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 condition is a single primitive (exists / not-exists / etag match / etag
mismatch / [un]modified-since). The caller reduces request semantics such as
RFC 7232 precedence to one primitive; the filer just compares. ETag comparison
mirrors the S3 gateway's precedence (stored Seaweed ETag attribute, then the
Md5/chunk fallback) so results match without coupling the filer to S3 handling.
Chris Lu
4e421400e4