A versioned write's only contended mutation is the .versions directory's latest
pointer; the version file itself goes to a unique <object>/.versions/<versionId>
path. Add a FinalizeVersionedWrite filer op that, under one exclusive lock on the
object key, evaluates the precondition against the current latest, stamps the
previous latest noncurrent (before the pointer flip so the lifecycle router
observes it), then merges the latest pointer / cached metadata into the .versions
entry. Key names are passed in, so the filer carries no S3 semantics.
Routing and the lock are keyed on the object (objectWriteOwner + lock_key), the
same key normal and suspended writes use, so all writes to one object resolve the
same owner and serialize on the same lock regardless of versioning state — a
versioned and a non-versioned write to the same object can't race on different
owners during a versioning-state change.
The gateway routes a versioned PutObject's finalize to that owner and tells
putToFiler the version path is unique, so it skips the object write lock and the
gateway precondition (the op does both atomically). When the owner is unknown or
the condition can't reduce to one primitive, it stays on the lock path; on op
error it returns InternalError. Versioned COPY, delete markers, suspended
versioning, and multipart completion still use the lock and adopt the same op as
follow-ups.
Chris Lu
2b8349f9e9