fix(recovery): a torn generation-log tail is a terminal verdict, never a wait

Two halves of one defect, found by a seeded-SIGKILL crash lane.

THE FALSE POSITIVE. stampEntityTree() recorded generationStore.generation()
— the ALLOCATED counter, a number a write in flight has claimed and may
never commit — while the JSDoc beside it already said the source is the
committed generation. Every crash inside a write window therefore produced
a spurious verdict at the next open: either 'sourceGeneration N is ahead of
the log head N-1' (the allocated generation died with the process) or
'rollup invariant nounCount: stamped X, observed Y' (the recovery fold
folded facts the stamp's counts predate). Both told the operator to run
repairIndex() — a whole-store recount — for a store that was coherent.
Measured before this commit: 4 of 11 SIGKILL cycles on a healthy store
raised one of the two. The stamp and the open now both read
committedGeneration(), which is what every other open-time watermark in the
class already reasons about.

THE TERMINAL VERDICT. A stamp still ahead of committed truth after the
recovery fold witnesses a generation that is not in the log — the stamp's
fsync outlived the tail's, and there is nothing to arrive. That is its own
verdict state now ('torn'), never folded in with 'incoherent': the two have
opposite cures. A writer open demotes it — the unusable stamped surface is
re-derived at the committed generation from the live counters, O(1),
straight-line, no loop and no await on external progress, narrated with
both count sets, the stamp's path and its committedAt. A read-only open
cannot re-stamp, so it says so and names the cure instead of guessing, and
still serves. Neither branch waits, and neither locks an owner out of a
canonical tree the stamp only describes.

Pins: the verifier returns the torn verdict with both generations; a
fabricated head-behind-source store narrates precisely, demotes inside a
bounded open, serves its rows, and is quiet at the next open (the demotion
converges); a read-only open narrates the same verdict and leaves the bytes
untouched.

src/brainy.ts

@@ -12497,6 +12497,18 @@ export class Brainy<T = any> implements BrainyInterface<T> {
    * healed by `repairIndex()`, whose unconditional recount rebuilds the
    * rollups from a canonical walk and re-stamps. Best-effort: a stamp-write
    * fault warns loudly but never fails the flush that carried real data.
+   *
+   * THE SOURCE IS `committedGeneration()`, NEVER `generation()`. The latter is
+   * the ALLOCATED counter — a number a write in flight has claimed and may
+   * never commit. Stamping it made the stamp's generation label a claim about
+   * counts it was not taken at, and every crash inside a write window then
+   * produced a spurious verdict at the next open: either `sourceGeneration N
+   * is ahead of the log head N-1` (the allocated generation died with the
+   * process) or `rollup invariant 'nounCount': stamped X, observed Y` (the
+   * recovery fold folded facts the stamp's counts predate). MEASURED on the
+   * crash-consistency lane before this line changed: 4 of 11 SIGKILL cycles on
+   * a coherent store raised one of those two verdicts, each of them naming
+   * `repairIndex()` — a whole-store recount — as the cure for nothing.
    */
   private async stampEntityTree(): Promise<void> {
     if (this.isReadOnly) return
@@ -12507,7 +12519,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       ])
       await writeFamilyStamp(this.storage, ENTITY_TREE_STAMP_PATH, {
         family: 'entity-tree',
-        sourceGeneration: this.generationStore.generation(),
+        sourceGeneration: this.generationStore.committedGeneration(),
         members: { mode: 'rollup', invariants: { nounCount, verbCount } }
       })
     } catch (error) {
@@ -12520,16 +12532,24 @@ export class Brainy<T = any> implements BrainyInterface<T> {
 
