decentraland / snapshots-fetcher / 30057773496

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fix: serialize evictions and re-verify stored bytes before deleting (#204)

* fix: serialize evictions and re-verify stored bytes before deleting

closes the concurrent-cache-heal race noted on #203: two callers can read the
same corrupt local entity; the first eviction plus a retry re-downloads a good
copy, and the second (stale) caller would then delete that good copy based on
the old bytes it already read.

evictions now run under a per-entity lock and re-verify the bytes CURRENTLY in
storage immediately before deleting: a copy that was healed in the meantime is
kept, a copy already removed is not deleted twice, and a failed re-read keeps
the copy (deleting on stale evidence alone is not safe). the eviction metric
still counts only actual deletes, and each outcome is reflected in the thrown
entity-scoped error.

* test: race two overlapping downloads on the same corrupt entity

addresses the review note that the eviction-lock behavior was only exercised
through sequential retrieve() simulations. two real
downloadEntityAndContentFiles calls now overlap on one id: a barrier holds both
until each has read the corrupt copy, and the heal is planted inside the
winner's delete, which the per-entity lock orders strictly before the loser's
re-check. asserts exactly one eviction across both callers, one
removed-the-copy outcome plus one kept-the-healed-copy outcome, and the healed
copy surviving.

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