Fix epoch block marshaling and add test for it

Epoch blocks previously were not json unmarshalable by the geth client,
they now are.

core/types/block.go

@@ -18,6 +18,7 @@
 package types
 
 import (
+	"encoding/json"
 	"fmt"
 	"io"
 	"math/big"
@@ -183,6 +184,37 @@ func (r *EpochSnarkData) IsEmpty() bool {
 	return len(r.Signature) == 0
 }
 
+// MarshalJSON marshals as JSON.
+func (r EpochSnarkData) MarshalJSON() ([]byte, error) {
+	type EpochSnarkData struct {
+		Bitmap    hexutil.Bytes
+		Signature hexutil.Bytes
+	}
+	var enc EpochSnarkData
+	enc.Bitmap = r.Bitmap.Bytes()
+	enc.Signature = r.Signature
+	return json.Marshal(&enc)
+}
+
+// UnmarshalJSON unmarshals from JSON.
+func (r *EpochSnarkData) UnmarshalJSON(input []byte) error {
+	type EpochSnarkData struct {
+		Bitmap    hexutil.Bytes
+		Signature hexutil.Bytes
+	}
+	var dec EpochSnarkData
+	if err := json.Unmarshal(input, &dec); err != nil {
+		return err
+	}
+	if dec.Bitmap != nil {
+		r.Bitmap = new(big.Int).SetBytes(dec.Bitmap)
+	}
+	if dec.Signature != nil {
+		r.Signature = dec.Signature
+	}
+	return nil
+}
+
 // Body is a simple (mutable, non-safe) data container for storing and moving
 // a block's data contents (transactions and uncles) together.
 type Body struct {

e2e_test/e2e_test.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/celo-org/celo-blockchain/test"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
@@ -37,3 +38,31 @@ func TestSendCelo(t *testing.T) {
 	err = network.AwaitTransactions(ctx, tx)
 	require.NoError(t, err)
 }
+
+// This test is intended to ensure that epoch blocks can be correctly marshalled.
+// We previously had an open bug for this https://github.com/celo-org/celo-blockchain/issues/1574
+func TestEpochBlockMarshaling(t *testing.T) {
+	accounts := test.Accounts(1)
+	gc, ec, err := test.BuildConfig(accounts)
+	require.NoError(t, err)
+
+	// Configure the shortest possible epoch, uptimeLookbackWindow minimum is 3
+	// and it needs to be < (epoch -2).
+	ec.Istanbul.Epoch = 6
+	ec.Istanbul.DefaultLookbackWindow = 3
+	network, err := test.NewNetwork(accounts, gc, ec)
+	require.NoError(t, err)
+	defer network.Shutdown()
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*30)
+	defer cancel()
+
+	// Wait for the whole network to process the transaction.
+	err = network.AwaitBlock(ctx, 6)
+	require.NoError(t, err)
+	b := network[0].Tracker.GetProcessedBlock(6)
+
+	// Check that epoch snark data was actually unmarshalled, I.E there was
+	// something there.
+	assert.True(t, len(b.EpochSnarkData().Signature) > 0)
+	assert.True(t, b.EpochSnarkData().Bitmap.Uint64() > 0)
+}