Add mechanism to retrieve missing blocks

ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/sync/BlockPropagationManager.java

@@ -66,7 +66,6 @@
 
 public class BlockPropagationManager {
   private static final Logger LOG = LoggerFactory.getLogger(BlockPropagationManager.class);
-
   private final SynchronizerConfiguration config;
   private final ProtocolSchedule protocolSchedule;
   private final ProtocolContext protocolContext;
@@ -364,6 +363,25 @@ private CompletableFuture<Block> processAnnouncedBlock(
     return getBlockFromPeers(Optional.of(peer), blockHash.number(), Optional.of(blockHash.hash()));
   }
 
+  private void requestParentBlock(final BlockHeader blockHeader) {
+    if (requestedBlocks.add(blockHeader.getParentHash())) {
+      retrieveParentBlock(blockHeader);
+    } else {
+      LOG.trace("Parent block with hash {} was already requested", blockHeader.getParentHash());
+    }
+  }
+
+  private CompletableFuture<Block> retrieveParentBlock(final BlockHeader blockHeader) {
+    final long targetParentBlockNumber = blockHeader.getNumber() - 1L;
+    final Hash targetParentBlockHash = blockHeader.getParentHash();
+    LOG.info(
+        "Retrieving parent {} of block #{} from peers",
+        targetParentBlockHash,
+        blockHeader.getNumber());
+    return getBlockFromPeers(
+        Optional.empty(), targetParentBlockNumber, Optional.of(targetParentBlockHash));
+  }
+
   private CompletableFuture<Block> getBlockFromPeers(
       final Optional<EthPeer> preferredPeer,
       final long blockNumber,
@@ -421,6 +439,10 @@ CompletableFuture<Block> importOrSavePendingBlock(final Block block, final Bytes
         if (pendingBlocksManager.registerPendingBlock(block, nodeId)) {
           LOG.info("Saving announced block {} for future import", block.toLogString());
         }
+
+        // Request parent of the lowest announced block
+        pendingBlocksManager.lowestAnnouncedBlock().ifPresent(this::requestParentBlock);
+
         return CompletableFuture.completedFuture(block);
       }
     }

ethereum/eth/src/test/java/org/hyperledger/besu/ethereum/eth/sync/AbstractBlockPropagationManagerTest.java

@@ -59,6 +59,7 @@
 import org.hyperledger.besu.testutil.TestClock;
 
 import java.util.Collections;
+import java.util.List;
 import java.util.concurrent.CompletableFuture;
 import java.util.function.Supplier;
 
@@ -634,6 +635,44 @@ public void shouldNotImportBlocksThatAreAlreadyBeingImported() {
     verify(ethScheduler, times(1)).scheduleSyncWorkerTask(any(Supplier.class));
   }
 
+  @Test
+  public void shouldRequestLowestAnnouncedPendingBlockParent() {
+    // test if block propagation manager can recover if one block is missed
+
+    blockchainUtil.importFirstBlocks(2);
+    final List<Block> blocks = blockchainUtil.getBlocks().subList(2, 4);
+
+    blockPropagationManager.start();
+
+    // Create peer and responder
+    final RespondingEthPeer peer = EthProtocolManagerTestUtil.createPeer(ethProtocolManager, 0);
+    final Responder responder = RespondingEthPeer.blockchainResponder(getFullBlockchain());
+
+    // skip first block then create messages from blocklist
+    blocks.stream()
+        .skip(1)
+        .map(this::createNewBlockHashMessage)
+        .forEach(
+            message -> { // Broadcast new block hash message
+              EthProtocolManagerTestUtil.broadcastMessage(ethProtocolManager, peer, message);
+            });
+
+    peer.respondWhile(responder, peer::hasOutstandingRequests);
+
+    // assert all blocks were imported
+    blocks.forEach(
+        block -> {
+          assertThat(blockchain.contains(block.getHash())).isTrue();
+        });
+  }
+
+  private NewBlockHashesMessage createNewBlockHashMessage(final Block block) {
+    return NewBlockHashesMessage.create(
+        Collections.singletonList(
+            new NewBlockHashesMessage.NewBlockHash(
+                block.getHash(), block.getHeader().getNumber())));
+  }
+
   @Test
   public void verifyBroadcastBlockInvocation() {
     blockchainUtil.importFirstBlocks(2);