First, when performing an upload using an upload session, data is streamed directly from the input stream to the request stream. (It is no longer read into an intermediate buffer first.)
Hopefully this will help these uploads respond to throttling behavior better.
Second, these backends now check for and respect a Retry-After header if it is present.
On 5xx errors, the existing exponential backoff is used for the intermediate retries.
A few other minor things have also been cleaned up (e.g., using Task.Delay instead of Thread.Sleep)
Tyler Gill
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