Upload chunks are buffered into a bytes.Reader and then, when transfer
accounting is active (always for a real copy), re-wrapped in accounting
before being handed to cachedReader. cachedReader only recognised a bare
*bytes.Reader, so the accounted chunk fell through to
readers.NewRepeatableReader, which copied the whole chunk again into an
append-grown slice. Every chunk (and the upload-cutoff prefix of every
file) therefore cost roughly twice its size in memory.
Look through the accounting wrapper when deciding whether the reader is
already a seekable buffer and, if so, seek the buffer underneath while
still reading through the accounting, so retries rewind without a copy.
Nick Craig-Wood
2a492cc355