feat(daemon): add ErrorResponse to allow external error creation (#303)

#265 provides the ability for the daemon REST API to be extended by appending new handlers to the API list.

For example, the following new reboot command can be added now:

// requestReboot asks the daemon to gracefully shut down the system
// and issue a reboot.
func requestReboot(d *daemon.Daemon) daemon.Response {
        d.HandleRestart(restart.RestartSystem)
        result := deviceResult{}
        return daemon.SyncResponse(result)
}

Note that SyncResponse was already public, which allows the code above to work.

Following this patch, we can now reply with a suitable error response:

:
    switch(...) {
    case reboot:
        requestReboot(...)
    default:    
        return daemon.ErrorResponse(http.StatusBadRequest,"invalid media type %q", mediaType)
    {
:

client/client.go

@@ -416,11 +416,15 @@ func (e *Error) Error() string {
 	return e.Message
 }
 
+// Error kinds for use as a response or maintenance result
 const (
 	ErrorKindLoginRequired     = "login-required"
+	ErrorKindNoDefaultServices = "no-default-services"
+	ErrorKindNotFound          = "not-found"
+	ErrorKindPermissionDenied  = "permission-denied"
+	ErrorKindGenericFileError  = "generic-file-error"
 	ErrorKindSystemRestart     = "system-restart"
 	ErrorKindDaemonRestart     = "daemon-restart"
-	ErrorKindNoDefaultServices = "no-default-services"
 )
 
 func (rsp *response) err(cli *Client) error {

internals/daemon/response.go

@@ -95,11 +95,11 @@ func (r *resp) ServeHTTP(w http.ResponseWriter, _ *http.Request) {
 	if err != nil {
 		logger.Noticef("Cannot marshal %#v to JSON: %v", *r, err)
 		bs = nil
-		status = 500
+		status = http.StatusInternalServerError
 	}
 
 	hdr := w.Header()
-	if r.Status == 202 || r.Status == 201 {
+	if r.Status == http.StatusAccepted || r.Status == http.StatusCreated {
 		if m, ok := r.Result.(map[string]interface{}); ok {
 			if location, ok := m["resource"]; ok {
 				if location, ok := location.(string); ok && location != "" {
@@ -116,22 +116,21 @@ func (r *resp) ServeHTTP(w http.ResponseWriter, _ *http.Request) {
 
 type errorKind string
 
+// Error kinds for use as a response or maintenance result
 const (
 	errorKindLoginRequired     = errorKind("login-required")
-	errorKindDaemonRestart     = errorKind("daemon-restart")
-	errorKindSystemRestart     = errorKind("system-restart")
 	errorKindNoDefaultServices = errorKind("no-default-services")
 	errorKindNotFound          = errorKind("not-found")
 	errorKindPermissionDenied  = errorKind("permission-denied")
 	errorKindGenericFileError  = errorKind("generic-file-error")
+	errorKindSystemRestart     = errorKind("system-restart")
+	errorKindDaemonRestart     = errorKind("daemon-restart")
 )
 
-type errorValue interface{}
-
 type errorResult struct {
-	Message string     `json:"message"` // note no omitempty
-	Kind    errorKind  `json:"kind,omitempty"`
-	Value   errorValue `json:"value,omitempty"`
+	Message string      `json:"message"` // note no omitempty
+	Kind    errorKind   `json:"kind,omitempty"`
+	Value   interface{} `json:"value,omitempty"`
 }
 
 func SyncResponse(result interface{}) Response {
@@ -145,15 +144,15 @@ func SyncResponse(result interface{}) Response {
 
 	return &resp{
 		Type:   ResponseTypeSync,
-		Status: 200,
+		Status: http.StatusOK,
 		Result: result,
 	}
 }
 
 func AsyncResponse(result map[string]interface{}, change string) Response {
 	return &resp{
 		Type:   ResponseTypeAsync,
-		Status: 202,
+		Status: http.StatusAccepted,
 		Result: result,
 		Change: change,
 	}
@@ -169,22 +168,30 @@ func (f fileResponse) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	http.ServeFile(w, r, string(f))
 }
 
