Handle container links

Ensures that linked containers are restarted if any of their
dependencies are restarted -- and makes sure that everything happens in
the correct order.

updater/sorter_test.go

@@ -0,0 +1,37 @@
+package updater
+
+import (
+	"testing"
+
+	"github.com/CenturyLinkLabs/watchtower/docker"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestContainerSorter_Success(t *testing.T) {
+	c1 := docker.NewTestContainer("1", []string{})
+	c2 := docker.NewTestContainer("2", []string{"1:"})
+	c3 := docker.NewTestContainer("3", []string{"2:"})
+	c4 := docker.NewTestContainer("4", []string{"3:"})
+	c5 := docker.NewTestContainer("5", []string{"4:"})
+	c6 := docker.NewTestContainer("6", []string{"5:", "3:"})
+	containers := []docker.Container{c6, c2, c4, c1, c3, c5}
+
+	cs := ContainerSorter{}
+	result, err := cs.Sort(containers)
+
+	assert.NoError(t, err)
+	assert.Equal(t, []docker.Container{c1, c2, c3, c4, c5, c6}, result)
+}
+
+func TestContainerSorter_Error(t *testing.T) {
+	c1 := docker.NewTestContainer("1", []string{"3:"})
+	c2 := docker.NewTestContainer("2", []string{"1:"})
+	c3 := docker.NewTestContainer("3", []string{"2:"})
+	containers := []docker.Container{c1, c2, c3}
+
+	cs := ContainerSorter{}
+	_, err := cs.Sort(containers)
+
+	assert.Error(t, err)
+	assert.EqualError(t, err, "Circular reference to 1")
+}