Only import block if it isn't already on the blockchain (#962)

ethereum/core/src/main/java/tech/pegasys/pantheon/ethereum/mainnet/MainnetBlockImporter.java

@@ -37,6 +37,9 @@ public synchronized boolean importBlock(
       final Block block,
       final HeaderValidationMode headerValidationMode,
       final HeaderValidationMode ommerValidationMode) {
+    if (context.getBlockchain().contains(block.getHash())) {
+      return true;
+    }
 
     final Optional<BlockProcessingOutputs> outputs =
         blockValidator.validateAndProcessBlock(

ethereum/core/src/test/java/tech/pegasys/pantheon/ethereum/mainnet/MainnetBlockImporterTest.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2019 ConsenSys AG.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+package tech.pegasys.pantheon.ethereum.mainnet;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+import static tech.pegasys.pantheon.ethereum.mainnet.HeaderValidationMode.FULL;
+
+import tech.pegasys.pantheon.ethereum.BlockValidator;
+import tech.pegasys.pantheon.ethereum.ProtocolContext;
+import tech.pegasys.pantheon.ethereum.chain.MutableBlockchain;
+import tech.pegasys.pantheon.ethereum.core.Block;
+import tech.pegasys.pantheon.ethereum.core.Hash;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+
+@RunWith(MockitoJUnitRunner.class)
+public class MainnetBlockImporterTest {
+  @Mock private BlockValidator<Object> blockValidator;
+  @Mock private ProtocolContext<Object> context;
+  @Mock private MutableBlockchain blockchain;
+  @Mock private Block block;
+  @Mock private Hash hash;
+  private MainnetBlockImporter<Object> blockImporter;
+
+  @Before
+  public void setup() {
+    blockImporter = new MainnetBlockImporter<>(blockValidator);
+    when(context.getBlockchain()).thenReturn(blockchain);
+    when(block.getHash()).thenReturn(hash);
+  }
+
+  @Test
+  public void doNotImportBlockIfBlockchainAlreadyHasBlock() {
+    when(blockchain.contains(hash)).thenReturn(true);
+
+    assertThat(blockImporter.importBlock(context, block, FULL, FULL)).isTrue();
+    verify(blockValidator, never()).validateAndProcessBlock(context, block, FULL, FULL);
+    verify(blockchain, never()).appendBlock(eq(block), any());
+  }
+}