbsent/upsert semantics pinned unchanged; N
identical concurrent blob writes → refCount === N; the blob survives until
the true last reference drops; interleaved write/delete storm never loses a
landed reference.
N concurrent update({ ifRev }) calls carrying the same expected revision all
fulfilled (zero RevisionConflictErrors, last-writer-wins) — the check ran
before the commit mutex, so interleaved callers all passed it before any apply
landed. Sequential calls conflicted correctly, which hid the race. A production
cutover rehearsal caught it: 8 parallel ifRev-guarded ledger writes all
"succeeded" and 7 were silently lost. Advisory locks built on exactly-one-winner
semantics could hand the same lock to two workers.
Fix: conditional commit. commitSingleOp and commitTransaction accept an
optional precommit(beforeImages) precondition, invoked under the commit mutex
against the just-read authoritative before-images and before anything is staged
or applied — the per-record analogue of ifAtGeneration, which always ran there.
A throw aborts the commit atomically (generation reservation returned, zero
staging I/O in the transaction path). The store stays domain-ignorant; update()
and transact() supply the ifRev predicate:
- update(): the planning-time check remains as a fast-fail (avoids embedding
cost on an obviously stale expectation); the authoritative check re-verifies
the before-image's _rev and re-stamps the update's _rev from it, so the
counter is monotonic even for concurrent non-CAS updates (N plain updates
advance _rev by N). CAS against a concurrently-removed entity now throws
EntityNotFoundError instead of silently resurrecting it; plain updates keep
their last-writer-wins re-create semantics.
- transact(): per-op ifRev expectations registered during planning
(PlannedTransact.casUpdates) are re-verified in the batch's precommit; any
conflict rejects the whole batch before staging. Multiple updates to one
entity sequence through a running rev; ids the batch itself adds
(PlannedTransact.createdNouns — add resets the rev baseline even on
overwrite) keep their exact plan-time sequencing.
Regression suite (tests/integration/ifrev-concurrent-cas.test.ts): 8 parallel
same-rev updates → exactly 1 winner + 7 conflicts; same for transact batches;
_rev monotonicity; the documented read→CAS→retry ledger loop converging exactly
(8 workers, 8 decrements, 0 lost); sequential behavior unchanged; forward-ref
add+update in one batch unchanged.
The one-time 7→8 layout migration rescues the head branch's entities
(branches/<head>/entities/* → entities/*), then drained the entire
branches/<head>/ directory. Any durable state a 7.x engine wrote under the head
branch OUTSIDE entities/ — a branch-scoped index, blob area, or field registry —
would be silently deleted by that drain, the same failure class as the VFS
content blobs a 7.x store kept in _cow/.
Guard it: after the entity move, list what remains under branches/<head>/ and
exclude entities/. If anything survives, it is non-entity durable state, so
PRESERVE the branch (skip removeRawPrefix) and warn loudly with the leftover
keys — recoverable, not lost. A clean branch (only the moved entities) still
drains exactly as before. Adds a PARITY GUARD test to the 7→8 migration suite.
Every persisted 8.0 store logged "📁 New installation: using depth 1 sharding"
on every open — even brains holding thousands of entities — which is alarming to
read during a restart or incident. 8.0 stores nouns in the canonical
`entities/nouns/<shard>/<id>/vectors.json` layout (the path `saveNoun`/`getNouns`
use), but the legacy sharding probe `detectExistingShardingDepth()` inspects
`entities/nouns/hnsw` — a 7.x directory the 8.0 write path never populates (its
only writer, `saveNode`, is dead code with zero callers). So the probe returned
null for every 8.0 store and concluded "new".
Drive the new-vs-existing log from the layout the database actually reads and
writes: a new `hasCanonicalEntities()` checks for a real 2-hex shard directory
under `entities/nouns/`, and the known noun count short-circuits it. An
established store now logs "Using depth 1 sharding (N entities)"; only a genuinely
empty store reports a new installation. Behavior is otherwise unchanged — the
probe only ever set a log line, never triggered a rebuild or migration.
The 7.x branch system stored Virtual Filesystem content as blobs in its
copy-on-write area (`_cow/`). 8.0 removed that system and stores content blobs
in the content-addressed store (`_cas/`), but the one-time layout migration only
moves entities — it never adopted the `_cow/` content blobs. A store that used
the VFS was left with every VFS-backed read throwing "Blob metadata not found"
and its pages 500ing, with no first-party recovery and the pre-upgrade backup
already removed on entity-migration "success".
