fix(docker): ensure connection is established with host

The `docker` and `docker_swarm` providers have been patched to ensure that the connection is properly established upon starting.

If the docker host is not available at starting time, then the application will stop. This will prevent from trying to register to the event stream on a non working client.

app/providers/docker_classic.go

@@ -25,10 +25,16 @@ type DockerClassicProvider struct {
 func NewDockerClassicProvider() (*DockerClassicProvider, error) {
 	cli, err := client.NewClientWithOpts(client.FromEnv, client.WithAPIVersionNegotiation())
 	if err != nil {
-		log.Fatal(fmt.Errorf("%+v", "Could not connect to docker API"))
-		return nil, err
+		return nil, fmt.Errorf("cannot create docker client: %v", err)
 	}
 
+	serverVersion, err := cli.ServerVersion(context.Background())
+	if err != nil {
+		return nil, fmt.Errorf("cannot connect to docker host: %v", err)
+	}
+
+	log.Trace(fmt.Sprintf("connection established with docker %s (API %s)", serverVersion.Version, serverVersion.APIVersion))
+
 	return &DockerClassicProvider{
 		Client:          cli,
 		desiredReplicas: 1,
@@ -144,14 +150,23 @@ func (provider *DockerClassicProvider) NotifyInstanceStopped(ctx context.Context
 	})
 	for {
 		select {
-		case msg := <-msgs:
+		case msg, ok := <-msgs:
+			if !ok {
+				log.Error("provider event stream is closed")
+				return
+			}
 			// Send the container that has died to the channel
 			instance <- strings.TrimPrefix(msg.Actor.Attributes["name"], "/")
-		case err := <-errs:
+		case err, ok := <-errs:
+			if !ok {
+				log.Error("provider event stream is closed", err)
+				return
+			}
 			if errors.Is(err, io.EOF) {
 				log.Debug("provider event stream closed")
 				return
 			}
+			log.Error("provider event stream error", err)
 		case <-ctx.Done():
 			return
 		}

app/providers/docker_swarm.go

@@ -23,8 +23,16 @@ type DockerSwarmProvider struct {
 func NewDockerSwarmProvider() (*DockerSwarmProvider, error) {
 	cli, err := client.NewClientWithOpts(client.FromEnv, client.WithAPIVersionNegotiation())
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("cannot create docker client: %v", err)
 	}
+
+	serverVersion, err := cli.ServerVersion(context.Background())
+	if err != nil {
+		return nil, fmt.Errorf("cannot connect to docker host: %v", err)
+	}
+
+	log.Trace(fmt.Sprintf("connection established with docker %s (API %s)", serverVersion.Version, serverVersion.APIVersion))
+
 	return &DockerSwarmProvider{
 		Client:          cli,
 		desiredReplicas: 1,
@@ -161,17 +169,26 @@ func (provider *DockerSwarmProvider) NotifyInstanceStopped(ctx context.Context,
 	go func() {
 		for {
 			select {
-			case msg := <-msgs:
+			case msg, ok := <-msgs:
+				if !ok {
+					log.Error("provider event stream is closed")
+					return
+				}
 				if msg.Actor.Attributes["replicas.new"] == "0" {
 					instance <- msg.Actor.Attributes["name"]
 				} else if msg.Action == "remove" {
 					instance <- msg.Actor.Attributes["name"]
 				}
-			case err := <-errs:
+			case err, ok := <-errs:
+				if !ok {
+					log.Error("provider event stream is closed", err)
+					return
+				}
 				if errors.Is(err, io.EOF) {
 					log.Debug("provider event stream closed")
 					return
 				}
+				log.Error("provider event stream error", err)
 			case <-ctx.Done():
 				return
 			}