registry: make defaultService.ServiceConfig() more idiomatic

The intent of this function is to return a copy of the service's configuration,
and to copy / dereference the options in its configuration.

The code was doing this in slightly complicated fashion. This patch;

- adds a `copy()` function to serviceConfig
- rewrites the code to use a slightly more idiomatic approach, using one of
  the approaches described in "golang SliceTricks" https://github.com/golang/go/wiki/SliceTricks#copy
- changes defaultService.ServiceConfig() to use this function, and updates
  its godoc to better describe that it returns a copy.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

registry/config.go

@@ -86,6 +86,21 @@ func newServiceConfig(options ServiceOptions) (*serviceConfig, error) {
 	return config, nil
 }
 
+// copy constructs a new ServiceConfig with a copy of the configuration in config.
+func (config *serviceConfig) copy() *registrytypes.ServiceConfig {
+	ic := make(map[string]*registrytypes.IndexInfo)
+	for key, value := range config.IndexConfigs {
+		ic[key] = value
+	}
+	return &registrytypes.ServiceConfig{
+		AllowNondistributableArtifactsCIDRs:     append([]*registrytypes.NetIPNet(nil), config.AllowNondistributableArtifactsCIDRs...),
+		AllowNondistributableArtifactsHostnames: append([]string(nil), config.AllowNondistributableArtifactsHostnames...),
+		InsecureRegistryCIDRs:                   append([]*registrytypes.NetIPNet(nil), config.InsecureRegistryCIDRs...),
+		IndexConfigs:                            ic,
+		Mirrors:                                 append([]string(nil), config.Mirrors...),
+	}
+}
+
 // loadAllowNondistributableArtifacts loads allow-nondistributable-artifacts registries into config.
 func (config *serviceConfig) loadAllowNondistributableArtifacts(registries []string) error {
 	cidrs := map[string]*registry.NetIPNet{}

registry/service.go

@@ -50,32 +50,11 @@ func NewService(options ServiceOptions) (Service, error) {
 	return &defaultService{config: config}, err
 }
 
-// ServiceConfig returns the public registry service configuration.
+// ServiceConfig returns a copy of the public registry service's configuration.
 func (s *defaultService) ServiceConfig() *registry.ServiceConfig {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
-
-	servConfig := registry.ServiceConfig{
-		AllowNondistributableArtifactsCIDRs:     make([]*(registry.NetIPNet), 0),
-		AllowNondistributableArtifactsHostnames: make([]string, 0),
-		InsecureRegistryCIDRs:                   make([]*(registry.NetIPNet), 0),
-		IndexConfigs:                            make(map[string]*(registry.IndexInfo)),
-		Mirrors:                                 make([]string, 0),
-	}
-
-	// construct a new ServiceConfig which will not retrieve s.Config directly,
-	// and look up items in s.config with mu locked
-	servConfig.AllowNondistributableArtifactsCIDRs = append(servConfig.AllowNondistributableArtifactsCIDRs, s.config.ServiceConfig.AllowNondistributableArtifactsCIDRs...)
-	servConfig.AllowNondistributableArtifactsHostnames = append(servConfig.AllowNondistributableArtifactsHostnames, s.config.ServiceConfig.AllowNondistributableArtifactsHostnames...)
-	servConfig.InsecureRegistryCIDRs = append(servConfig.InsecureRegistryCIDRs, s.config.ServiceConfig.InsecureRegistryCIDRs...)
-
-	for key, value := range s.config.ServiceConfig.IndexConfigs {
-		servConfig.IndexConfigs[key] = value
-	}
-
-	servConfig.Mirrors = append(servConfig.Mirrors, s.config.ServiceConfig.Mirrors...)
-
-	return &servConfig
+	return s.config.copy()
 }
 
 // LoadAllowNondistributableArtifacts loads allow-nondistributable-artifacts registries for Service.