Handle no auditor tree head in stored state

db/shared.go

@@ -12,5 +12,8 @@ func deserializeStoredTreeHead(raw []byte) (*TransparencyTreeHead, map[string]*A
 	if err := json.Unmarshal(raw, data); err != nil {
 		return nil, nil, err
 	}
+	if data.AuditorHeads == nil {
+		data.AuditorHeads = make(map[string]*AuditorTreeHead)
+	}
 	return data.TreeHead, data.AuditorHeads, nil
 }

db/shared_test.go

@@ -11,6 +11,7 @@ import (
 	"testing"
 
 	"github.com/kinbiko/jsonassert"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestSerializeNewStoredTreeHead(t *testing.T) {
@@ -54,6 +55,30 @@ func TestSerializeNewStoredTreeHead(t *testing.T) {
 	jsonassert.New(t).Assertf(actualJsonString, "%s", expectedJsonString)
 }
 
+func TestDeserializeStoredTreeHeadWithNoAuditorHeads(t *testing.T) {
+	storedTreeHead := &storedTreeHead{
+		TreeHead: &TransparencyTreeHead{
+			TreeSize:  123,
+			Timestamp: 123456,
+			Signatures: []*Signature{
+				{random(32), random(32)},
+			},
+		},
+	}
+
+	bytes, err := json.Marshal(storedTreeHead)
+	if err != nil {
+		t.Fatal("expected no error marshaling stored tree head")
+	}
+
+	_, auditorHeads, err := deserializeStoredTreeHead(bytes)
+	if err != nil {
+		t.Fatal("expected no error deserializing stored tree head")
+	}
+
+	assert.NotNil(t, auditorHeads)
+}
+
 func random(numBytes int) []byte {
 	out := make([]byte, numBytes)
 	if _, err := rand.Read(out); err != nil {