GODRIVER-2133 Error on semantic single documents as aggregate pipelines

mongo/mongo.go

@@ -228,6 +228,17 @@ func transformAggregatePipeline(registry *bsoncodec.Registry, pipeline interface
 		aidx, arr := bsoncore.AppendArrayStart(nil)
 		var hasOutputStage bool
 		valLen := val.Len()
+
+		// Explicitly forbid non-empty pipelines that are semantically single documents
+		// and are implemented as slices.
+		switch t := pipeline.(type) {
+		case bson.D, bson.Raw, bsoncore.Document:
+			if valLen > 0 {
+				return nil, false,
+					fmt.Errorf("%T is not an allowed pipeline type as it represents a single document. Use bson.A or mongo.Pipeline instead", t)
+			}
+		}
+
 		for idx := 0; idx < valLen; idx++ {
 			doc, err := transformBsoncoreDocument(registry, val.Index(idx).Interface(), true, fmt.Sprintf("pipeline stage :%v", idx))
 			if err != nil {

mongo/mongo_test.go

@@ -126,6 +126,10 @@ func TestMongoHelpers(t *testing.T) {
 		arr, _ = bsoncore.AppendDocumentEnd(arr, dindex)
 		arr, _ = bsoncore.AppendArrayEnd(arr, index)
 
+		index, doc := bsoncore.AppendDocumentStart(nil)
+		doc = bsoncore.AppendInt32Element(doc, "x", 1)
+		doc, _ = bsoncore.AppendDocumentEnd(doc, index)
+
 		testCases := []struct {
 			name           string
 			pipeline       interface{}
@@ -342,6 +346,48 @@ func TestMongoHelpers(t *testing.T) {
 				true,
 				nil,
 			},
+			{
+				"semantic single document/bson.D",
+				bson.D{{"x", 1}},
+				nil,
+				false,
+				errors.New("primitive.D is not an allowed pipeline type as it represents a single document. Use bson.A or mongo.Pipeline instead"),
+			},
+			{
+				"semantic single document/bson.Raw",
+				bson.Raw(doc),
+				nil,
+				false,
+				errors.New("bson.Raw is not an allowed pipeline type as it represents a single document. Use bson.A or mongo.Pipeline instead"),
+			},
+			{
+				"semantic single document/bsoncore.Document",
+				bsoncore.Document(doc),
+				nil,
+				false,
+				errors.New("bsoncore.Document is not an allowed pipeline type as it represents a single document. Use bson.A or mongo.Pipeline instead"),
+			},
+			{
+				"semantic single document/empty bson.D",
+				bson.D{},
+				bson.A{},
+				false,
+				nil,
+			},
+			{
+				"semantic single document/empty bson.Raw",
+				bson.Raw{},
+				bson.A{},
+				false,
+				nil,
+			},
+			{
+				"semantic single document/empty bsoncore.Document",
+				bsoncore.Document{},
+				bson.A{},
+				false,
+				nil,
+			},
 		}
 
 		for _, tc := range testCases {