Save an unnecessary allocation of the result base type

We were doing an extra allocation just to get the result base type. With
some re-wiring, we don't need that extra allocation

reflect/reflect.go

@@ -36,7 +36,7 @@ func StructToMap(val interface{}) (map[string]interface{}, bool) {
 	return mapVal, true
 }
 
-// StructFieldMap takes a struct and extracts the Field info into a map by
+// StructFieldMap takes a struct type and extracts the Field info into a map by
 // field name. The "cql" key in the struct field's tag value is the key
 // name. Examples:
 //
@@ -50,15 +50,12 @@ func StructToMap(val interface{}) (map[string]interface{}, bool) {
 //   Field int "myName"
 //
 // If lowercaseFields is set to true, field names are lowercased in the map
-func StructFieldMap(val interface{}, lowercaseFields bool) (map[string]Field, error) {
-	// indirect so function works with both structs and pointers to them
-	structVal := r.Indirect(r.ValueOf(val))
-	kind := structVal.Kind()
-	if kind != r.Struct {
-		return nil, fmt.Errorf("expected val to be a struct, got %T", val)
+func StructFieldMap(structType r.Type, lowercaseFields bool) (map[string]Field, error) {
+	if structType.Kind() != r.Struct {
+		return nil, fmt.Errorf("expected val to be a struct, got %v", structType)
 	}
 
-	structFields := cachedTypeFields(structVal.Type())
+	structFields := cachedTypeFields(structType)
 	mapVal := make(map[string]Field, len(structFields))
 	for _, info := range structFields {
 		name := info.name

reflect/reflect_test.go

@@ -138,7 +138,7 @@ func TestMapToStruct(t *testing.T) {
 }
 
 func TestStructFieldMap(t *testing.T) {
-	m, err := StructFieldMap(Tweet{}, false)
+	m, err := StructFieldMap(reflect.TypeOf(Tweet{}), false)
 	if err != nil {
 		t.Fatalf("expected field map to be created, err: %v", err)
 	}
@@ -185,7 +185,7 @@ func TestStructFieldMap(t *testing.T) {
 	}
 
 	// Test lowercasing fields
-	m2, err := StructFieldMap(Tweet{}, true)
+	m2, err := StructFieldMap(reflect.TypeOf(Tweet{}), true)
 	if err != nil {
 		t.Fatalf("expected field map to be created, err: %v", err)
 	}
@@ -203,7 +203,7 @@ func TestStructFieldMapEmbeddedStruct(t *testing.T) {
 		Embedder string
 	}
 
-	m, err := StructFieldMap(EmbeddedTweet{}, false)
+	m, err := StructFieldMap(reflect.TypeOf(EmbeddedTweet{}), false)
 	if err != nil {
 		t.Fatalf("expected field map to be created, err: %v", err)
 	}
@@ -234,7 +234,7 @@ func TestStructFieldMapEmbeddedStruct(t *testing.T) {
 }
 
 func TestStructFieldMapNonStruct(t *testing.T) {
-	_, err := StructFieldMap(42, false)
+	_, err := StructFieldMap(reflect.TypeOf(42), false)
 	if err == nil {
 		t.Fatalf("expected StructFieldMap to have an error, got nil error")
 	}

scanner.go

@@ -81,6 +81,7 @@ func (s *scanner) iterSlice(iter Scannable) (int, error) {
 		if err != nil {
 			return rowsScanned, err
 		}
+		fillInZeroedPtrs(ptrs)
 
 		sliceElem.Set(reflect.Append(sliceElem, wrapPtrValue(outVal, sliceElemType)))
 		rowsScanned++
@@ -124,6 +125,7 @@ func (s *scanner) iterSingle(iter Scannable) (int, error) {
 	if err != nil {
 		return 0, err
 	}
+	fillInZeroedPtrs(ptrs)
 
 	s.rowsScanned++
 	return 1, nil
@@ -132,10 +134,9 @@ func (s *scanner) iterSingle(iter Scannable) (int, error) {
 // structFields matches the SelectStatement field names selected to names of
 // fields within the target struct type
 func (s *scanner) structFields(structType reflect.Type) ([]*r.Field, error) {
-	fmPtr := reflect.New(structType).Interface()
-	m, err := r.StructFieldMap(fmPtr, true)
+	m, err := r.StructFieldMap(structType, true)
 	if err != nil {
-		return nil, fmt.Errorf("could not decode struct of type %T: %v", fmPtr, err)
+		return nil, fmt.Errorf("could not decode struct of type %v: %v", structType, err)
 	}
 
 	structFields := make([]*r.Field, len(s.stmt.fields))
@@ -185,16 +186,47 @@ func generatePtrs(structFields []*r.Field, structVal reflect.Value) []interface{
 
 		switch elem.Kind() {
 		case reflect.Map:
-			elem.Set(reflect.MakeMap(elem.Type()))
+			if elem.IsNil() {
+				elem.Set(reflect.MakeMap(elem.Type()))
+			}
 		case reflect.Slice:
-			elem.Set(reflect.MakeSlice(elem.Type(), 0, 0))
+			if elem.IsNil() {
+				elem.Set(reflect.MakeSlice(elem.Type(), 0, 0))
+			}
 		}
 
 		ptrs[i] = elem.Addr().Interface()
 	}
 	return ptrs
 }
 
+// fillInZeroedPtrs is necessary to re-allocate nil slices/maps in our ptr
+// list. Gocql unfortunately sees no data as an opportunity to zero out the
+// entire slice rather than leaving it as the empty slice. This means something
+// like []string{} will get turned into []string(nil) which aren't technically
+// the same
+func fillInZeroedPtrs(ptrs []interface{}) {
+	for _, ptr := range ptrs {
+		if _, ok := ptr.(*ignoreFieldType); ok {
+			continue
+		}
+
+		elem := reflect.ValueOf(ptr).Elem()
+
+		switch elem.Kind() {
+		case reflect.Map:
+			if elem.IsNil() || elem.IsZero() {
+				elem.Set(reflect.MakeMap(elem.Type()))
+			}
+		case reflect.Slice:
+			if elem.IsNil() || elem.IsZero() {
+				elem.Set(reflect.MakeSlice(elem.Type(), 0, 0))
+			}
+		}
+
+	}
+}
+
 // allocateNilReference checks to see if the in is not nil itself but points to
 // an object which itself is nil. Note that it only checks one depth down.
 // Returns true if any allocation has happened, false if no allocation was needed

scanner_test.go

@@ -254,6 +254,27 @@ func TestScanIterEmbedded(t *testing.T) {
 	iter.Reset()
 }
 
+func TestFillInZeroedPtrs(t *testing.T) {
+	str := ""
+	strSlice := []string{}
+	strMap := map[string]string{}
+	strSliceNil := []string(nil)
+	strMapNil := map[string]string(nil)
+
+	// Test with already allocated
+	fillInZeroedPtrs([]interface{}{&str, &strSlice, &strMap})
+	assert.Equal(t, "", str)
+	assert.Equal(t, []string{}, strSlice)
+	assert.Equal(t, map[string]string{}, strMap)
+
+	// Test with nil allocated
+	assert.NotEqual(t, []string{}, strSliceNil)
+	assert.NotEqual(t, map[string]string{}, strMapNil)
+	fillInZeroedPtrs([]interface{}{&strSliceNil, &strMapNil})
+	assert.Equal(t, []string{}, strSliceNil)
+	assert.Equal(t, map[string]string{}, strMapNil)
+}
+
 func TestAllocateNilReference(t *testing.T) {
 	// Test non pointer, should do nothing
 	var a string