The message length is pre-calculated in advance to provide correct
content-length request header. This math is not obvious and deserves a
comment.
Also, the final message preparation code is also implicitly checking
if any part failed to upload. There's a comment in the upload_sink's
upload_part() method about it, but the finalization place deserves one
too.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
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