fix: do not retry if context has been cancelled

The additional retry logic introduced by #337 caused any error to become
retry-able if the request did not make it to the back end. This included
errors that came from context cancellation. This commit makes requests
non-retry-able as soon as the context has been cancelled.

Co-Authored-By: Dominik Froehlich <[email protected]>

Author: maxmoehl

proxy/round_tripper/proxy_round_tripper.go

@@ -441,6 +441,11 @@ func isIdempotent(request *http.Request) bool {
 }
 
 func (rt *roundTripper) isRetriable(request *http.Request, err error, trace *requestTracer) (bool, error) {
+	// if the context has been cancelled we do not perform further retries
+	if request.Context().Err() != nil {
+		return false, request.Context().Err()
+	}
+
 	// io.EOF errors are considered safe to retry for certain requests
 	// Replace the error here to track this state when classifying later.
 	if err == io.EOF && isIdempotent(request) {