Add an on-open recovery pass (`autoAdoptLegacyVfsBlobsIfNeeded`) that runs right
after the layout migration and adopts every orphaned `_cow/` blob into `_cas/` —
copying both the bytes and the metadata via the raw object primitives,
idempotently and non-destructively (the `_cow/` originals are never deleted). It
is a separate phase, not folded into the migration, so it also heals a store
already upgraded by an earlier 8.0.x that stranded the blobs (whose layout-
migration marker is stamped): it is gated on the presence of `_cow/` and its own
`_system/vfs-blob-adoption.json` marker. Filesystem-only; native-8.0 and non-VFS
stores no-op on a cheap existence check.
- `BaseStorage.adoptLegacyCowBlobs()` — the scan/copy primitive, returning
`{ cowBlobs, adopted, alreadyPresent, incomplete }`; skips (does not half-adopt)
a blob missing its bytes or metadata.
- `brain.vfs.adoptOrphanedBlobs()` — the explicit force path; self-initializes.
- Backup retention: the automatic pre-upgrade backup is now kept if any blob
can't be fully adopted (`incomplete > 0`), instead of being removed on
entity-migration success alone — a blob-parity gate the migration signal
cannot provide.
Adds an end-to-end integration test (storage-level adopt with idempotency and
incomplete handling; self-heal on reopen; the explicit API) and an upgrade guide.
`find({ where })` had two gaps that could silently return nothing. The in-memory
matcher (`matchesQuery`, used for egress re-validation and historical reads) was
missing several documented operators — `in`, `greaterThanOrEqual`,
`lessThanOrEqual` — so it disagreed with the index path, which does handle them.
And an unknown operator key (a typo, or `notIn` written where `not: { in }` was
meant) fell through to a nested-object-field interpretation and matched nothing.
- Align `matchesQuery` to the full documented operator set (add the missing
aliases; the default branch now throws instead of treating an unknown key as a
nested field — dotted paths remain the supported nested form).
- Validate the `where` filter up front (`validateWhereFilter`), throwing a typed
`BrainyError('INVALID_QUERY')` that names the unknown operator, recursing into
`allOf` / `anyOf` / `not`.
- Stop excluding a user field literally named `level` from the metadata index
(it collided with an internal never-index key), so numeric/exact filters on it
resolve instead of returning 0.
Adds unit coverage for the operator set and the throw-on-unknown behavior.
- data-storage-architecture.md described transact() as 'the unit of history'
(rc.2 era). Single-op retention has been the model since 8.0 — every write
gets its own generation; transact() only groups several into one. Corrected.
- RELEASE-GUIDE.md now records the hard ordering constraint: no embedding-model
change ships before vector model-version stamping + hard-error-on-mismatch
lands.
A production deployment measured ~48 seconds on EVERY reopen of an
11k-entity brain. Root cause: brainy's rebuild gate decided from in-memory
size()/count, which read 0 for a durable-but-not-resident index, so it
re-read every entity file to rebuild from scratch. At GA we gave only the
GRAPH provider a readiness contract (init() eager cold-load + isReady()
honest signal) so it would never eat that spurious rebuild; the vector and
metadata providers never got it, and brainy never even eager-inited the
vector provider.
Complete the contract symmetrically:
- plugin.ts: VectorIndexProvider gains optional init()+isReady();
MetadataIndexProvider gains isReady() — mirroring GraphIndexProvider.
Additive and optional; a provider that exposes nothing keeps today's
behavior.
- brainy.ts: eager-init every provider that exposes init() (after metadata
init() so the id-mapper is hydrated first), then decide per leg in
precedence order — migrating (skip) -> epoch drift (rebuild) -> isReady()
-> a per-leg empty fallback. The old instant fast-path keyed off
this.index.size()>0, a dishonest proxy that skipped the metadata/graph
checks whenever the vector was warm and never fired on a real cold process
anyway; removed.
The per-leg fallbacks differ because "empty" means different things: the JS
vector's rebuild() IS its load, so size()===0 correctly triggers it; the
id-mapper backs metadata, so totalEntries===0 (past the empty-store return)
is a real load failure; but entities do not imply edges, so a graph
size()===0 is a valid empty state, not a load failure.
- The JS graph now COLD-LOADS its durable LSM instead of re-deriving from a
full canonical verb scan on every boot (baseStorage._initializeGraphIndex
loads the persisted SSTables via a new GraphAdjacencyIndex.init(); it
self-heals from canonical only when the durable state is genuinely missing).
This removes an O(E)-per-open cost every filesystem consumer paid.
- LSMTree.loadManifest loads its SSTables BEFORE publishing the relationship
count, and resets to an honest-empty state on load failure — a tree can no
longer claim persisted relationships while holding none (the silent-empty
cold-load class the query-time guards exist to prevent).
Verified end-to-end against a built brain: a warm reopen (with edges and
edgeless) reloads only the JS vector; the graph and metadata cold-load with
no rebuild, and queries return correct results. New tests in
cold-open-rebuild-gate.test.ts pin the contract (isReady() defers, self-heal
still fires); migration-deference updated to drive size-based deference
through the vector, the leg where empty->rebuild remains correct.