   /**
    * @description Open-time coherence check for the entity tree's family stamp:
-   * compare `sourceGeneration` against the log head and the stamped rollup
-   * invariants against the live counters. Verdicts:
+   * compare `sourceGeneration` against the store's COMMITTED generation and
+   * the stamped rollup invariants against the live counters. Verdicts:
    * - `coherent` / `absent` (legacy store; first flush stamps) → silent.
    * - `behind` → benign for the tree (it is written BY the commit; only the
    *   stamp is stale — a crash landed between commit and flush). Refreshed at
    *   the next flush.
+   * - `torn` → a TORN GENERATION-LOG TAIL, handled by
+   *   {@link demoteTornEntityTreeStamp}: terminal, never a wait.
    * - `incoherent` → LOUD: the tree or its counters diverged from what was
    *   stamped — `repairIndex()` recounts from canonical and re-stamps.
    * Never blocks open; a fault reading the stamp is surfaced as unverifiable,
    * never conflated with absence.
+   *
+   * THE COMPARISON IS AGAINST `committedGeneration()`, matching what
+   * {@link stampEntityTree} writes and what every other open-time watermark in
+   * this class already reasons about (the fact-scan capability, the metadata /
+   * graph / HNSW watermark verdicts). Comparing against the allocated counter
+   * was the one place that disagreed, and disagreeing was the whole defect.
    */
   private async verifyEntityTreeStamp(): Promise<void> {
     let stamp: FamilyStamp | null
@@ -12546,11 +12566,16 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       this.storage.getNounCount(),
       this.storage.getVerbCount()
     ])
-    const verdict = verifyFamilyStamp(stamp, this.generationStore.generation(), {
+    const verdict = verifyFamilyStamp(stamp, this.generationStore.committedGeneration(), {
       nounCount,
       verbCount
     })
-    if (verdict.state === 'incoherent') {
+    if (verdict.state === 'torn') {
+      await this.demoteTornEntityTreeStamp(stamp as FamilyStamp, verdict.stampSource, verdict.head, {
+        nounCount,
+        verbCount
+      })
+    } else if (verdict.state === 'incoherent') {
       prodLog.warn(
         `[Brainy] entity-tree stamp INCOHERENT at open: ${verdict.failures.join('; ')}. ` +
           `The canonical tree or its counters diverged from the stamped state — run ` +
@@ -12564,6 +12589,92 @@ export class Brainy<T = any> implements BrainyInterface<T> {
     }
   }
 
