SingleFileBackingStore.PushAll replaced the file without reading it, so a
truncated store or one in a format this runtime does not read was clobbered,
where Rust, TypeScript and Python refuse. PushAll now reads the header of a
non-empty file under the lock before it writes; a file it cannot read throws
and is left as it was. An empty file and a legacy envelope array carry no
header and are still replaced.
The header is read by the store's codec: SingleFileBackingStore gains a
protected virtual ReadStoreHeader (default: read the whole snapshot), and the
MessagePack store overrides it with
CultDocumentMessagePackSerialization.ReadStoreHeader. An unconditional commit
reads the file it rewrites through the same check.
Doc text: last-writer-wins now says it holds only over a file the writer can
read (CultCache.cs PushAll comment, cultcache-persistence-format.md
"Conditional commit is the only safe multi-process write" and writer rule 4,
cultcache-store-composition.md "A plain flush and an unconditional commit are
last-writer-wins").
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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