User registration return M_USER_IN_USE when username is already taken

src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go

@@ -103,6 +103,12 @@ func InvalidUsername(msg string) *MatrixError {
 	return &MatrixError{"M_INVALID_USERNAME", msg}
 }
 
+// InvalidUsername is an error returned when the client tries to register an
+// username that already exists
+func UserInUse(msg string) *MatrixError {
+	return &MatrixError{"M_USER_IN_USE", msg}
+}
+
 // GuestAccessForbidden is an error which is returned when the client is
 // forbidden from accessing a resource as a guest.
 func GuestAccessForbidden(msg string) *MatrixError {

src/github.com/matrix-org/dendrite/clientapi/routing/register.go

@@ -457,6 +457,19 @@ func completeRegistration(
 		}
 	}
 
+	avail, err := accountDB.CheckAccountAvailability(ctx, username)
+	if err == nil && avail == false {
+		return util.JSONResponse{
+			Code: 400,
+			JSON: jsonerror.UserInUse("Desired user ID is already taken."),
+		}
+	} else if err != nil {
+		return util.JSONResponse{
+			Code: 500,
+			JSON: jsonerror.Unknown("Failed to check account availability: " + err.Error()),
+		}
+	}
+
 	acc, err := accountDB.CreateAccount(ctx, username, password)
 	if err != nil {
 		return util.JSONResponse{