Gracefully take care of the error case when a container

retries to attach to a network, it is already connected to

Fixes - https://github.com/docker/for-linux/issues/632

Signed-off-by: Arko Dasgupta <arko.dasgupta@docker.com>

daemon/container_operations.go

@@ -354,6 +354,14 @@ func (daemon *Daemon) findAndAttachNetwork(container *container.Container, idOrN
 		if container.Managed || !n.Info().Dynamic() {
 			return n, nil, nil
 		}
+		// Throw an error if the container is already attached to the network
+		if container.NetworkSettings.Networks != nil {
+			networkName := n.Name()
+			containerName := strings.TrimPrefix(container.Name, "/")
+			if network, ok := container.NetworkSettings.Networks[networkName]; ok && network.EndpointID != "" {
+				return n, nil, types.ForbiddenErrorf("%s is already attached to network %s", containerName, networkName)
+			}
+		}
 	}
 
 	var addresses []string

integration/service/network_test.go

@@ -75,3 +75,43 @@ func TestDockerNetworkConnectAlias(t *testing.T) {
 	assert.Check(t, is.Equal(len(ng2.NetworkSettings.Networks[name].Aliases), 2))
 	assert.Check(t, is.Equal(ng2.NetworkSettings.Networks[name].Aliases[0], "bbb"))
 }
+
+func TestDockerNetworkReConnect(t *testing.T) {
+	skip.If(t, testEnv.DaemonInfo.OSType == "windows")
+	defer setupTest(t)()
+	d := swarm.NewSwarm(t, testEnv)
+	defer d.Stop(t)
+	client := d.NewClientT(t)
+	defer client.Close()
+	ctx := context.Background()
+
+	name := t.Name() + "dummyNet"
+	net.CreateNoError(t, ctx, client, name,
+		net.WithDriver("overlay"),
+		net.WithAttachable(),
+	)
+
+	c1 := container.Create(t, ctx, client, func(c *container.TestContainerConfig) {
+		c.NetworkingConfig = &network.NetworkingConfig{
+			EndpointsConfig: map[string]*network.EndpointSettings{
+				name: {},
+			},
+		}
+	})
+
+	err := client.NetworkConnect(ctx, name, c1, &network.EndpointSettings{})
+	assert.NilError(t, err)
+
+	err = client.ContainerStart(ctx, c1, types.ContainerStartOptions{})
+	assert.NilError(t, err)
+
+	n1, err := client.ContainerInspect(ctx, c1)
+	assert.NilError(t, err)
+
+	err = client.NetworkConnect(ctx, name, c1, &network.EndpointSettings{})
+	assert.ErrorContains(t, err, "is already attached to network")
+
+	n2, err := client.ContainerInspect(ctx, c1)
+	assert.NilError(t, err)
+	assert.Check(t, is.DeepEqual(n1, n2))
+}