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

@@ -504,13 +504,20 @@ func finishSettingUp(ctx Context, cfg *Config) error {
 
 		if cfg.Admin.Config.LoadDelay > 0 {
 			go func() {
+				// 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("loading dynamic config failed", zap.Error(err))
-						return
+							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():
@@ -519,6 +526,8 @@ func finishSettingUp(ctx Context, cfg *Config) error {
 						}
 						Log().Info("stopping dynamic config loading")
 					}
+					break
+				}
 			}()
 		} else {
 			// if no LoadDelay is provided, will load config synchronously
@@ -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)
 }