add pre/post update check lifecycle hooks (#373)

* add pre/post update check lifecycle hooks

* update docs for lifecycle hooks

* Fix phrasing

docs/lifecycle-hooks.md

@@ -1,41 +1,49 @@
-
 ## Executing commands before and after updating
 
-> **DO NOTE**: Both commands are shell commands executed with `sh`, and therefore require the 
+> **DO NOTE**: These are shell commands executed with `sh`, and therefore require the
 > container to provide the `sh` executable.
 
-It is possible to execute a *pre-update* command and a *post-update* command 
-**inside** every container updated by watchtower. The *pre-update* command is 
-executed before stopping the container, and the *post-update* command is 
-executed after restarting the container.
+It is possible to execute _pre/post\-check_ and _pre/post\-update_ commands
+**inside** every container updated by watchtower.
+
+- The _pre-check_ command is executed for each container prior to every update cycle.
+- The _pre-update_ command is executed before stopping the container when an update is about to start.
+- The _post-update_ command is executed after restarting the updated container
+- The _post-check_ command is executed for each container post every update cycle.
 
 This feature is disabled by default. To enable it, you need to set the option
 `--enable-lifecycle-hooks` on the command line, or set the environment variable
 `WATCHTOWER_LIFECYCLE_HOOKS` to `true`.
 
- 
-
 ### Specifying update commands
 
-The commands are specified using docker container labels, with 
-`com.centurylinklabs.watchtower.lifecycle.pre-update-command` for the *pre-update* 
-command and `com.centurylinklabs.watchtower.lifecycle.post-update` for the
-*post-update* command.
+The commands are specified using docker container labels, the following are currently available:
+
+| Type        | Docker Container Label                                 |
+| ----------- | ------------------------------------------------------ | 
+| Pre Check   | `com.centurylinklabs.watchtower.lifecycle.pre-check`   |
+| Pre Update  | `com.centurylinklabs.watchtower.lifecycle.pre-update`  | 
+| Post Update | `com.centurylinklabs.watchtower.lifecycle.post-update` |
+| Post Check  | `com.centurylinklabs.watchtower.lifecycle.post-check`  |
 
-These labels can be declared as instructions in a Dockerfile:
+These labels can be declared as instructions in a Dockerfile (with some example .sh files):
 
 ```docker
+LABEL com.centurylinklabs.watchtower.lifecycle.pre-check="/sync.sh"
 LABEL com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh"
 LABEL com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh"
+LABEL com.centurylinklabs.watchtower.lifecycle.post-check="/send-heartbeat.sh"
 ```
 
 Or be specified as part of the `docker run` command line:
 
 ```bash
 docker run -d \
+  --label=com.centurylinklabs.watchtower.lifecycle.pre-check="/sync.sh" \
   --label=com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh" \
   --label=com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh" \
   someimage
+  --label=com.centurylinklabs.watchtower.lifecycle.post-check="/send-heartbeat.sh" \
 ```
 
 ### Execution failure

internal/actions/update.go

@@ -13,6 +13,8 @@ import (
 func Update(client container.Client, params UpdateParams) error {
 	log.Debug("Checking containers for updated images")
 
+	executePreCheck(client, params)
+
 	containers, err := client.ListContainers(params.Filter)
 	if err != nil {
 		return err
@@ -36,12 +38,14 @@ func Update(client container.Client, params UpdateParams) error {
 	checkDependencies(containers)
 
 	if params.MonitorOnly {
+		executePostCheck(client, params)
 		return nil
 	}
 
 	stopContainersInReversedOrder(containers, client, params)
 	restartContainersInSortedOrder(containers, client, params)
 
+	executePostCheck(client, params)
 	return nil
 }
 
@@ -123,11 +127,58 @@ func checkDependencies(containers []container.Container) {
 	}
 }
 
+func executePreCheck(client container.Client, params UpdateParams) {
+	containers, err := client.ListContainers(params.Filter)
+	if err != nil {
+		return
+	}
+	for _, container := range containers {
+		executePreCheckCommand(client, container)
+	}
+}
+
+func executePostCheck(client container.Client, params UpdateParams) {
+	containers, err := client.ListContainers(params.Filter)
+	if err != nil {
+		return
+	}
+	for _, container := range containers {
+		executePostCheckCommand(client, container)
+	}
+}
+
+func executePreCheckCommand(client container.Client, container container.Container) {
+	command := container.GetLifecyclePreCheckCommand()
+	if len(command) == 0 {
+		log.Debug("No pre-check command supplied. Skipping")
+		return
+	}
+
+	log.Info("Executing pre-check command.")
+	if err := client.ExecuteCommand(container.ID(), command); err != nil {
+		log.Error(err)
+	}
+}
+
+func executePostCheckCommand(client container.Client, container container.Container) {
+	command := container.GetLifecyclePostCheckCommand()
+	if len(command) == 0 {
+		log.Debug("No post-check command supplied. Skipping")
+		return
+	}
+
+	log.Info("Executing post-check command.")
+	if err := client.ExecuteCommand(container.ID(), command); err != nil {
+		log.Error(err)
+	}
+}
+
 func executePreUpdateCommand(client container.Client, container container.Container) {
 
 	command := container.GetLifecyclePreUpdateCommand()
 	if len(command) == 0 {
 		log.Debug("No pre-update command supplied. Skipping")
+		return
 	}
 
 	log.Info("Executing pre-update command.")
@@ -146,6 +197,7 @@ func executePostUpdateCommand(client container.Client, newContainerID string) {
 	command := newContainer.GetLifecyclePostUpdateCommand()
 	if len(command) == 0 {
 		log.Debug("No post-update command supplied. Skipping")
+		return
 	}
 
 	log.Info("Executing post-update command.")

pkg/container/metadata.go

@@ -5,10 +5,22 @@ const (
 	signalLabel     = "com.centurylinklabs.watchtower.stop-signal"
 	enableLabel     = "com.centurylinklabs.watchtower.enable"
 	zodiacLabel     = "com.centurylinklabs.zodiac.original-image"
+	preCheckLabel   = "com.centurylinklabs.watchtower.lifecycle.pre-check"
+	postCheckLabel  = "com.centurylinklabs.watchtower.lifecycle.post-check"
 	preUpdateLabel  = "com.centurylinklabs.watchtower.lifecycle.pre-update"
 	postUpdateLabel = "com.centurylinklabs.watchtower.lifecycle.post-update"
 )
 
+// GetLifecyclePreCheckCommand returns the pre-check command set in the container metadata or an empty string
+func (c Container) GetLifecyclePreCheckCommand() string {
+	return c.getLabelValueOrEmpty(preCheckLabel)
+}
+
+// GetLifecyclePostCheckCommand returns the post-check command set in the container metadata or an empty string
+func (c Container) GetLifecyclePostCheckCommand() string {
+	return c.getLabelValueOrEmpty(postCheckLabel)
+}
+
 // GetLifecyclePreUpdateCommand returns the pre-update command set in the container metadata or an empty string
 func (c Container) GetLifecyclePreUpdateCommand() string {
 	return c.getLabelValueOrEmpty(preUpdateLabel)