Monitor-only for individual containers (#652)

* Add monitor-only label

* Add tests for monitor-only

* Treat missing monitor-only label as if the option was set to false

* Add docs for container-based monitor-only

* Add function doc

* Fix monitor-only logic

docs/arguments.md

@@ -162,7 +162,7 @@ Environment Variable: WATCHTOWER_LABEL_ENABLE
 **Do not** update containers that have `com.centurylinklabs.watchtower.enable` label set to false and no `--label-enable` argument is passed. Note that only one or the other (targeting by enable label) can be used at the same time to target containers.
 
 ## Without updating containers
-Will only monitor for new images, not update the containers.
+Will only monitor for new images, send notifications and invoke the [pre-check/post-check hooks](https://containrrr.dev/watchtower/lifecycle-hooks/), but will **not** update the containers.
 
 > ### ⚠️ Please note
 >
@@ -175,6 +175,8 @@ Environment Variable: WATCHTOWER_MONITOR_ONLY
              Default: false
 ```
 
+Note that monitor-only can also be specified on a per-container basis with the `com.centurylinklabs.watchtower.monitor-only` label set on those containers.
+
 ## Without restarting containers
 Do not restart containers after updating. This option can be useful when the start of the containers
 is managed by an external system such as systemd.

docs/container-selection.md

@@ -1,5 +1,12 @@
 By default, watchtower will watch all containers. However, sometimes only some containers should be updated.
 
+There are two options:
+
+- **Fully exclude**: You can choose to exclude containers entirely from being watched by watchtower.
+- **Monitor only**: In this mode, watchtower checks for container updates, sends notifications and invokes the [pre-check/post-check hooks](https://containrrr.dev/watchtower/lifecycle-hooks/) on the containers but does **not** perform the update.
+
+## Full Exclude 
+
 If you need to exclude some containers, set the _com.centurylinklabs.watchtower.enable_ label to `false`.
 
 ```docker
@@ -29,3 +36,23 @@ If you wish to create a monitoring scope, you will need to [run multiple instanc
 Watchtower filters running containers by testing them against each configured criteria. A container is monitored if all criteria are met. For example:
 - If a container's name is on the monitoring name list (not empty `--name` argument) but it is not enabled (_centurylinklabs.watchtower.enable=false_), it won't be monitored;
 - If a container's name is not on the monitoring name list (not empty `--name` argument), even if it is enabled (_centurylinklabs.watchtower.enable=true_ and `--label-enable` flag is set), it won't be monitored;
+
+## Monitor Only
+
+Individual containers can be marked to only be monitored (without being updated).
+
+To do so, set the *com.centurylinklabs.watchtower.monitor-only* label to `true` on that container.
+
+```docker
+LABEL com.centurylinklabs.watchtower.monitor-only="true"
+```
+
+Or, it can be specified as part of the `docker run` command line:
+
+```bash
+docker run -d --label=com.centurylinklabs.watchtower.monitor-only=true someimage
+```
+
+When the label is specified on a container, watchtower treats that container exactly as if [`WATCHTOWER_MONITOR_ONLY`](https://containrrr.dev/watchtower/arguments/#without_updating_containers) was set, but the effect is limited to the individual container. 
+
+

internal/actions/mocks/container.go

@@ -27,3 +27,22 @@ func CreateMockContainer(id string, name string, image string, created time.Time
 		},
 	)
 }
+
+// CreateMockContainerWithConfig creates a container substitute valid for testing
+func CreateMockContainerWithConfig(id string, name string, image string, created time.Time, config *container2.Config) container.Container {
+	content := types.ContainerJSON{
+		ContainerJSONBase: &types.ContainerJSONBase{
+			ID:      id,
+			Image:   image,
+			Name:    name,
+			Created: created.String(),
+		},
+		Config: config,
+	}
+	return *container.NewContainer(
+		&content,
+		&types.ImageInspect{
+			ID: image,
+		},
+	)
+}

internal/actions/update.go

@@ -28,7 +28,7 @@ func Update(client container.Client, params types.UpdateParams) error {
 
 	for i, targetContainer := range containers {
 		stale, err := client.IsContainerStale(targetContainer)
-		if stale && !params.NoRestart && !params.MonitorOnly && !targetContainer.HasImageInfo() {
+		if stale && !params.NoRestart && !params.MonitorOnly && !targetContainer.IsMonitorOnly() && !targetContainer.HasImageInfo() {
 			err = errors.New("no available image info")
 		}
 		if err != nil {
@@ -45,18 +45,20 @@ func Update(client container.Client, params types.UpdateParams) error {
 
 	checkDependencies(containers)
 