+  /**
+   * @description THE TERMINAL VERDICT for a torn generation-log tail.
+   *
+   * A stamp whose `sourceGeneration` sits ABOVE the store's committed
+   * watermark witnesses a generation that is not in the log: the stamp's fsync
+   * outlived the tail's. By the time this runs, log-authority recovery has
+   * already folded every intact fact above the manifest and advanced the
+   * watermark to cover them — so if the stamp is STILL ahead, the generation
+   * it names is not merely late, it is GONE. There is nothing to wait for.
+   *
+   * That is the whole point of this method. A field report of this class
+   * (single-process store, abrupt termination mid-fold) described a reopen
+   * that narrated the tear and then held 100% CPU with zero log growth for
+   * eight minutes before an operator wiped the directory. A recovery that
+   * cannot say what it is waiting for has no business spinning; the honest
+   * answer here is a verdict, taken now, at O(1) cost.
+   *
+   * WHAT THE VERDICT DOES — the stamped surface is UNUSABLE, so it is
+   * discarded rather than believed: the stamped counts describe a generation
+   * that never became durable, and comparing them against live counters can
+   * only produce noise. The tree itself is not in question (it IS canonical —
+   * every commit writes it, and the fold re-applied every after-image the log
+   * still holds), so the demotion is a re-derivation of this family's verified
+   * surface at the generation the store can actually show:
+   *
+   * - WRITER open → re-stamp at `committedGeneration()` from the live
+   *   counters — exactly what the next flush would write, taken now so the
+   *   tear cannot re-narrate on every subsequent open. Both count sets are
+   *   logged so an operator can see whether anything really moved.
+   * - READER open → a reader cannot re-stamp. Narrate the same terminal
+   *   verdict with the named cure and carry on serving; a read-only inspector
+   *   is never locked out of a store, and never left waiting either.
+   *
+   * BOUNDEDNESS: straight-line code. No loop, no retry, no await on any
+   * external progress signal — the two counter reads and one stamp write are
+   * the entire cost, and none of them scales with the store.
+   */
+  private async demoteTornEntityTreeStamp(
+    stamp: FamilyStamp,
+    stampSource: number,
+    head: number,
+    observed: { nounCount: number; verbCount: number }
+  ): Promise<void> {
+    const stamped = stamp.members.mode === 'rollup' ? stamp.members.invariants : {}
+    const detail =
+      `[Brainy] TORN GENERATION-LOG TAIL at open: ${ENTITY_TREE_STAMP_PATH} witnesses source ` +
+      `generation ${stampSource} (stamped ${stamp.committedAt}), but the store's committed ` +
+      `generation is ${head} after crash recovery — the stamp's fsync outlived the log tail's, ` +
+      `and generation ${stampSource} is not in the log to arrive. Stamped rollups ` +
+      `${JSON.stringify(stamped)}; observed ${JSON.stringify(observed)}.`
+
+    if (this.isReadOnly) {
+      prodLog.warn(
+        `${detail} This open is READ-ONLY, so the stamp cannot be re-derived: the entity-tree ` +
+          `family stays UNVERIFIED for this session (reads are unaffected — the canonical tree ` +
+          `is the truth this stamp only describes). Cure: open the store with a writer, or run ` +
+          `brain.repairIndex() there, to recount from canonical and re-stamp.`
+      )
+      return
+    }
+
+    const startedAt = Date.now()
+    try {
+      await writeFamilyStamp(this.storage, ENTITY_TREE_STAMP_PATH, {
+        family: 'entity-tree',
+        sourceGeneration: head,
+        members: {
+          mode: 'rollup',
+          invariants: { nounCount: observed.nounCount, verbCount: observed.verbCount }
+        }
+      })
+      prodLog.warn(
+        `${detail} DEMOTED: the unusable stamp was re-derived at committed generation ${head} ` +
+          `from the live counters in ${Date.now() - startedAt}ms — terminal, not a wait. If the ` +
+          `observed counts above look wrong for your data, run brain.repairIndex() to recount ` +
+          `from canonical.`
+      )
+    } catch (error) {
+      prodLog.warn(
+        `${detail} The demotion's re-stamp FAILED (${(error as Error).message}) — the tear will ` +
+          `narrate again at the next open, which is the honest outcome; the store still serves ` +
+          `from canonical. Cure: run brain.repairIndex() to recount from canonical and re-stamp.`
+      )
+    }
+  }
+
   /**
    * Ask the writer process serving this data directory to flush its in-memory
    * indexes to disk, so a read-only inspector can observe fresh state.

src/db/familyStamp.ts

@@ -12,9 +12,11 @@
  *   the verified surface is a small set of rollup invariants (entity/
  *   relationship counts) plus `sourceGeneration`.
  *
- * `sourceGeneration` is the generation of the source-of-truth log this
- * projection reflects — open-time coherence becomes a COMPARISON (stamp vs
- * log head), not a walk:
+ * `sourceGeneration` is the COMMITTED generation of the source-of-truth log
+ * this projection reflects — never the allocated counter, which names a
+ * generation that may never commit (see {@link StampVerdict.torn}) — so
+ * open-time coherence becomes a COMPARISON (stamp vs committed head), not a
+ * walk:
  *
  * - equal + invariants hold → coherent, serve.
  * - behind → the projection missed the tail (crash between commit and stamp);
@@ -24,6 +26,9 @@
  * - invariants FAIL at equal generation → genuine incoherence: loud, and the
  *   repair ritual (`repairIndex()`, whose recount rebuilds the rollups from a
  *   canonical walk) heals it.
+ * - AHEAD → a torn generation-log tail: the stamp's fsync outlived the log
+ *   tail's. TERMINAL, never a wait — the generation the stamp names does not
+ *   exist to arrive.
  *
  * Stamps are JSON on purpose — every incident gets debugged by reading a
  * stamp in a terminal.
@@ -70,6 +75,12 @@ export type StampVerdict =
   | { state: 'coherent' }
   | { state: 'absent' } // legacy store — first stamp writes at the next flush
   | { state: 'behind'; stampSource: number; head: number }
+  /**
+   * TORN GENERATION-LOG TAIL: the stamp witnesses a source generation the
+   * store's committed watermark can no longer show. TERMINAL — there is no
+   * generation to wait for, so the open demotes (or refuses) and never spins.
+   */
+  | { state: 'torn'; stampSource: number; head: number }
   | { state: 'incoherent'; failures: string[] }
   | { state: 'unverifiable'; reason: string } // a FAULT reading the stamp — never conflated with absence
 
