http: use Retry-After HTTP header to optimize retries

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. This logic only becomes active if the
option Acquire::Retries::HandleRetryAfter is enabled.

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

1 files changed, 1 insertions, 0 deletions

diff --git a/doc/examples/configure-index b/doc/examples/configure-index
index 72ce1fe3a..feadca8d2 100644
--- a/doc/examples/configure-index
+++ b/doc/examples/configure-index
@@ -249,6 +249,7 @@ Acquire
       Delay "<BOOL>" {   // whether to backoff between retries using the delay: method
         Maximum "<INT>"; // maximum number of seconds to delay an item per retry
       };
+      HandleRetryAfter "<BOOL>"; // whether to respect the wait time of the Retry-After header
   };
   Source-Symlinks "<BOOL>";
   ForceHash "<STRING>"; // hashmethod used for expected hash: sha256, sha1 or md5sum