THRIFT-5806: Count zero set fields on a nil union rather than dereferencing it

compiler/cpp/src/thrift/generate/t_go_generator.cc

@@ -1768,6 +1768,11 @@ void t_go_generator::generate_countsetfields_helper(ostream& out,
   out << indent() << "func (p *" << tstruct_name << ") CountSetFields" << tstruct_name << "() int {"
       << '\n';
   indent_up();
+  out << indent() << "if p == nil {" << '\n';
+  indent_up();
+  out << indent() << "return 0" << '\n';
+  indent_down();
+  out << indent() << "}" << '\n';
   out << indent() << "count := 0" << '\n';
   for (f_iter = fields.begin(); f_iter != fields.end(); ++f_iter) {
     if ((*f_iter)->get_req() == t_field::T_REQUIRED)

lib/go/test/UnionDefaultValueTest.thrift

@@ -32,3 +32,8 @@ union Descendant {
 struct TestStruct {
   1: optional Descendant descendant = { "option1": {}}
 }
+
+struct StructWithUnsetUnion {
+  1: required bool f_1 = 1,
+  2: Descendant f_2,
+}

lib/go/test/tests/union_default_value_test.go

@@ -20,9 +20,12 @@
 package tests
 
 import (
+	"context"
+	"strings"
 	"testing"
 
 	"github.com/apache/thrift/lib/go/test/gopath/src/uniondefaultvaluetest"
+	"github.com/apache/thrift/lib/go/thrift"
 )
 
 func TestUnionDefaultValue(t *testing.T) {
@@ -32,3 +35,33 @@ func TestUnionDefaultValue(t *testing.T) {
 		t.Error("Default Union value not set!")
 	}
 }
+
+func TestNilUnion(t *testing.T) {
+	var d *uniondefaultvaluetest.Descendant
+	if count := d.CountSetFieldsDescendant(); count != 0 {
+		t.Errorf("Expected 0 set fields for nil union, got %d", count)
+	}
+
+	proto := thrift.NewTBinaryProtocolConf(thrift.NewTMemoryBuffer(), nil)
+	err := d.Write(context.Background(), proto)
+	if err == nil {
+		t.Error("Expected error when writing nil union, got nil")
+	}
+}
+
+func TestStructWithUnsetUnion(t *testing.T) {
+	s := uniondefaultvaluetest.NewStructWithUnsetUnion()
+	buf := thrift.NewTMemoryBuffer()
+	proto := thrift.NewTBinaryProtocolConf(buf, nil)
+
+	// Writing a struct whose union field is nil used to dereference the nil
+	// receiver inside CountSetFields and panic. It now reports which union
+	// could not be written instead.
+	err := s.Write(context.Background(), proto)
+	if err == nil {
+		t.Fatal("Expected an error writing a struct with an unset union, got nil")
+	}
+	if !strings.Contains(err.Error(), "exactly one field must be set") {
+		t.Errorf("Expected the union arity error, got %v", err)
+	}
+}