@@ -118,12 +129,15 @@ export function verifyFamilyStamp(
 ): StampVerdict {
   if (stamp === null) return { state: 'absent' }
   if (stamp.sourceGeneration > head) {
-    // A stamp AHEAD of the log claims state that never committed — the
-    // projection was stamped against truth that a crash rolled back.
-    return {
-      state: 'incoherent',
-      failures: [`sourceGeneration ${stamp.sourceGeneration} is ahead of the log head ${head}`]
-    }
+    // A stamp AHEAD of committed truth witnesses a generation the store can no
+    // longer show: the stamp's fsync survived a crash that the log tail did
+    // not. This is the TORN GENERATION-LOG TAIL — its own class, never folded
+    // in with `incoherent` (a count that drifted at a generation both sides
+    // agree on), because the two have opposite cures: incoherence is recounted,
+    // a tear is DEMOTED. It is also terminal by construction — there is no
+    // generation the open can wait for, because the one the stamp names is
+    // gone.
+    return { state: 'torn', stampSource: stamp.sourceGeneration, head }
   }
   if (stamp.sourceGeneration < head) {
     return { state: 'behind', stampSource: stamp.sourceGeneration, head }

tests/integration/entity-tree-stamp.test.ts

@@ -57,7 +57,11 @@ describe('entity-tree family stamp', () => {
     const invariants = (stamp.members as any).invariants
     expect(invariants.nounCount).toBe(await brain.storage.getNounCount())
     expect(invariants.verbCount).toBe(await brain.storage.getVerbCount())
-    expect(stamp.sourceGeneration).toBe(brain.generation())
+    // THE SOURCE IS COMMITTED TRUTH, never the allocated counter. Stamping the
+    // counter labelled the stamp with a generation a write in flight had merely
+    // claimed, so every crash inside a write window produced a spurious verdict
+    // at the next open (see the torn-tail pins below).
+    expect(stamp.sourceGeneration).toBe(brain.generationStore.committedGeneration())
     expect(stamp.generation).toBeGreaterThanOrEqual(1)
   })
 
@@ -112,6 +116,96 @@ describe('entity-tree family stamp', () => {
     expect(stillIncoherent).toEqual([])
   })
 
