Fix data race on dynFields length in Builder.Clone (#72)

Builder.Clone was reading the length of dynFields without holding
dynFieldsLock. This moves the read to be covered by the lock.

libhoney.go

@@ -915,7 +915,6 @@ func (b *Builder) Clone() *Builder {
 		Dataset:    b.Dataset,
 		SampleRate: b.SampleRate,
 		APIHost:    b.APIHost,
-		dynFields:  make([]dynamicField, 0, len(b.dynFields)),
 		client:     b.client,
 	}
 	newB.data = make(map[string]interface{})
@@ -927,9 +926,8 @@ func (b *Builder) Clone() *Builder {
 	// copy dynamic metric generators
 	b.dynFieldsLock.RLock()
 	defer b.dynFieldsLock.RUnlock()
-	for _, dynFd := range b.dynFields {
-		newB.dynFields = append(newB.dynFields, dynFd)
-	}
+	newB.dynFields = make([]dynamicField, len(b.dynFields))
+	copy(newB.dynFields, b.dynFields)
 	return newB
 }
 

libhoney_test.go

@@ -526,6 +526,32 @@ func TestBuilderStaticFields(t *testing.T) {
 	testEquals(t, ev4.data["floatF"], nil)
 }
 
+func TestBuilderDynFieldsCloneRace(t *testing.T) {
+	resetPackageVars()
+
+	b := NewBuilder()
+
+	const interations = 100
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for i := 0; i < interations; i++ {
+			b.Clone()
+		}
+	}()
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for i := 0; i < interations; i++ {
+			b.AddDynamicField("dyn_field", nil)
+		}
+	}()
+
+	wg.Wait()
+}
+
 func TestOutputInterface(t *testing.T) {
 	resetPackageVars()
 	testTx := &MockOutput{}