refactor(controller): use an atomic.Value for storing routes (#1486)

This avoids having to acquire the lock to access the routing table,
which can be easily forgotten.

backend/controller/controller.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"connectrpc.com/connect"
+	"github.com/alecthomas/atomic"
 	"github.com/alecthomas/concurrency"
 	"github.com/alecthomas/kong"
 	"github.com/alecthomas/types/optional"
@@ -164,8 +165,7 @@ type Service struct {
 	// Map from endpoint to client.
 	clients *ttlcache.Cache[string, clients]
 
-	routesMu      sync.RWMutex
-	routes        map[string][]dal.Route
+	routes        atomic.Value[map[string][]dal.Route]
 	config        Config
 	runnerScaling scaling.RunnerScaling
 
@@ -192,11 +192,11 @@ func New(ctx context.Context, db *dal.DAL, config Config, runnerScaling scaling.
 		key:                     key,
 		deploymentLogsSink:      newDeploymentLogsSink(ctx, db),
 		clients:                 ttlcache.New[string, clients](ttlcache.WithTTL[string, clients](time.Minute)),
-		routes:                  map[string][]dal.Route{},
 		config:                  config,
 		runnerScaling:           runnerScaling,
 		increaseReplicaFailures: map[string]int{},
 	}
+	svc.routes.Store(map[string][]dal.Route{})
 
 	cronSvc := cronjobs.New(ctx, key, svc.config.Advertise.Host, cronjobs.Config{Timeout: config.CronJobTimeout}, db, svc.tasks, svc.callWithRequest)
 	svc.cronJobs = cronSvc
@@ -303,8 +303,8 @@ func (s *Service) Status(ctx context.Context, req *connect.Request[ftlv1.StatusR
 	if err != nil {
 		return nil, fmt.Errorf("could not get status: %w", err)
 	}
-	s.routesMu.RLock()
-	routes := slices.FlatMap(maps.Values(s.routes), func(routes []dal.Route) (out []*ftlv1.StatusResponse_Route) {
+	sroutes := s.routes.Load()
+	routes := slices.FlatMap(maps.Values(sroutes), func(routes []dal.Route) (out []*ftlv1.StatusResponse_Route) {
 		out = make([]*ftlv1.StatusResponse_Route, len(routes))
 		for i, route := range routes {
 			out[i] = &ftlv1.StatusResponse_Route{
@@ -316,7 +316,6 @@ func (s *Service) Status(ctx context.Context, req *connect.Request[ftlv1.StatusR
 		}
 		return out
 	})
-	s.routesMu.RUnlock()
 	replicas := map[string]int32{}
 	protoRunners, err := slices.MapErr(status.Runners, func(r dal.Runner) (*ftlv1.StatusResponse_Runner, error) {
 		var deployment *string
@@ -543,9 +542,7 @@ func (s *Service) RegisterRunner(ctx context.Context, stream *connect.ClientStre
 		} else if err != nil {
 			return nil, err
 		}
-		s.routesMu.Lock()
-		s.routes = routes
-		s.routesMu.Unlock()
+		s.routes.Store(routes)
 	}
 	if stream.Err() != nil {
 		return nil, stream.Err()
@@ -743,9 +740,7 @@ func (s *Service) callWithRequest(
 	}
 
 	module := verbRef.Module
-	s.routesMu.RLock()
-	routes, ok := s.routes[module]
-	s.routesMu.RUnlock()
+	routes, ok := s.routes.Load()[module]
 	if !ok {
 		return nil, connect.NewError(connect.CodeNotFound, fmt.Errorf("no routes for module %q", module))
 	}
@@ -1342,9 +1337,7 @@ func (s *Service) syncRoutes(ctx context.Context) (time.Duration, error) {
 	} else if err != nil {
 		return 0, err
 	}
-	s.routesMu.Lock()
-	s.routes = routes
-	s.routesMu.Unlock()
+	s.routes.Store(routes)
 	return time.Second, nil
 }