Add ability to overrider depending containers with special label (#489)

* Add ability to overrider depending containers with special label

* Add documentation of how to override container dependencies with special label

docs/linked-containers.md

@@ -1,3 +1,5 @@
 Watchtower will detect if there are links between any of the running containers and ensures that things are stopped/started in a way that won't break any of the links. If an update is detected for one of the dependencies in a group of linked containers, watchtower will stop and start all of the containers in the correct order so that the application comes back up correctly.
 
-For example, imagine you were running a _mysql_ container and a _wordpress_ container which had been linked to the _mysql_ container. If watchtower were to detect that the _mysql_ container required an update, it would first shut down the linked _wordpress_ container followed by the _mysql_ container. When restarting the containers it would handle _mysql_ first and then _wordpress_ to ensure that the link continued to work.
+For example, imagine you were running a _mysql_ container and a _wordpress_ container which had been linked to the _mysql_ container. If watchtower were to detect that the _mysql_ container required an update, it would first shut down the linked _wordpress_ container followed by the _mysql_ container. When restarting the containers it would handle _mysql_ first and then _wordpress_ to ensure that the link continued to work.
+
+If you want to override existing links you can use special `com.centurylinklabs.watchtower.depends-on` label with dependent container names, separated by a comma.

pkg/container/container.go

@@ -95,6 +95,13 @@ func (c Container) Enabled() (bool, bool) {
 func (c Container) Links() []string {
 	var links []string
 
+	dependsOnLabelValue := c.getLabelValueOrEmpty(dependsOnLabel)
+
+	if dependsOnLabelValue != "" {
+		links := strings.Split(dependsOnLabelValue, ",")
+		return links
+	}
+
 	if (c.containerInfo != nil) && (c.containerInfo.HostConfig != nil) {
 		for _, link := range c.containerInfo.HostConfig.Links {
 			name := strings.Split(link, ":")[0]

pkg/container/container_test.go

@@ -181,6 +181,42 @@ var _ = Describe("the container", func() {
 				Expect(imageName).To(Equal(name + ":latest"))
 			})
 		})
+
+		When("fetching container links", func() {
+			When("the depends on label is present", func() {
+				It("should fetch depending containers from it", func() {
+					c = mockContainerWithLabels(map[string]string{
+						"com.centurylinklabs.watchtower.depends-on": "postgres",
+					})
+					links := c.Links()
+					Expect(links).To(SatisfyAll(ContainElement("postgres"), HaveLen(1)))
+				})
+				It("should fetch depending containers if there are many", func() {
+					c = mockContainerWithLabels(map[string]string{
+						"com.centurylinklabs.watchtower.depends-on": "postgres,redis",
+					})
+					links := c.Links()
+					Expect(links).To(SatisfyAll(ContainElement("postgres"), ContainElement("redis"), HaveLen(2)))
+				})
+				It("should fetch depending containers if label is blank", func() {
+					c = mockContainerWithLabels(map[string]string{
+						"com.centurylinklabs.watchtower.depends-on": "",
+					})
+					links := c.Links()
+					Expect(links).To(HaveLen(0))
+				})
+			})
+			When("the depends on label is not present", func() {
+				It("should fetch depending containers from host config links", func() {
+					c = mockContainerWithLinks([]string{
+						"redis:test-containrrr",
+						"postgres:test-containrrr",
+					})
+					links := c.Links()
+					Expect(links).To(SatisfyAll(ContainElement("redis"), ContainElement("postgres"), HaveLen(2)))
+				})
+			})
+		})
 	})
 })
 
@@ -190,6 +226,23 @@ func mockContainerWithImageName(name string) *Container {
 	return container
 }
 
+func mockContainerWithLinks(links []string) *Container {
+	content := types.ContainerJSON{
+		ContainerJSONBase: &types.ContainerJSONBase{
+			ID:    "container_id",
+			Image: "image",
+			Name:  "test-containrrr",
+			HostConfig: &container.HostConfig{
+				Links: links,
+			},
+		},
+		Config: &container.Config{
+			Labels: map[string]string{},
+		},
+	}
+	return NewContainer(&content, nil)
+}
+
 func mockContainerWithLabels(labels map[string]string) *Container {
 	content := types.ContainerJSON{
 		ContainerJSONBase: &types.ContainerJSONBase{

pkg/container/metadata.go

@@ -4,6 +4,7 @@ const (
 	watchtowerLabel       = "com.centurylinklabs.watchtower"
 	signalLabel           = "com.centurylinklabs.watchtower.stop-signal"
 	enableLabel           = "com.centurylinklabs.watchtower.enable"
+	dependsOnLabel        = "com.centurylinklabs.watchtower.depends-on"
 	zodiacLabel           = "com.centurylinklabs.zodiac.original-image"
 	preCheckLabel         = "com.centurylinklabs.watchtower.lifecycle.pre-check"
 	postCheckLabel        = "com.centurylinklabs.watchtower.lifecycle.post-check"