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.
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