-	if params.MonitorOnly {
-		if params.LifecycleHooks {
-			lifecycle.ExecutePostChecks(client, params)
+	containersToUpdate := []container.Container{}
+	if !params.MonitorOnly {
+		for i := len(containers) - 1; i >= 0; i-- {
+			if !containers[i].IsMonitorOnly() {
+				containersToUpdate = append(containersToUpdate, containers[i])
+			}
 		}
-		return nil
 	}
 
 	if params.RollingRestart {
-		performRollingRestart(containers, client, params)
+		performRollingRestart(containersToUpdate, client, params)
 	} else {
-		stopContainersInReversedOrder(containers, client, params)
-		restartContainersInSortedOrder(containers, client, params)
+		stopContainersInReversedOrder(containersToUpdate, client, params)
+		restartContainersInSortedOrder(containersToUpdate, client, params)
 	}
 	if params.LifecycleHooks {
 		lifecycle.ExecutePostChecks(client, params)

internal/actions/update_test.go

@@ -5,6 +5,7 @@ import (
 	"github.com/containrrr/watchtower/pkg/container"
 	"github.com/containrrr/watchtower/pkg/container/mocks"
 	"github.com/containrrr/watchtower/pkg/types"
+	container2 "github.com/docker/docker/api/types/container"
 	cli "github.com/docker/docker/client"
 	"time"
 
@@ -80,4 +81,73 @@ var _ = Describe("the update action", func() {
 			})
 		})
 	})
+
+	When("watchtower has been instructed to monitor only", func() {
+		When("certain containers are set to monitor only", func() {
+			BeforeEach(func() {
+				client = CreateMockClient(
+					&TestData{
+						NameOfContainerToKeep: "test-container-02",
+						Containers: []container.Container{
+							CreateMockContainer(
+								"test-container-01",
+								"test-container-01",
+								"fake-image1:latest",
+								time.Now()),
+							CreateMockContainerWithConfig(
+								"test-container-02",
+								"test-container-02",
+								"fake-image2:latest",
+								time.Now(),
+								&container2.Config{
+									Labels: map[string]string{
+										"com.centurylinklabs.watchtower.monitor-only": "true",
+									},
+								}),
+						},
+					},
+					dockerClient,
+					false,
+					false,
+				)
+			})
+
+			It("should not update those containers", func() {
+				err := actions.Update(client, types.UpdateParams{Cleanup: true})
+				Expect(err).NotTo(HaveOccurred())
+				Expect(client.TestData.TriedToRemoveImageCount).To(Equal(1))
+			})
+		})
+
+		When("monitor only is set globally", func() {
+			BeforeEach(func() {
+				client = CreateMockClient(
+					&TestData{
+						Containers: []container.Container{
+							CreateMockContainer(
+								"test-container-01",
+								"test-container-01",
+								"fake-image:latest",
+								time.Now()),
+							CreateMockContainer(
+								"test-container-02",
+								"test-container-02",
+								"fake-image:latest",
+								time.Now()),
+						},
+					},
+					dockerClient,
+					false,
+					false,
+				)
+			})
+
+			It("should not update any containers", func() {
+				err := actions.Update(client, types.UpdateParams{MonitorOnly: true})
+				Expect(err).NotTo(HaveOccurred())
+				Expect(client.TestData.TriedToRemoveImageCount).To(Equal(0))
+			})
+		})
+
+	})
 })

pkg/container/container.go

@@ -90,6 +90,22 @@ func (c Container) Enabled() (bool, bool) {
 	return parsedBool, true
 }
 
+// IsMonitorOnly returns the value of the monitor-only label. If the label
+// is not set then false is returned.
+func (c Container) IsMonitorOnly() bool {
+	rawBool, ok := c.getLabelValue(monitorOnlyLabel)
+	if !ok {
+		return false
+	}
+
+	parsedBool, err := strconv.ParseBool(rawBool)
+	if err != nil {
+		return false
+	}
+
+	return parsedBool
+}
+
 // Scope returns the value of the scope UID label and if the label
 // was set.
 func (c Container) Scope() (string, bool) {

pkg/container/metadata.go

@@ -4,6 +4,7 @@ const (
 	watchtowerLabel       = "com.centurylinklabs.watchtower"
 	signalLabel           = "com.centurylinklabs.watchtower.stop-signal"
 	enableLabel           = "com.centurylinklabs.watchtower.enable"
+	monitorOnlyLabel      = "com.centurylinklabs.watchtower.monitor-only"
 	dependsOnLabel        = "com.centurylinklabs.watchtower.depends-on"
 	zodiacLabel           = "com.centurylinklabs.zodiac.original-image"
 	scope                 = "com.centurylinklabs.watchtower.scope"