GODRIVER-3470 Correct BSON unmarshaling logic for null values

bson/bsoncodec/default_value_decoders.go

@@ -1521,7 +1521,13 @@ func (dvd DefaultValueDecoders) ValueUnmarshalerDecodeValue(_ DecodeContext, vr
 		return ValueDecoderError{Name: "ValueUnmarshalerDecodeValue", Types: []reflect.Type{tValueUnmarshaler}, Received: val}
 	}
 
-	if vr.Type() == bsontype.Null {
+	// If BSON value is null and the go value is a pointer, then don't call
+	// UnmarshalBSONValue. Even if the Go pointer is already initialized (i.e.,
+	// non-nil), encountering null in BSON will result in the pointer being
+	// directly set to nil here. Since the pointer is being replaced with nil,
+	// there is no opportunity (or reason) for the custom UnmarshalBSONValue logic
+	// to be called.
+	if vr.Type() == bsontype.Null && val.Kind() == reflect.Ptr {
 		val.Set(reflect.Zero(val.Type()))
 
 		return vr.ReadNull()

bson/unmarshal.go

@@ -41,6 +41,9 @@ type ValueUnmarshaler interface {
 // Unmarshal parses the BSON-encoded data and stores the result in the value
 // pointed to by val. If val is nil or not a pointer, Unmarshal returns
 // InvalidUnmarshalError.
+//
+// When unmarshaling BSON, if the BSON value is null and the Go value is a
+// pointer, the pointer is set to nil without calling UnmarshalBSONValue.
 func Unmarshal(data []byte, val interface{}) error {
 	return UnmarshalWithRegistry(DefaultRegistry, data, val)
 }

bson/unmarshal_value_test.go

@@ -14,6 +14,7 @@ import (
 	"go.mongodb.org/mongo-driver/bson/bsoncodec"
 	"go.mongodb.org/mongo-driver/bson/bsontype"
 	"go.mongodb.org/mongo-driver/internal/assert"
+	"go.mongodb.org/mongo-driver/internal/require"
 	"go.mongodb.org/mongo-driver/x/bsonx/bsoncore"
 )
 
@@ -93,6 +94,29 @@ func TestUnmarshalValue(t *testing.T) {
 	})
 }
 
+func TestInitializedPointerDataWithBSONNull(t *testing.T) {
+	// Set up the test case with initialized pointers.
+	tc := unmarshalBehaviorTestCase{
+		BSONValuePtrTracker: &unmarshalBSONValueCallTracker{},
+		BSONPtrTracker:      &unmarshalBSONCallTracker{},
+	}
+
+	// Create BSON data where the '*_ptr_tracker' fields are explicitly set to
+	// null.
+	bytes := docToBytes(D{
+		{Key: "bv_ptr_tracker", Value: nil},
+		{Key: "b_ptr_tracker", Value: nil},
+	})
+
+	// Unmarshal the BSON data into the test case struct. This should set the
+	// pointer fields to nil due to the BSON null value.
+	err := Unmarshal(bytes, &tc)
+	require.NoError(t, err)
+
+	assert.Nil(t, tc.BSONValuePtrTracker)
+	assert.Nil(t, tc.BSONPtrTracker)
+}
+
 // tests covering GODRIVER-2779
 func BenchmarkSliceCodecUnmarshal(b *testing.B) {
 	benchmarks := []struct {

bson/unmarshaling_cases_test.go

@@ -114,6 +114,26 @@ func unmarshalingTestCases() []unmarshalingTestCase {
 			},
 			data: docToBytes(D{{"fooBar", int32(10)}}),
 		},
+		{
+			name:  "nil pointer and non-pointer type with literal null BSON",
+			sType: reflect.TypeOf(unmarshalBehaviorTestCase{}),
+			want: &unmarshalBehaviorTestCase{
+				BSONValueTracker: unmarshalBSONValueCallTracker{
+					called: true,
+				},
+				BSONValuePtrTracker: nil,
+				BSONTracker: unmarshalBSONCallTracker{
+					called: true,
+				},
+				BSONPtrTracker: nil,
+			},
+			data: docToBytes(D{
+				{Key: "bv_tracker", Value: nil},
+				{Key: "bv_ptr_tracker", Value: nil},
+				{Key: "b_tracker", Value: nil},
+				{Key: "b_ptr_tracker", Value: nil},
+			}),
+		},
 		// GODRIVER-2252
 		// Test that a struct of pointer types with UnmarshalBSON functions defined marshal and
 		// unmarshal to the same Go values when the pointer values are "nil".
@@ -269,3 +289,39 @@ func (ms *myString) UnmarshalBSON(bytes []byte) error {
 	*ms = myString(s)
 	return nil
 }
+
+// unmarshalBSONValueCallTracker is a test struct that tracks whether the
+// UnmarshalBSONValue method has been called.
+type unmarshalBSONValueCallTracker struct {
+	called bool // called is set to true when UnmarshalBSONValue is invoked.
+}
+
+var _ ValueUnmarshaler = &unmarshalBSONValueCallTracker{}
+
+// unmarshalBSONCallTracker is a test struct that tracks whether the
+// UnmarshalBSON method has been called.
+type unmarshalBSONCallTracker struct {
+	called bool // called is set to true when UnmarshalBSON is invoked.
+}
+
+// Ensure unmarshalBSONCallTracker implements the Unmarshaler interface.
+var _ Unmarshaler = &unmarshalBSONCallTracker{}
+
+// unmarshalBehaviorTestCase holds instances of call trackers for testing BSON
+// unmarshaling behavior.
+type unmarshalBehaviorTestCase struct {
+	BSONValueTracker    unmarshalBSONValueCallTracker  `bson:"bv_tracker"`     // BSON value unmarshaling by value.
+	BSONValuePtrTracker *unmarshalBSONValueCallTracker `bson:"bv_ptr_tracker"` // BSON value unmarshaling by pointer.
+	BSONTracker         unmarshalBSONCallTracker       `bson:"b_tracker"`      // BSON unmarshaling by value.
+	BSONPtrTracker      *unmarshalBSONCallTracker      `bson:"b_ptr_tracker"`  // BSON unmarshaling by pointer.
+}
+
+func (tracker *unmarshalBSONValueCallTracker) UnmarshalBSONValue(bsontype.Type, []byte) error {
+	tracker.called = true
+	return nil
+}
+
+func (tracker *unmarshalBSONCallTracker) UnmarshalBSON([]byte) error {
+	tracker.called = true
+	return nil
+}