feat(spanner): add row.String() and refine error message for decoding a struct array

spanner/read_test.go

@@ -152,11 +152,6 @@ var (
 	}
 )
 
-// String implements fmt.stringer.
-func (r *Row) String() string {
-	return fmt.Sprintf("{fields: %s, val: %s}", r.fields, r.vals)
-}
-
 func describeRows(l []*Row) string {
 	// generate a nice test failure description
 	var s = "["

spanner/row.go

@@ -88,6 +88,11 @@ type Row struct {
 	vals   []*proto3.Value // keep decoded for now
 }
 
+// String implements fmt.stringer.
+func (r *Row) String() string {
+	return fmt.Sprintf("{fields: %s, values: %s}", r.fields, r.vals)
+}
+
 // errNamesValuesMismatch returns error for when columnNames count is not equal
 // to columnValues count.
 func errNamesValuesMismatch(columnNames []string, columnValues []interface{}) error {

spanner/row_test.go

@@ -1672,6 +1672,24 @@ func TestToStructEmbedded(t *testing.T) {
 	}
 }
 
+func TestRowToString(t *testing.T) {
+	r := Row{
+		[]*sppb.StructType_Field{
+			{Name: "F1", Type: stringType()},
+			{Name: "F2", Type: stringType()},
+		},
+		[]*proto3.Value{
+			stringProto("v1"),
+			stringProto("v2"),
+		},
+	}
+	got := r.String()
+	want := `{fields: [name:"F1" type:{code:STRING} name:"F2" type:{code:STRING}], values: [string_value:"v1" string_value:"v2"]}`
+	if !testEqual(r.String(), want) {
+		t.Errorf("got %+v, want %+v", got, want)
+	}
+}
+
 // Test helpers for getting column names.
 func TestColumnNameAndIndex(t *testing.T) {
 	// Test Row.Size().

spanner/value.go

@@ -1355,8 +1355,8 @@ func decodeValue(v *proto3.Value, t *sppb.Type, ptr interface{}) error {
 			return errNilDst(p)
 		}
 		if !isPtrStructPtrSlice(vp.Type()) {
-			// The container is not a pointer to a struct pointer slice.
-			return errTypeMismatch(code, acode, ptr)
+			// The container is not a slice of struct pointers.
+			return fmt.Errorf("the container is not a slice of struct pointers: %v", errTypeMismatch(code, acode, ptr))
 		}
 		// Only use reflection for nil detection on slow path.
 		// Also, IsNil panics on many types, so check it after the type check.