Pairs with the native provider's isReady()/init() implementation — brainy's
gate defers only to a signal the provider exposes.
A consumer's clean-room verification of the 8.0.10 fix found relate() still
hanging their scripts. Root-causing the CLASS instead of the repro found two
mechanisms:
1. The 'beforeExit' auto-flush hook looped forever on any script that never
reaches close(): Node re-emits beforeExit after every event-loop drain, and
the async flush schedules new work — flush, drain, flush, forever. The
listener now self-deregisters BEFORE its one flush, so the next drain exits.
(Empirically: process.on('beforeExit', async () => await anything) alone
never exits — this was the deepest root of the whole hang class.)
2. Every background-maintenance interval is now unref'd at creation — graph
auto-flush, LSM compaction, metadata write-buffer flush, VFS cache
maintenance, PathResolver cache maintenance, statistics debounce (the
writer-lock heartbeat, flush watcher, and cache monitors already were).
Durability is owned by close() and the beforeExit flush, both deterministic;
a best-effort interval must never keep the host process alive.
Proof: the reported shape (add x2 + relate, filesystem) exits cleanly BOTH
with close() (~0.5 s) and with NO teardown at all — and in the no-teardown
case the beforeExit flush still lands the data (verified by reopen: both
nouns + the edge present). New per-op-class sweep test asserts no ref'd
timer survives close() for add / relate / graph find / metadata update /
vfs — turning this bug class off permanently instead of per-repro.
A consumer report on the GA pair: any minimal add/find/close script hangs
forever. Root cause was FOUR ref'd keep-alives brainy never released:
1+2. The global SIGTERM/SIGINT shutdown hooks (registered once at init) were
anonymous and never removed — a process.on signal listener holds a ref'd
signal handle. They are now named statics, deregistered when the LAST live
instance closes (Brainy.instances also stopped leaking closed instances);
a later init re-registers them.
3. UnifiedCache.startFairnessMonitor() created an interval it never stored,
cleared, or unref'd — unclearable by construction. Now stored + unref'd
(same posture as the existing memory-pressure timer): a cache monitor must
never keep the host process alive.
4. brainy.close() never called VirtualFileSystem.close(), stranding the VFS
background-maintenance interval and the PathResolver maintenance interval.
close() now shuts the VFS down.
Proof: a bare script (init/add/close, no process.exit) exits 1 ms after
close() returns; before the fix it hung until killed. 3 new lifecycle tests
pin the hook semantics (once globally, survive while other instances live,
removed on last close, re-register after).
With plugins unset (the default), init() probes for the first-party
accelerator: not installed means plain brainy with zero noise; installed and
healthy means it loads and announces itself; installed but broken (import
failure, invalid shape, failed activation, version mismatch) makes init()
THROW. An installed accelerator never silently vanishes behind the JS engines
- the anti-drift posture of the explicit list, applied to detection.
plugins: []/false stays a true opt-out (no probe); an explicit list keeps its
required-and-loud semantics. The import runs through an importPluginPackage
seam (variable specifier - bundlers cannot static-resolve the optional
package; tests simulate all outcomes without it installed).
Also: when a plugin activates but registers zero native providers (e.g. a
licensing gate declining to engage), the provider summary now warns loudly
instead of leaving every query silently on the JS engines.
Supersedes 8.0.8's explicit-opt-in wording; README/PLUGINS/types now document
the guarded contract. 7 new tests (tests/unit/plugin-autodetect.test.ts).
A clean-room install smoke of the published GA pair caught the README telling
scale-up users the native provider is auto-detected. It is not, by design: the
loader (loadPlugins) does no auto-detection — undefined means no plugins, and
only an explicit plugins: ['@soulcraft/cor'] loads the native engine (loud
failure if it can't). The stale comment on the plugins config field in the
public types claimed auto-detection and is corrected to match the loader; one
phrase in the plugin-author guide likewise.
Rewrites the README around the feature-showcase structure: write->index->query
table, runnable quick start (subtype-correct on the 8.0 strict default, real
VerbTypes), feature tour with per-pillar code, the scale-up path to
@soulcraft/cor, and measured-only performance claims with benchmark citations.
GA retargets to 8.0.1: npm permanently retired the 8.0.0 version number after
a January development-cycle publish/unpublish, so the first stable 8.x release
is 8.0.1. RELEASES.md documents this; the phantom 8.0.0 CHANGELOG entry is
removed (the release run regenerates it as 8.0.1).
The 3.0 native engine ships as @soulcraft/cor (brainy 8.x <-> cor 3.x are a
version-matched pair); the old @soulcraft/cortex package stays on the 2.x/7.x
line. Public docs and .d.ts-visible comments now name the correct package.
