NC-1815 Rinkeby import can stall with too many fragments revision

As recommended in #231 alter the API so that the
subclasses always return their list from executePeerTask in the
`CompletableFuture` and reset the retry in `#executeTask()` when any
non-empty list is returned.

ethereum/eth/src/main/java/tech/pegasys/pantheon/ethereum/eth/manager/AbstractRetryingPeerTask.java

@@ -18,6 +18,7 @@ import tech.pegasys.pantheon.ethereum.eth.sync.tasks.WaitForPeerTask;
 import tech.pegasys.pantheon.util.ExceptionUtils;
 
 import java.time.Duration;
+import java.util.Collection;
 import java.util.concurrent.CompletableFuture;
 
 import org.apache.logging.log4j.LogManager;
@@ -25,16 +26,12 @@ import org.apache.logging.log4j.Logger;
 
 /**
  * A task that will retry a fixed number of times before completing the associated CompletableFuture
- * exceptionally with a new {@link MaxRetriesReachedException}.
+ * exceptionally with a new {@link MaxRetriesReachedException}. If the future returned from {@link
+ * #executePeerTask()} is complete with a non-empty list the retry counter is reset.
  *
- * <p>As an additional semantic subclasses may call {@link #resetRetryCounter} so that if they have
- * partial success they can reset the retry counter and only count zero progress retries against the
- * exception limit. If this facility is used only consecutive zero progress retries count against
- * the maximum retries limit.
- *
- * @param <T> The type of the CompletableFuture this task will return.
+ * @param <T> The type as a typed list that the peer task can get partial or full results in.
  */
-public abstract class AbstractRetryingPeerTask<T> extends AbstractEthTask<T> {
+public abstract class AbstractRetryingPeerTask<T extends Collection<?>> extends AbstractEthTask<T> {
 
   private static final Logger LOG = LogManager.getLogger();
   private final EthContext ethContext;
@@ -68,12 +65,16 @@ public abstract class AbstractRetryingPeerTask<T> extends AbstractEthTask<T> {
               if (error != null) {
                 handleTaskError(error);
               } else {
+                // If we get a partial success reset the retry counter.
+                if (peerResult.size() > 0) {
+                  retryCount = 0;
+                }
                 executeTask();
               }
             });
   }
 
-  protected abstract CompletableFuture<?> executePeerTask();
+  protected abstract CompletableFuture<T> executePeerTask();
 
   private void handleTaskError(final Throwable error) {
     final Throwable cause = ExceptionUtils.rootCause(error);
@@ -106,13 +107,5 @@ public abstract class AbstractRetryingPeerTask<T> extends AbstractEthTask<T> {
             ethContext.getScheduler().scheduleFutureTask(this::executeTask, Duration.ofSeconds(1)));
   }
 
-  /**
-   * Reset the retryCounter. Once called executeTask will get a fresh set of retries to complete the
-   * task.
-   */
-  protected void resetRetryCounter() {
-    retryCount = 0;
-  }
-
   protected abstract boolean isRetryableError(Throwable error);
 }

ethereum/eth/src/main/java/tech/pegasys/pantheon/ethereum/eth/sync/tasks/CompleteBlocksTask.java

@@ -83,7 +83,7 @@ public class CompleteBlocksTask<C> extends AbstractRetryingPeerTask<List<Block>>
   }
 
   @Override
-  protected CompletableFuture<?> executePeerTask() {
+  protected CompletableFuture<List<Block>> executePeerTask() {
     return requestBodies().thenCompose(this::processBodiesResult);
   }
 
@@ -117,26 +117,18 @@ public class CompleteBlocksTask<C> extends AbstractRetryingPeerTask<List<Block>>
         });
   }
 
-  private CompletableFuture<Void> processBodiesResult(
+  private CompletableFuture<List<Block>> processBodiesResult(
       final PeerTaskResult<List<Block>> blocksResult) {
-    final int startingIncompleteHeaders = incompleteHeaders().size();
     blocksResult.getResult().forEach((block) -> blocks.put(block.getHeader().getNumber(), block));
 
-    final int endingIncompleteHeaders = incompleteHeaders().size();
-    final boolean done = endingIncompleteHeaders == 0;
-    if (done) {
+    if (incompleteHeaders().size() == 0) {
       result
           .get()
           .complete(
               headers.stream().map(h -> blocks.get(h.getNumber())).collect(Collectors.toList()));
-    } else if (endingIncompleteHeaders < startingIncompleteHeaders) {
-      // If we made any progress reset the retry counter
-      resetRetryCounter();
     }
 
-    final CompletableFuture<Void> future = new CompletableFuture<>();
-    future.complete(null);
-    return future;
+    return CompletableFuture.completedFuture(blocksResult.getResult());
   }
 
   private List<BlockHeader> incompleteHeaders() {

ethereum/eth/src/main/java/tech/pegasys/pantheon/ethereum/eth/sync/tasks/DownloadHeaderSequenceTask.java

@@ -106,7 +106,7 @@ public class DownloadHeaderSequenceTask<C> extends AbstractRetryingPeerTask<List
   }
 
   @Override
-  protected CompletableFuture<?> executePeerTask() {
+  protected CompletableFuture<List<BlockHeader>> executePeerTask() {
     LOG.debug(
         "Downloading headers from {} to {}.", startingBlockNumber, referenceHeader.getNumber() - 1);
     final CompletableFuture<List<BlockHeader>> task =
@@ -163,7 +163,6 @@ public class DownloadHeaderSequenceTask<C> extends AbstractRetryingPeerTask<List
           BlockHeader child = null;
           boolean firstSkipped = false;
           for (final BlockHeader header : headersResult.getResult()) {
-            resetRetryCounter();
             final int headerIndex =
                 Ints.checkedCast(
                     segmentLength - (referenceHeader.getNumber() - header.getNumber()));