+  /**
+   * Rewrite the on-disk stamp so its `sourceGeneration` sits ABOVE the store's
+   * committed watermark — the durable shape a torn generation-log tail leaves
+   * behind (the stamp's fsync outlived the tail's). Fabricated rather than
+   * crash-produced so the pin is deterministic; the seeded-SIGKILL lane
+   * (`scripts/crash-consistency.mjs` in the engine repo) produces the same
+   * shape from a real abrupt termination.
+   */
+  const fabricateTear = (ahead: number): FamilyStamp => {
+    const file = path.join(dir, `${ENTITY_TREE_STAMP_PATH}.gz`)
+    const zlib = require('node:zlib')
+    const raw = JSON.parse(zlib.gunzipSync(fs.readFileSync(file)).toString('utf-8')) as FamilyStamp
+    const torn: FamilyStamp = { ...raw, sourceGeneration: raw.sourceGeneration + ahead }
+    fs.writeFileSync(file, zlib.gzipSync(JSON.stringify(torn)))
+    return torn
+  }
+
+  it('a torn generation-log tail is a TERMINAL VERDICT at open: narrated, demoted, never a wait', async () => {
+    for (let i = 0; i < 3; i++)
+      await brain.add({ data: `torn${i}`, type: 'document', metadata: { i } })
+    await brain.close()
+    const torn = fabricateTear(5)
+
+    const warn = vi.spyOn(prodLog, 'warn')
+    const startedAt = Date.now()
+    brain = await open()
+    const openMs = Date.now() - startedAt
+
+    const tearLines = warn.mock.calls.filter((c) => String(c[0]).includes('TORN GENERATION-LOG TAIL'))
+    expect(tearLines.length).toBe(1)
+    const said = String(tearLines[0][0])
+    // Narrated PRECISELY: both generations, the file, and the named cure.
+    expect(said).toContain(`source generation ${torn.sourceGeneration}`)
+    expect(said).toContain(`committed generation ${brain.generationStore.committedGeneration()}`)
+    expect(said).toContain(ENTITY_TREE_STAMP_PATH)
+    expect(said).toContain('DEMOTED')
+    expect(said).toMatch(/repairIndex\(\)/)
+    // Terminal, not a wait: the demotion is O(1) straight-line work, so a tear
+    // cannot turn an open into the 8-minute spin this class was reported as.
+    expect(openMs).toBeLessThan(30_000)
+
+    // The store SERVES — a tear in a stamp never locks an owner out of the
+    // canonical tree the stamp merely describes.
+    expect((await brain.find({ type: 'document', limit: 100 })).length).toBe(3)
+
+    // The demotion CONVERGED: the stamp now names committed truth, and the
+    // next open is quiet. A verdict that re-narrates every open is a wait
+    // wearing a different hat.
+    const restamped = (await readFamilyStamp(brain.storage, ENTITY_TREE_STAMP_PATH)) as FamilyStamp
+    expect(restamped.sourceGeneration).toBe(brain.generationStore.committedGeneration())
+    await brain.close()
+    const warn2 = vi.spyOn(prodLog, 'warn')
+    brain = await open()
+    expect(warn2.mock.calls.filter((c) => String(c[0]).includes('TORN'))).toEqual([])
+  })
+
+  it('a READ-ONLY open on a torn tail refuses to guess: terminal verdict + named cure, no re-stamp', async () => {
+    await brain.add({ data: 'ro', type: 'document', metadata: {} })
+    await brain.close()
+    const torn = fabricateTear(3)
+
+    const warn = vi.spyOn(prodLog, 'warn')
+    const reader: any = await Brainy.openReadOnly({
+      requireSubtype: false,
+      storage: { type: 'filesystem', path: dir },
+      silent: true,
+      dimensions: 384
+    })
+    const tearLines = warn.mock.calls.filter((c) => String(c[0]).includes('TORN GENERATION-LOG TAIL'))
+    expect(tearLines.length).toBe(1)
+    const said = String(tearLines[0][0])
+    expect(said).toContain('READ-ONLY')
+    expect(said).toContain('UNVERIFIED')
+    expect(said).toMatch(/repairIndex\(\)/)
+    await reader.close()
+
+    // A reader never rewrites the store: read the bytes back off disk (not
+    // through a writer open, which would demote them) — the torn stamp is
+    // exactly as it was found.
+    const onDisk = JSON.parse(
+      require('node:zlib')
+        .gunzipSync(fs.readFileSync(path.join(dir, `${ENTITY_TREE_STAMP_PATH}.gz`)))
+        .toString('utf-8')
+    ) as FamilyStamp
+    expect(onDisk.sourceGeneration).toBe(torn.sourceGeneration)
+    expect(onDisk.generation).toBe(torn.generation)
+
+    brain = await open()
+  })
+
   it('the one verifier handles both member modes', () => {
     const rollup: FamilyStamp = {
       family: 'x',
@@ -127,7 +221,13 @@ describe('entity-tree family stamp', () => {
       stampSource: 5,
       head: 9
     })
-    expect(verifyFamilyStamp(rollup, 3, { nounCount: 10 }).state).toBe('incoherent') // ahead of head
+    // AHEAD is its own class — a torn generation-log tail, never folded in
+    // with `incoherent`: the two have opposite cures (recount vs demote).
+    expect(verifyFamilyStamp(rollup, 3, { nounCount: 10 })).toEqual({
+      state: 'torn',
+      stampSource: 5,
+      head: 3
+    })
     expect(verifyFamilyStamp(null, 5, {})).toEqual({ state: 'absent' })
 
     const enumerated: FamilyStamp = {