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/unmarshaling_cases_test.go

@@ -8,9 +8,12 @@ package bson
 
 import (
 	"reflect"
+	"testing"
 
 	"go.mongodb.org/mongo-driver/bson/bsonrw"
 	"go.mongodb.org/mongo-driver/bson/bsontype"
+	"go.mongodb.org/mongo-driver/internal/assert"
+	"go.mongodb.org/mongo-driver/internal/require"
 )
 
 type unmarshalingTestCase struct {
@@ -114,6 +117,17 @@ 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{
+				Tracker: unmarshalCallTracker{
+					unmarshalCalled: true,
+				},
+				PtrTracker: nil,
+			},
+			data: docToBytes(D{{Key: "tracker", Value: nil}, {Key: "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 +283,35 @@ func (ms *myString) UnmarshalBSON(bytes []byte) error {
 	*ms = myString(s)
 	return nil
 }
+
+type unmarshalCallTracker struct {
+	unmarshalCalled bool
+}
+
+type unmarshalBehaviorTestCase struct {
+	Tracker    unmarshalCallTracker  `bson:"tracker"`
+	PtrTracker *unmarshalCallTracker `bson:"ptr_tracker"`
+}
+
+func (ms *unmarshalCallTracker) UnmarshalBSONValue(bsontype.Type, []byte) error {
+	ms.unmarshalCalled = true
+
+	return nil
+}
+
+func TestInitializedPointerDataWithBSONNull(t *testing.T) {
+	// Set up the test case with an initialized pointer.
+	tc := unmarshalBehaviorTestCase{
+		PtrTracker: &unmarshalCallTracker{},
+	}
+
+	// Create BSON data where the 'ptr_tracker' field is explicitly set to null.
+	bytes := docToBytes(D{{Key: "ptr_tracker", Value: nil}})
+
+	// Unmarshal the BSON data into the test case struct.
+	// This should set PtrTracker to nil due to the BSON null value.
+	err := Unmarshal(bytes, &tc)
+	require.NoError(t, err)
+
+	assert.Nil(t, tc.PtrTracker)
+}