[MINOR] Eliminate redundant header validation (#943)

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

ethereum/eth/src/main/java/tech/pegasys/pantheon/ethereum/eth/sync/BlockPropagationManager.java

@@ -109,32 +109,6 @@ public class BlockPropagationManager<C> {
         .subscribe(EthPV62.NEW_BLOCK_HASHES, this::handleNewBlockHashesFromNetwork);
   }
 
-  private void validateAndBroadcastBlock(final Block block) {
-    final ProtocolSpec<C> protocolSpec =
-        protocolSchedule.getByBlockNumber(block.getHeader().getNumber());
-    final BlockHeaderValidator<C> blockHeaderValidator = protocolSpec.getBlockHeaderValidator();
-    final BlockHeader parent =
-        protocolContext
-            .getBlockchain()
-            .getBlockHeader(block.getHeader().getParentHash())
-            .orElseThrow(
-                () ->
-                    new IllegalArgumentException(
-                        "Incapable of retrieving header from non-existent parent of "
-                            + block.getHeader().getNumber()
-                            + "."));
-    if (blockHeaderValidator.validateHeader(
-        block.getHeader(), parent, protocolContext, HeaderValidationMode.FULL)) {
-      final UInt256 totalDifficulty =
-          protocolContext
-              .getBlockchain()
-              .getTotalDifficultyByHash(parent.getHash())
-              .get()
-              .plus(block.getHeader().getDifficulty());
-      blockBroadcaster.propagate(block, totalDifficulty);
-    }
-  }
-
   private void onBlockAdded(final BlockAddedEvent blockAddedEvent, final Blockchain blockchain) {
     // Check to see if any of our pending blocks are now ready for import
     final Block newBlock = blockAddedEvent.getBlock();
@@ -263,6 +237,16 @@ public class BlockPropagationManager<C> {
     return getBlockTask.run().thenCompose((r) -> importOrSavePendingBlock(r.getResult()));
   }
 
+  private void broadcastBlock(final Block block, final BlockHeader parent) {
+    final UInt256 totalDifficulty =
+        protocolContext
+            .getBlockchain()
+            .getTotalDifficultyByHash(parent.getHash())
+            .get()
+            .plus(block.getHeader().getDifficulty());
+    blockBroadcaster.propagate(block, totalDifficulty);
+  }
+
   @VisibleForTesting
   CompletableFuture<Block> importOrSavePendingBlock(final Block block) {
     // Synchronize to avoid race condition where block import event fires after the
@@ -292,19 +276,54 @@ public class BlockPropagationManager<C> {
       return CompletableFuture.completedFuture(block);
     }
 
-    ethContext.getScheduler().scheduleSyncWorkerTask(() -> validateAndBroadcastBlock(block));
+    final BlockHeader parent =
+        protocolContext
+            .getBlockchain()
+            .getBlockHeader(block.getHeader().getParentHash())
+            .orElseThrow(
+                () ->
+                    new IllegalArgumentException(
+                        "Incapable of retrieving header from non-existent parent of "
+                            + block.getHeader().getNumber()
+                            + "."));
 
-    // Import block
-    final PersistBlockTask<C> importTask =
-        PersistBlockTask.create(
-            protocolSchedule, protocolContext, block, HeaderValidationMode.FULL, metricsSystem);
+    final ProtocolSpec<C> protocolSpec =
+        protocolSchedule.getByBlockNumber(block.getHeader().getNumber());
+    final BlockHeaderValidator<C> blockHeaderValidator = protocolSpec.getBlockHeaderValidator();
     return ethContext
         .getScheduler()
-        .scheduleSyncWorkerTask(importTask::run)
+        .scheduleSyncWorkerTask(
+            () -> validateAndProcessPendingBlock(blockHeaderValidator, block, parent));
+  }
+
+  private CompletableFuture<Block> validateAndProcessPendingBlock(
+      final BlockHeaderValidator<C> blockHeaderValidator,
+      final Block block,
+      final BlockHeader parent) {
+    if (blockHeaderValidator.validateHeader(
+        block.getHeader(), parent, protocolContext, HeaderValidationMode.FULL)) {
+      ethContext.getScheduler().scheduleSyncWorkerTask(() -> broadcastBlock(block, parent));
+      return runImportTask(block);
+    } else {
+      importingBlocks.remove(block.getHash());
+      LOG.warn(
+          "Failed to import announced block {} ({}).",
+          block.getHeader().getNumber(),
+          block.getHash());
+      return CompletableFuture.completedFuture(block);
+    }
+  }
+
+  private CompletableFuture<Block> runImportTask(final Block block) {
+    final PersistBlockTask<C> importTask =
+        PersistBlockTask.create(
+            protocolSchedule, protocolContext, block, HeaderValidationMode.NONE, metricsSystem);
+    return importTask
+        .run()
         .whenComplete(
-            (r, t) -> {
+            (result, throwable) -> {
               importingBlocks.remove(block.getHash());
-              if (t != null) {
+              if (throwable != null) {
                 LOG.warn(
                     "Failed to import announced block {} ({}).",
                     block.getHeader().getNumber(),