Use simple string instead of JSON string for pre/post update commands.

README.md

@@ -280,35 +280,23 @@ docker run -d \
 For every container that could be updated by watchtower, it is possible to 
 specify a command that will be executed before stopping the container (a 
 `pre-update` command), and a command that will be executed after restarting the
-container (a `post-update` command).
+container (a `post-update` command). These commands are specified using the 
-
+*com.centurylinklabs.watchtower.pre-update-command* and the 
-These commands are specified using the *com.centurylinklabs.watchtower.pre-update-command*
+*com.centurylinklabs.watchtower.post-update-command* labels.
-and the *com.centurylinklabs.watchtower.post-update-command* labels. The values
-of these labels are JSON formatted strings that describes the commands.
 
 These labels can be declared as instructions in a Dockerfile:
 
 ```docker
-LABEL com.centurylinklabs.watchtower.pre-update-command='{"Cmd": ["sh", "-c", "/dump-data.sh"]}'
+LABEL com.centurylinklabs.watchtower.pre-update-command="/dump-data.sh"
-LABEL com.centurylinklabs.watchtower.post-update-command='{"Cmd": ["sh", "-c", "/restore-data.sh"]}'
+LABEL com.centurylinklabs.watchtower.post-update-command="/restore-data.sh"
 ```
 
 Or be specified as part of the `docker run` command line:
 
 ```bash
 docker run -d \
-  --label=com.centurylinklabs.watchtower.pre-update-command='{"Cmd": ["sh", "-c", "/dump-data.sh"]}' \
+  --label=com.centurylinklabs.watchtower.pre-update-command="/dump-data.sh" \
-  --label=com.centurylinklabs.watchtower.post-update-command='{"Cmd": ["sh", "-c", "/restore-data.sh"]}' \
+  --label=com.centurylinklabs.watchtower.post-update-command="/restore-data.sh" \
   someimage
 ```
 
-The JSON object is made of the following fields:
-
-* `Cmd`: the command to execute, as an array of strings (required)
-* `User`: a string representing a username or a UID. It is of the form `user`,
-  `user:group`, `uid`, or `uid:gid` (optional)
-* `Privileged`: `true` if the command should be given extended privileges, 
-  `false` otherwise (optional, default to `false`)
-* `Env`: a list of instructions to set enviroment variables, for example 
-  `["FOO=bar", BAZ=quux]` (optional)
-

actions/update.go

@@ -51,13 +51,10 @@ func Update(client container.Client, filter container.Filter, cleanup bool, noRe
 		if container.Stale {
 
                         // Execute the pre-update command if it is defined.
-                        preUpdateCommandInfo, err := container.PreUpdateCommandInfo()
+                        preUpdateCommand := container.PreUpdateCommand()
-                        if err != nil {
+                        if len(preUpdateCommand) > 0 {
-                                log.Error("Error while reading pre-update command info.")
-                                log.Error(err)
-                        } else if preUpdateCommandInfo.IsDefined() {
                                 log.Info("Executing pre-update command.")
-		                if err := client.ExecuteCommand(container, preUpdateCommandInfo); err != nil {
+		                if err := client.ExecuteCommand(container, preUpdateCommand); err != nil {
 				        log.Error(err)
 			        }
                         }
@@ -87,13 +84,10 @@ func Update(client container.Client, filter container.Filter, cleanup bool, noRe
 					log.Error(err)
 				} else {
 				        // Execute the post-update command if it is defined.
-                                        postUpdateCommandInfo, err := container.PostUpdateCommandInfo()
+                                        postUpdateCommand := container.PostUpdateCommand()
-                                        if err != nil {
+                                        if len(postUpdateCommand) > 0 {
-                                                 log.Error("Error while reading post-update command info.")
-                                                 log.Error(err)
-                                        } else if postUpdateCommandInfo.IsDefined() {
                                                 log.Info("Executing post-update command.")
-		                                if err := client.ExecuteCommand(container, postUpdateCommandInfo); err != nil {
+		                                if err := client.ExecuteCommand(container, postUpdateCommand); err != nil {
 			                                log.Error(err)
 			                        }
                                         }

container/client.go

@@ -25,7 +25,7 @@ type Client interface {
 	RenameContainer(Container, string) error
 	IsContainerStale(Container) (bool, error)
 	RemoveImage(Container) error
-	ExecuteCommand(Container, CommandInfo) error
+	ExecuteCommand(Container, string) error
 }
 