+// ErrorResponse builds an error Response that returns the status and formatted message.
+//
+// If no arguments are provided, formatting is disabled, and the format string
+// is used as is and not interpreted in any way.
+func ErrorResponse(status int, format string, v ...interface{}) Response {
+	res := &errorResult{}
+	if len(v) == 0 {
+		res.Message = format
+	} else {
+		res.Message = fmt.Sprintf(format, v...)
+	}
+	if status == http.StatusUnauthorized {
+		res.Kind = errorKindLoginRequired
+	}
+	return &resp{
+		Type:   ResponseTypeError,
+		Result: res,
+		Status: status,
+	}
+}
+
 func makeErrorResponder(status int) errorResponder {
 	return func(format string, v ...interface{}) Response {
-		res := &errorResult{}
-		if len(v) == 0 {
-			res.Message = format
-		} else {
-			res.Message = fmt.Sprintf(format, v...)
-		}
-		if status == 401 {
-			res.Kind = errorKindLoginRequired
-		}
-		return &resp{
-			Type:   ResponseTypeError,
-			Result: res,
-			Status: status,
-		}
+		return ErrorResponse(status, format, v...)
 	}
 }
 
@@ -194,11 +201,11 @@ type errorResponder func(string, ...interface{}) Response
 
 // Standard error responses.
 var (
-	statusBadRequest       = makeErrorResponder(400)
-	statusUnauthorized     = makeErrorResponder(401)
-	statusForbidden        = makeErrorResponder(403)
-	statusNotFound         = makeErrorResponder(404)
-	statusMethodNotAllowed = makeErrorResponder(405)
-	statusInternalError    = makeErrorResponder(500)
-	statusGatewayTimeout   = makeErrorResponder(504)
+	statusBadRequest       = makeErrorResponder(http.StatusBadRequest)
+	statusUnauthorized     = makeErrorResponder(http.StatusUnauthorized)
+	statusForbidden        = makeErrorResponder(http.StatusForbidden)
+	statusNotFound         = makeErrorResponder(http.StatusNotFound)
+	statusMethodNotAllowed = makeErrorResponder(http.StatusMethodNotAllowed)
+	statusInternalError    = makeErrorResponder(http.StatusInternalServerError)
+	statusGatewayTimeout   = makeErrorResponder(http.StatusGatewayTimeout)
 )

internals/daemon/response_test.go

@@ -34,7 +34,7 @@ func (s *responseSuite) TestRespSetsLocationIfAccepted(c *check.C) {
 	rec := httptest.NewRecorder()
 
 	rsp := &resp{
-		Status: 202,
+		Status: http.StatusAccepted,
 		Result: map[string]interface{}{
 			"resource": "foo/bar",
 		},
@@ -49,7 +49,7 @@ func (s *responseSuite) TestRespSetsLocationIfCreated(c *check.C) {
 	rec := httptest.NewRecorder()
 
 	rsp := &resp{
-		Status: 201,
+		Status: http.StatusCreated,
 		Result: map[string]interface{}{
 			"resource": "foo/bar",
 		},
@@ -64,7 +64,7 @@ func (s *responseSuite) TestRespDoesNotSetLocationIfOther(c *check.C) {
 	rec := httptest.NewRecorder()
 
 	rsp := &resp{
-		Status: 418, // I'm a teapot
+		Status: http.StatusTeapot, // I'm a teapot
 		Result: map[string]interface{}{
 			"resource": "foo/bar",
 		},
@@ -104,7 +104,7 @@ func (s *responseSuite) TestRespJSONWithNullResult(c *check.C) {
 }
 
 func (s *responseSuite) TestErrorResponderPrintfsWithArgs(c *check.C) {
-	teapot := makeErrorResponder(418)
+	teapot := makeErrorResponder(http.StatusTeapot)
 
 	rec := httptest.NewRecorder()
 	rsp := teapot("system memory below %d%%.", 1)
@@ -119,7 +119,7 @@ func (s *responseSuite) TestErrorResponderPrintfsWithArgs(c *check.C) {
 }
 
 func (s *responseSuite) TestErrorResponderDoesNotPrintfAlways(c *check.C) {
-	teapot := makeErrorResponder(418)
+	teapot := makeErrorResponder(http.StatusTeapot)
 
 	rec := httptest.NewRecorder()
 	rsp := teapot("system memory below 1%.")