fix: honor retry after header (#94)

open-vulnerability-clients/src/main/java/io/github/jeremylong/openvulnerability/client/nvd/NvdApiRetryStrategy.java

@@ -17,6 +17,9 @@
 package io.github.jeremylong.openvulnerability.client.nvd;
 
 import org.apache.hc.client5.http.impl.DefaultHttpRequestRetryStrategy;
+import org.apache.hc.client5.http.utils.DateUtils;
+import org.apache.hc.core5.http.Header;
+import org.apache.hc.core5.http.HttpHeaders;
 import org.apache.hc.core5.http.HttpRequest;
 import org.apache.hc.core5.http.HttpResponse;
 import org.apache.hc.core5.http.protocol.HttpContext;
@@ -25,12 +28,13 @@
 import org.slf4j.LoggerFactory;
 
 import java.io.IOException;
+import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.concurrent.TimeUnit;
 
 /**
- * Implements a back-off delay retry strategy.
+ * Implements a back-off delay retry strategy that honors the retry-after header.
  */
 public class NvdApiRetryStrategy extends DefaultHttpRequestRetryStrategy {
 
@@ -70,11 +74,29 @@ public boolean retryRequest(HttpResponse response, int execCount, HttpContext co
 
     @Override
     public TimeValue getRetryInterval(final HttpResponse response, final int execCount, final HttpContext context) {
-
+        TimeValue value;
         if (execCount < maxRetries / 2) {
-            return TimeValue.of(delay * execCount, TimeUnit.MILLISECONDS);
+            value = TimeValue.of(delay * execCount, TimeUnit.MILLISECONDS);
+        } else {
+            value = TimeValue.of(delay * execCount / 2, TimeUnit.MILLISECONDS);
         }
-
-        return TimeValue.of(delay * execCount / 2, TimeUnit.MILLISECONDS);
+        //check retry after header
+        final Header header = response.getFirstHeader(HttpHeaders.RETRY_AFTER);
+        if (header != null) {
+            TimeValue retryAfter = null;
+            final String headerValue = header.getValue();
+            try {
+                retryAfter = TimeValue.ofSeconds(Long.parseLong(headerValue));
+            } catch (final NumberFormatException ignore) {
+                final Instant retryAfterDate = DateUtils.parseStandardDate(headerValue);
+                if (retryAfterDate != null) {
+                    retryAfter = TimeValue.ofMilliseconds(retryAfterDate.toEpochMilli() - System.currentTimeMillis());
+                }
+            }
+            if (TimeValue.isPositive(retryAfter) && retryAfter.compareTo(value) < 0) {
+                return retryAfter;
+            }
+        }
+        return value;
     }
 }