Fixes inability to authenticate through the older cmdQuery mechanism

internal/handler/cmd_query.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"strings"
 
+	"github.com/FerretDB/FerretDB/internal/clientconn/conninfo"
 	"github.com/FerretDB/FerretDB/internal/handler/common"
 	"github.com/FerretDB/FerretDB/internal/handler/handlererrors"
 	"github.com/FerretDB/FerretDB/internal/types"
@@ -41,6 +42,16 @@ func (h *Handler) CmdQuery(ctx context.Context, query *wire.OpQuery) (*wire.OpRe
 	// database name typically is either "$external" or "admin"
 
 	if cmd == "saslStart" && strings.HasSuffix(collection, ".$cmd") {
+		mechanism, err := common.GetRequiredParam[string](query.Query(), "mechanism")
+		if err == nil && mechanism == "PLAIN" {
+			username, password, err := saslStartPlain(query.Query())
+			if err != nil {
+				return nil, err
+			}
+
+			conninfo.Get(ctx).SetAuth(username, password)
+		}
+
 		var emptyPayload types.Binary
 		reply := wire.OpReply{
 			NumberReturned: 1,