Historical 2.x contract references (e.g. the 2.3.1 read-side fallback) keep
the old name deliberately.
Brings the u64-id core to main as a merge of feat/8.0-u64-ids. main's tree now
matches the gated 8.0 line exactly; the 7.31.8-7.33.5 hotfix history is retained
as the merge's first parent (every one of those fixes is already in the 8.0 line).
cor confirmed the migration write-new / tmp+rename invariant the hard-link backup
relies on — with one exception: the native shared mmap `_id_mapper/*` is mutated
IN PLACE (msync + a rebuild truncate+re-inject), the same category as the tx-log.
A hard-linked snapshot of it would be corrupted by a live in-place mutation
reaching through the shared inode, so the backup would not be a faithful
pre-upgrade copy.
Add a SNAPSHOT_BYTE_COPY_DIRS set (`_id_mapper`) so snapshotToDirectory byte-copies
every file under it (the directory analogue of SNAPSHOT_BYTE_COPY_PATHS); the id
map is bounded, so the copy cost is small. Everything else (SSTables / .dkann /
manifests / objects, all tmp+rename) still hard-links. Test: `_id_mapper/*` gets
an independent inode + faithful content; a regular file stays hard-linked.
Companion to 7.33.4's graph cold-read guard, now for the metadata field index. A
downstream deployment reported find({where}) returning a silent [] on a
freshly-opened brain (a native metadata provider that reports data but hasn't
loaded its field postings), blanking filtered pages after every restart.
verifyMetadataLive() — one-shot per brain on the first filtered find(): probe a
known persisted field value; if served, live (one O(1) probe on a warm brain). If
not, rebuild from canonical + re-probe; if still unserved, throw the new exported
MetadataIndexNotReadyError rather than let a silent [] pass. Inconclusive cases
(empty store, no plain field, shared-store foreign entity) resolve to live, no
false rebuild. Also exports BrainyError + GraphIndexNotReadyError (were internal).
The JS index cold-loads correctly; this guards the native path (durable cure is
cortex-side, same shape as the graph 2.7.8 cure). 4 unit tests. tsc 0, guard 4/4,
full unit 1538 pass (+1 pre-existing flaky VFS perf test, green in isolation).
A downstream deployment reported find({where}) returning a silent [] on a
freshly-opened brain (a native metadata provider that reports data but hasn't
loaded its field postings), blanking filtered pages after every restart. Brainy
had a cold-read guard for GRAPH reads (verifyGraphAdjacencyLive → self-heal or a
loud GraphIndexNotReadyError) but no equivalent for metadata `where`.
Add verifyMetadataLive() — the field-index counterpart, one-shot per brain on the
first filtered find(): probe a known persisted entity's plain field value; if the
index serves it, live (the only cost on a warm brain — one O(1) probe). If not,
the postings didn't load: rebuild from canonical + re-probe; if still unserved,
throw the new exported MetadataIndexNotReadyError rather than let a silent [] pass.
Inconclusive cases (empty store, no plain field, shared-store foreign entity,
migrating provider) resolve to live — never a false rebuild.
The 8.0 open-core JS index already cold-loads correctly (verified: cold where 3/3,
cold related 2/2) — this guards the NATIVE path, where the durable cold-load cure
is cortex-side (same shape as the graph 2.7.8 cure). 4 unit tests (warm no-rebuild,
self-heal, loud-fail, no-filter-no-probe). Gates: typecheck 0, build 0, test:unit
1757/1757.
David's ask: back up the brain before the one-time upgrade, drop it after. On
open, when the on-disk format is stale (a 7.x→8.0 rebuild will run) and the store
holds data, brainy hard-link-snapshots the brain dir to a sibling
`<brainDir>.migration-backup` BEFORE any provider rebuilds — near-zero cost/space
(shared inodes; the store is immutable-by-rename), instant even at scale. Removed
automatically once the upgrade verifies + stamps the marker; retained on failure
for rollback. Default-on; opt out with `migrationBackup: false`.
- Reuses the existing snapshotToDirectory() hard-link farm via new optional
adapter methods createMigrationBackup()/removeMigrationBackup() (filesystem
only — feature-detected; memory + non-fs are a graceful no-op).
- Taken before the native provider inits, so it captures true pre-migration
state; a prior failed upgrade's backup is reused, not overwritten.
- Best-effort: a backup failure never blocks the upgrade (the migration is itself
safe — reads canonical, reconstructable, self-heals). Catastrophe-insurance
against a migration bug, not a data-loss guard or an off-device backup.
4 lifecycle tests (adapter hard-link / reuse / remove-without-touching-live /
empty→null; stale-epoch reopen create+remove; opt-out; memory no-op). Gates:
typecheck 0, build 0, test:unit 1753/1753, layout-migration suite 5/5.
