core: Retry dynamic config load if error or no-op (#4603)

Also fix ineffectual assignment (unrelated)

admin.go

@@ -262,7 +262,7 @@ func (admin *AdminConfig) newAdminHandler(addr NetworkAddress, remote bool) admi
 
 // provisionAdminRouters provisions all the router modules
 // in the admin.api namespace that need provisioning.
-func (admin AdminConfig) provisionAdminRouters(ctx Context) error {
+func (admin *AdminConfig) provisionAdminRouters(ctx Context) error {
 	for _, router := range admin.routers {
 		provisioner, ok := router.(Provisioner)
 		if !ok {
@@ -277,6 +277,7 @@ func (admin AdminConfig) provisionAdminRouters(ctx Context) error {
 
 	// We no longer need the routers once provisioned, allow for GC
 	admin.routers = nil
+
 	return nil
 }
 

caddy.go

@@ -504,20 +504,29 @@ func finishSettingUp(ctx Context, cfg *Config) error {
 
 		if cfg.Admin.Config.LoadDelay > 0 {
 			go func() {
-				timer := time.NewTimer(time.Duration(cfg.Admin.Config.LoadDelay))
-				select {
-				case <-timer.C:
-					loadedConfig, err := val.(ConfigLoader).LoadConfig(ctx)
-					if err != nil {
-						Log().Error("loading dynamic config failed", zap.Error(err))
-						return
+				// the loop is here only to iterate if there is an error or a no-op config load,
+				// in which case we simply wait the delay and try again
+				for {
+					timer := time.NewTimer(time.Duration(cfg.Admin.Config.LoadDelay))
+					select {
+					case <-timer.C:
+						loadedConfig, err := val.(ConfigLoader).LoadConfig(ctx)
+						if err != nil {
+							Log().Error("failed loading dynamic config; will retry", zap.Error(err))
+							continue
+						}
+						if loadedConfig == nil {
+							Log().Info("dynamically-loaded config was nil; will retry")
+							continue
+						}
+						runLoadedConfig(loadedConfig)
+					case <-ctx.Done():
+						if !timer.Stop() {
+							<-timer.C
+						}
+						Log().Info("stopping dynamic config loading")
 					}
-					runLoadedConfig(loadedConfig)
-				case <-ctx.Done():
-					if !timer.Stop() {
-						<-timer.C
-					}
-					Log().Info("stopping dynamic config loading")
+					break
 				}
 			}()
 		} else {
@@ -534,8 +543,10 @@ func finishSettingUp(ctx Context, cfg *Config) error {
 	return nil
 }
 
-// ConfigLoader is a type that can load a Caddy config. The
-// returned config must be valid Caddy JSON.
+// ConfigLoader is a type that can load a Caddy config. If
+// the return value is non-nil, it must be valid Caddy JSON;
+// if nil or with non-nil error, it is considered to be a
+// no-op load and may be retried later.
 type ConfigLoader interface {
 	LoadConfig(Context) ([]byte, error)
 }