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b18d3dc96c
Add an optional WriteCondition to CreateEntryRequest. When set, the filer evaluates it against the current entry while holding the per-path lock, so the check and the write are atomic on this filer, and returns PRECONDITION_FAILED when it does not hold. The caller must route the key's writes to the owner filer for the check to be authoritative. A condition is a list of clauses that all must hold (logical AND). One clause is the common case; several express what a single comparison cannot: an ETag set (If-Match / If-None-Match with multiple values), weak-ETag comparison, and compound conditions. ETag comparison mirrors the S3 gateway's precedence (stored Seaweed ETag attribute, then the Md5/chunk fallback) and follows RFC 7232 strong/weak rules, so results match without coupling the filer to S3 handling. Condition parsing and evaluation live in filer_grpc_server_condition.go.