 // NewClient returns a new Client instance which can be used to interact with
@@ -223,7 +223,7 @@ func (client dockerClient) IsContainerStale(c Container) (bool, error) {
 	return false, nil
 }
 
-func (client dockerClient) ExecuteCommand(c Container, commandInfo CommandInfo) error {
+func (client dockerClient) ExecuteCommand(c Container, command string) error {
 	bg := context.Background()
 
         // Get the id of the actual container
@@ -235,14 +235,11 @@ func (client dockerClient) ExecuteCommand(c Container, commandInfo CommandInfo)
 
 	// Create the exec
 	execConfig := types.ExecConfig{
-	        User: commandInfo.User,
-                Privileged: commandInfo.Privileged,
                 Tty: true,
                 AttachStderr: true,
                 AttachStdout: true,
                 Detach: false,
-                Env: commandInfo.Env,
+                Cmd: []string{"sh", "-c", command},
-                Cmd: commandInfo.Cmd,
 	}
 	exec, err := client.api.ContainerExecCreate(bg, containerID, execConfig)
 	if err != nil {
@@ -257,13 +254,13 @@ func (client dockerClient) ExecuteCommand(c Container, commandInfo CommandInfo)
 	}
 
         // Inspect the exec to get the exit code and print a message if the
-        // exit code is not success
+        // exit code is not success.
 	execInspect, err := client.api.ContainerExecInspect(bg, exec.ID)
         if err != nil {
 		return err
 	}
         if execInspect.ExitCode > 0 {
-	        log.Error("Failed to execute command.")
+	        log.Errorf("Command exited with code %v.", execInspect.ExitCode)
         }
 
 	return nil

container/command_info.go

@@ -1,33 +0,0 @@
-package container
-
-import (
-        "encoding/json"
-)
-
-// CommandInfo is the data structure used to encode information about a
-// pre/post-update command.
-type CommandInfo struct {
-	User         string
-        Privileged   bool
-        Env          []string
-        Cmd          []string
-}
-
-// ReadCommandInfoFromJSON takes a JSON formatted description of a
-// pre/post-update as input and returns the parsed data as a CommandInfo.
-func ReadCommandInfoFromJSON(commandInfoJSON string) (CommandInfo, error) {
-        commandInfo := CommandInfo{}
-
-        err := json.Unmarshal([]byte(commandInfoJSON), &commandInfo)
-        if err != nil {
-		return CommandInfo{}, err
-	}
-
-        return commandInfo, nil
-}
-
-// IsDefined returns true if a CommandInfo actually contains a command to
-// execute or false otherwise.
-func (commandInfo CommandInfo) IsDefined() bool {
-        return commandInfo.Cmd != nil && len(commandInfo.Cmd) > 0
-}

container/container.go

@@ -119,34 +119,26 @@ func (c Container) StopSignal() string {
 	return ""
 }
 
-// PreUpdateCommandInfo returns the pre-update command set in the container's
+// PreUpdateCommand returns the pre-update command set in the container's
 // metadata or an empty string if the user did not set it.
-func (c Container) PreUpdateCommandInfo() (CommandInfo, error) {
+func (c Container) PreUpdateCommand() string {
 	if val, ok := c.containerInfo.Config.Labels[preUpdateCommandLabel]; ok {
 
-		commandInfo, err := ReadCommandInfoFromJSON(val)
+		return val
-                if err != nil {
-		        return CommandInfo{}, err
-	        }
-		return commandInfo, nil
 	}
 
-	return CommandInfo{}, nil
+	return ""
 }
 
-// PostUpdateCommandInfo returns the post-update command set in the container's
+// PostUpdateCommand returns the post-update command set in the container's
 // metadata or an empty string if the user did not set it.
-func (c Container) PostUpdateCommandInfo() (CommandInfo, error) {
+func (c Container) PostUpdateCommand() string {
 	if val, ok := c.containerInfo.Config.Labels[postUpdateCommandLabel]; ok {
 
-		commandInfo, err := ReadCommandInfoFromJSON(val)
+		return val
-                if err != nil {
-		        return CommandInfo{}, err
-	        }
-		return commandInfo, nil
 	}
 
-	return CommandInfo{}, nil
+	return ""
 }
 
 // Ideally, we'd just be able to take the ContainerConfig from the old container