Single part uploads with Object Lock parameters need a Content-MD5
header, which the SDK can't compute from a stream, so the whole body
was read into memory with io.ReadAll to hash it - up to
--s3-upload-cutoff per file. prepareUpload already sets Content-MD5
from the source object's hash when it has one, so skip the buffering
entirely in that case and only buffer when the hash is unavailable.
When buffering is needed, read the body into a multipart.NewRW buffer
from the global pool, hashing in transit, so the memory is reused
across uploads and released after the request. The presigned request
path hands the body straight to http.NewRequest, so wrap it in
readers.NoCloser there to stop the transport closing the pooled buffer.
Nick Craig-Wood
891fddc28c