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Replaces the placeholder MetadataStore.Update (which delegated the entire read-modify-write to the fallback datastore) with a wrapper-level CAS retry loop that always lands writes in primary. When the doc is in primary or the doc is absent from both stores, it delegates to primary.Update; when the doc lives only in fallback, it feeds the fallback value to the caller's callback and inserts the result into primary via WriteCas, retrying on CAS race. A delete returned by the callback for a fallback-only doc is a no-op in primary - the metadata migration sweep tombstones the fallback copy. WriteUpdateWithXattrs gets the same dual-read pattern for symmetry (callers today are application-data only, but a future metadata caller would otherwise silently skip fallback data). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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CBG-5291
MetadataStore.Updatepreviously delegated the entire read-modify-write to the fallback datastoreImplements a
MetadataStorewrapper-level CAS retry loop that always writes to primary, even when the doc lives only in the fallback store.MetadataStore.WriteUpdateWithXattrsisDeletefor fallback-only doc (no-op — migration sweep handles cleanup), post-SetMigrationCompletebehaviour, and the xattr analogue of the migration case.Why this is CAS-safe
The wrapper invariant — no writes ever go to fallback while the dual-store is in use — means the only writer to either store is this wrapper writing to primary. So the loop:
primary.Exists. If found (or migration is sealed), delegate toprimary.Update/primary.WriteUpdateWithXattrs— those already have correct CAS retry loops.WriteCas/WriteWithXattrswithcas=0(insert) into primary.IsCasMismatch(covers bothgocb.ErrDocumentExistsand rosmarCasMismatchErr), retry from step 1 — by now primary holds the doc, so we go through the standard primary CAS path.Integration Tests