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, 28 insertions, 1 deletions

diff --git a/methods/basehttp.cc b/methods/basehttp.cc
index 26b633cba..d335e6fa0 100644
--- a/methods/basehttp.cc
+++ b/methods/basehttp.cc
@@ -263,6 +263,21 @@ bool RequestState::HeaderLine(string const &Line)			/*{{{*/
       return true;
    }
 
+   if (stringcasecmp(Tag, "Retry-After:") == 0)
+   {
+      unsigned long _retry_after_s;
+      if (RFC1123StrToTime(Val, RetryAfter))
+      {
+	 return true;
+      }
+      if (StrToNum(Val.c_str(), _retry_after_s, 4, 10) == 1)
+      {
+	 RetryAfter = time(nullptr) + _retry_after_s;
+	 return true;
+      }
+      return _error->Error(_("Unknown date format"));
+   }
+
    if (Server->RangesAllowed && stringcasecmp(Tag, "Via:") == 0)
    {
       auto const parts = VectorizeString(Val, ' ');
@@ -884,7 +899,19 @@ int BaseHttpMethod::Loop()
 	       599, // Network Connect Timeout Error
 	    };
 	    if (std::find(std::begin(TransientCodes), std::end(TransientCodes), Req.Result) != std::end(TransientCodes))
-	       Fail(true);
+	    {
+	       if (Req.RetryAfter)
+	       {
+		  std::unordered_map<std::string, std::string> fields;
+		  fields["Retry-After"] = std::to_string(Req.RetryAfter);
+		  SetFailReason("TooManyRequests");
+		  FailWithContext(std::string("TooManyRequests"), true, fields);
+	       }
+	       else
+	       {
+		  Fail(true);
+	       }
+	    }
 	    else
 	       Fail();
 	    break;