Presigned PutObject size limit

RustFS V1 supports an optional, RustFS-specific capability on a SigV4 presigned PutObject URL:

x-rustfs-max-content-length=<unsigned 64-bit integer>

The backend that creates the URL must add this query parameter to the request URI before calculating the SigV4 presign. It is part of the canonical query; adding, removing, or changing it after signing invalidates the signature. A browser can then upload with a plain PUT and does not need a custom size header.

RustFS validates the capability after SigV4 authentication and enforces it on the decoded request body. A declared Content-Length above the limit is rejected before storage. If the body produces more bytes than the limit while streaming, RustFS returns EntityTooLarge and does not publish the object.

The V1 contract is deliberately narrow:

• The parameter is accepted only on a SigV4 presigned PutObject request.
• Duplicate, case-variant, malformed, negative, or overflowing values return InvalidRequest.
• Requests without the parameter, including ordinary authenticated or anonymous PUT, keep the existing behavior.
• The parameter on CopyObject, multipart, GET, HEAD, DELETE, bucket, or other operations returns InvalidRequest.
• Unknown-length and SigV4 streaming-chunked uploads remain unsupported by the existing PutObject admission contract and are not enabled by this feature.

This capability is per request; it is not a cumulative multipart-upload cap. Multipart session limits are planned for V2 under a separate query/API contract.