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http: use Retry-After HTTP header to optimize retries

Felix Moessbauer requested to merge fmoessbauer/apt:fm/apt-rate-limit into main

Some mirrors like snapshot.debian.org apply strict http rate limits. While apt already has an exponential backoff mechanism implemented, this is not sufficient due to the following reasons:

  1. all retries happen roughly at the same time
  2. the retry-after information from the server is not used

We fix this by improving the algorithm: First, if present, the timestamp or duration of the Retry-After header is added to the exponential backoff, optimizing for success-on-second-try. Second, a random delay is added (delay += [0, delay]) to distribute the retries to not immediately run into the limit again. To avoid super-long delays, this is capped by the configured maximum delay.

Signed-off-by: Felix Moessbauer felix.moessbauer@siemens.com

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