added a hostname flag/var and Docker usage docs

Now Wadsworth should be a bit easier to use within a container. This change adds documentation for Docker usage as well as a solution to a problem introduced by using a container: the lack of access to the host's hostname. This change introduces a new flag/environment variable named `hostname`/`HOSTNAME` so the parent machine's hostname can be passed in to wadsworth's container and used during configuration execution.

Author: Southclaws

README.md

@@ -35,6 +35,33 @@ Currently, Wadsworth has a single command: `run` and it takes a single parameter
 this way instead of just using the target repos to define what Wadsworth should do is 1. to consolidate Wadsworth config
 into one place, 2. separate the config of the tools from the applications and 3. keep your target repos clean.
 
+## Usage as a Docker Container
+
+See the `docker-compose.yml` file for an example and read below for details.
+
+You can run Wadsworth as a Docker container. If you're using it to deploy Docker containers via compose, this makes the
+most sense. This is quite simple and is best done by writing a Docker Compose configuration for Wadsworth in order to
+bootstrap your deployment.
+
+The Wadsworth image is built on the `docker/compose` image, since most use-cases will use Docker or Compose to deploy
+services. This means you must mount the Docker API socket into the container, just like Portainer or cAdvisor or any of
+the other Docker tools that also run inside a container.
+
+The socket is located by default at `/var/run/docker.sock` and the `docker/compose` image expects this path too, so you
+just need to add a volume mount to your compose that specifies `/var/run/docker.sock:/var/run/docker.sock`.
+
+Another minor detail you should know is that Wadsworth exposes a `HOSTNAME` variable for the configuration script.
+However, when in a container, this hostname is a randomised string such as `b50fa67783ad`. This means, if your
+configuration performs checks such as `if (HOSTNAME === 'server031')`, this won't work. To resolve this, Wadsworth will
+attempt to read the environment variable `HOSTNAME` and use that instead of using `/etc/hostname`.
+
+This means, you can bootstrap a Wadsworth deployment with only two variables:
+
+```env
+VAULT_TOKEN=abcxyz
+HOSTNAME=server012
+```
+
 ### Configuration
 
 The precursor to Wadsworth used JSON for configuration, this was fine for simple tasks but the ability to provide a

main.go

@@ -59,6 +59,7 @@ this repository has new commits, Wadsworth will automatically reconfigure.`,
 			Usage:     "argument `target` specifies Git repository for configuration.",
 			ArgsUsage: "target",
 			Flags: []cli.Flag{
+				cli.StringFlag{Name: "hostname", EnvVar: "HOSTNAME"},
 				cli.StringFlag{Name: "directory", EnvVar: "DIRECTORY", Value: "./cache/"},
 				cli.BoolFlag{Name: "no-ssh", EnvVar: "NO_SSH"},
 				cli.DurationFlag{Name: "check-interval", EnvVar: "CHECK_INTERVAL", Value: time.Second * 10},
@@ -75,10 +76,20 @@ this repository has new commits, Wadsworth will automatically reconfigure.`,
 				ctx, cancel := context.WithCancel(context.Background())
 				defer cancel()
 
+				// If no hostname is provided, use the actual host's hostname
+				hostname := c.String("hostname")
+				if hostname == "" {
+					hostname, err = os.Hostname()
+					if err != nil {
+						return errors.Wrap(err, "failed to get hostname")
+					}
+				}
+
 				svc, err := service.Initialise(ctx, service.Config{
 					Target:        c.Args().First(),
-					NoSSH:         c.Bool("no-ssh"),
+					Hostname:      hostname,
 					Directory:     c.String("directory"),
+					NoSSH:         c.Bool("no-ssh"),
 					CheckInterval: c.Duration("check-interval"),
 					VaultAddress:  c.String("vault-addr"),
 					VaultToken:    c.String("vault-token"),

service/config/config.go

@@ -21,7 +21,7 @@ type State struct {
 
 // ConfigFromDirectory searches a directory for configuration files and
 // constructs a desired state from the declarations.
-func ConfigFromDirectory(dir string) (state State, err error) {
+func ConfigFromDirectory(dir, hostname string) (state State, err error) {
 	files, err := ioutil.ReadDir(dir)
 	if err != nil {
 		err = errors.Wrap(err, "failed to read config directory")
@@ -46,7 +46,7 @@ func ConfigFromDirectory(dir string) (state State, err error) {
 		scripts: sources,
 	}
 
-	err = cb.construct()
+	err = cb.construct(hostname)
 	if err != nil {
 		return
 	}
@@ -61,7 +61,7 @@ type configBuilder struct {
 	scripts []string
 }
 
-func (cb *configBuilder) construct() (err error) {
+func (cb *configBuilder) construct(hostname string) (err error) {
 	//nolint:errcheck
 	cb.vm.Run(`'use strict';
 var STATE = {
@@ -86,10 +86,6 @@ function E(k, v) {
 }
 `)
 
-	hostname, err := os.Hostname()
-	if err != nil {
-		return errors.Wrap(err, "failed to get hostname")
-	}
 	cb.vm.Set("HOSTNAME", hostname) //nolint:errcheck
 
 	env := make(map[string]string)

service/reconfigure.go

@@ -18,7 +18,7 @@ import (
 // read the configuration file(s) from the repository, gather all the targets
 // and set up the target watcher. This should always happen in sync with the
 // rest of the service to prevent a reconfiguration during an event handler.
-func (app *App) reconfigure() (err error) {
+func (app *App) reconfigure(hostname string) (err error) {
 	zap.L().Debug("reconfiguring")
 
 	err = app.watchConfig()
@@ -31,7 +31,12 @@ func (app *App) reconfigure() (err error) {
 	if err != nil {
 		return
 	}
-	state := getNewState(path, app.state)
+	state := getNewState(path, hostname, app.state)
+
+	// Set the HOSTNAME config environment variable if necessary.
+	if app.config.Hostname != "" {
+		state.Env["HOSTNAME"] = app.config.Hostname
+	}
 
 	// diff targets
 	additions, removals := diffTargets(app.targets, state.Targets)
@@ -113,8 +118,8 @@ func (app *App) watchTargets() (err error) {
 
 // getNewState attempts to obtain a new desired state from the given path, if
 // any failures occur, it simply returns a fallback state and logs an error
-func getNewState(path string, fallback config.State) (state config.State) {
-	state, err := config.ConfigFromDirectory(path)
+func getNewState(path, hostname string, fallback config.State) (state config.State) {
+	state, err := config.ConfigFromDirectory(path, hostname)
 	if err != nil {
 		zap.L().Error("failed to construct config from repo, falling back to original state",
 			zap.Error(err))

service/service.go

@@ -33,6 +33,7 @@ type App struct {
 
 type Config struct {
 	Target        string
+	Hostname      string
 	NoSSH         bool
 	Directory     string
 	CheckInterval time.Duration
@@ -72,7 +73,7 @@ func Initialise(ctx context.Context, c Config) (app *App, err error) {
 		}
 	}
 
-	err = app.reconfigure()
+	err = app.reconfigure(c.Hostname)
 	if err != nil {
 		return
 	}
@@ -89,7 +90,7 @@ func (app *App) Start() (final error) {
 	f := func() (err error) {
 		select {
 		case <-app.configWatcher.Events:
-			err = app.reconfigure()
+			err = app.reconfigure(app.config.Hostname)
 
 		case event := <-app.targetsWatcher.Events:
 			e := app.handle(event)