Fix estimate gas RPC failing for clique when no blocks have been created (#1528)

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

consensus/clique/src/main/java/tech/pegasys/pantheon/consensus/clique/CliqueBlockHashing.java

@@ -23,7 +23,6 @@ import tech.pegasys.pantheon.util.bytes.BytesValue;
 import java.util.function.Supplier;
 
 public class CliqueBlockHashing {
-
   /**
    * Constructs a hash of the block header, suitable for use when creating the proposer seal. The
    * extra data is modified to have a null proposer seal and empty list of committed seals.
@@ -49,6 +48,9 @@ public class CliqueBlockHashing {
   public static Address recoverProposerAddress(
       final BlockHeader header, final CliqueExtraData cliqueExtraData) {
     if (!cliqueExtraData.getProposerSeal().isPresent()) {
+      if (header.getNumber() == BlockHeader.GENESIS_BLOCK_NUMBER) {
+        return Address.ZERO;
+      }
       throw new IllegalArgumentException(
           "Supplied cliqueExtraData does not include a proposer " + "seal");
     }

consensus/clique/src/test/java/tech/pegasys/pantheon/consensus/clique/CliqueBlockHashingTest.java

@@ -55,6 +55,16 @@ public class CliqueBlockHashingTest {
     assertThat(VALIDATORS_IN_HEADER.contains(proposerAddress)).isTrue();
   }
 
+  @Test
+  public void recoverProposerAddressForGenesisBlockReturnsAddressZero() {
+    final BlockHeader genesisBlockHeader = createGenesisBlock();
+    final CliqueExtraData cliqueExtraData = CliqueExtraData.decode(genesisBlockHeader);
+    final Address proposerAddress =
+        CliqueBlockHashing.recoverProposerAddress(genesisBlockHeader, cliqueExtraData);
+
+    assertThat(proposerAddress).isEqualTo(Address.ZERO);
+  }
+
   @Test
   public void readValidatorListFromExtraData() {
     final CliqueExtraData cliqueExtraData = CliqueExtraData.decode(expectedHeader);
@@ -101,4 +111,33 @@ public class CliqueBlockHashingTest {
 
     return builder.buildBlockHeader();
   }
+
+  private BlockHeader createGenesisBlock() {
+    // The following was taken from the Rinkeby genesis file
+    final BlockHeaderBuilder builder = new BlockHeaderBuilder();
+    builder.difficulty(UInt256.of(1));
+    builder.extraData(
+        BytesValue.fromHexString(
+            "0x52657370656374206d7920617574686f7269746168207e452e436172746d616e42eb768f2244c8811c63729a21a3569731535f067ffc57839b00206d1ad20c69a1981b489f772031b279182d99e65703f0076e4812653aab85fca0f00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"));
+    builder.gasLimit(4700000);
+    builder.gasUsed(0);
+    // Do not do Hash.
+    builder.logsBloom(
+        LogsBloomFilter.fromHexString(
+            "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"));
+    builder.coinbase(Address.fromHexString("0x0000000000000000000000000000000000000000"));
+    builder.mixHash(Hash.ZERO);
+    builder.nonce(0);
+    builder.number(0);
+    builder.parentHash(Hash.ZERO);
+    builder.receiptsRoot(Hash.ZERO);
+    builder.ommersHash(Hash.ZERO);
+    builder.stateRoot(Hash.ZERO);
+    builder.timestamp(1492009146);
+    builder.transactionsRoot(Hash.ZERO);
+
+    builder.blockHeaderFunctions(new CliqueBlockHeaderFunctions());
+
+    return builder.buildBlockHeader();
+  }
 }

ethereum/core/src/main/java/tech/pegasys/pantheon/ethereum/core/Address.java

@@ -42,6 +42,8 @@ public class Address extends DelegatingBytesValue {
   public static final Integer PRIVACY = Byte.MAX_VALUE - 1;
   public static final Address DEFAULT_PRIVACY = Address.precompiled(PRIVACY);
 
+  public static final Address ZERO = Address.fromHexString("0x0");
+
   protected Address(final BytesValue bytes) {
     super(bytes);
     checkArgument(