Test empty objects, conflicts, errors in a 200, and object sizes on multipart writes

[tiptenbrink]

Summary

• An empty object through multipart, on S3 and S3 Express:

  • stage-empty-part stages a part with an empty body, which S3 takes.
  • commit-one-empty-part completes an upload of that one part, which writes an empty object.
  • S3 refuses a completion of no parts, and commit-nothing lets a client refuse one locally. Its neighbor that a client must send is now commit-one-empty-part, so a client that refuses no parts must still write an empty object this way.

• A 409 conflict on a conditional write, on S3. S3 answers ConditionalRequestConflict when a conditional write meets a concurrent one. AWS advises retrying a PutObject, but not a CompleteMultipartUpload: after the conflict, the caller starts a new upload and stages every part again. So:

  • put-conditional-conflict accepts the error, and up to five retries before it, which it never requires.
  • commit-conditional-conflict accepts no retry: a client must not send the completion again.
  • S3 sends the 409 only during such a race, so both cases are transient. Their requests are verified against the 412 that S3 answers them now.

• An error in a 200, on S3 and S3 Express. AWS documents that CopyObject, UploadPartCopy and CompleteMultipartUpload may answer 200 with an <Error> body, and advises treating an InternalError there as a server error. copy-200-internal-error and commit-200-internal-error accept the error, with up to five optional retries before it, and never a success.

• A stated object size, on S3 and S3 Express. A completion may send the size it expects in x-amz-mp-object-size, the call's new object_size. commit-object-size succeeds. In commit-object-size-mismatch the parts add up to 3 bytes and the header says 4: a general-purpose bucket answers 400 InvalidRequest, and a directory bucket sends the same error in a 200.

All cases were verified live. The AWS C++ SDK passes the empty-part, conflict and object-size cases on S3. It reports an InternalError in a 200 with an empty code, and an Express completion error with status -1, as it does elsewhere.

For clients

• A client that retries a completion after a 409 fails commit-conditional-conflict.

• A retry is never required: every retry exchange is optional.

• object_size is a new field of s3.complete_multipart.

Test plan

• object-tests validate cases/operations.json passes (931 cases).