Adding in diff plugin execution

- Simple copy-paste for the most part from sync job.  I had started down
the path of adding in a meta PluginCommand directive and trying to make it
more modular, but in the end there are some small differences between
the execution that were a bit difficult to model and it just got ugly.
Figured keeping it simple would be easier to manage

helmexec/exec.go

@@ -76,6 +76,18 @@ func (helm *execer) SyncChart(name, chart string, flags ...string) error {
 	return err
 }
 
+func (helm *execer) DiffChart(name, chart string, flags ...string) error {
+	chart, err := normalizeChart(chart)
+	if err != nil {
+		return err
+	}
+	out, err := helm.exec(append([]string{"diff", name, chart}, flags...)...)
+	if helm.writer != nil {
+		helm.writer.Write(out)
+	}
+	return err
+}
+
 func (helm *execer) DeleteChart(name string) error {
 	out, err := helm.exec("delete", name)
 	if helm.writer != nil {

helmexec/helmexec.go

@@ -7,5 +7,6 @@ type Interface interface {
 	UpdateRepo() error
 
 	SyncChart(name, chart string, flags ...string) error
+	DiffChart(name, chart string, flags ...string) error
 	DeleteChart(name string) error
 }

main.go

@@ -107,6 +107,45 @@ func main() {
 				return nil
 			},
 		},
+		{
+			Name:  "diff",
+			Usage: "diff charts from state file against env (helm diff)",
+			Flags: []cli.Flag{
+				cli.StringSliceFlag{
+					Name:  "values",
+					Usage: "additional value files to be merged into the command",
+				},
+				cli.BoolFlag{
+					Name:  "sync-repos",
+					Usage: "enable a repo sync prior to diffing",
+				},
+			},
+			Action: func(c *cli.Context) error {
+				state, helm, err := before(c)
+				if err != nil {
+					return err
+				}
+
+				if c.Bool("sync-repos") {
+					if errs := state.SyncRepos(helm); errs != nil && len(errs) > 0 {
+						for _, err := range errs {
+							fmt.Printf("err: %s\n", err.Error())
+						}
+						os.Exit(1)
+					}
+				}
+
+				values := c.StringSlice("values")
+
+				if errs := state.DiffCharts(helm, values); errs != nil && len(errs) > 0 {
+					for _, err := range errs {
+						fmt.Printf("err: %s\n", err.Error())
+					}
+					os.Exit(1)
+				}
+				return nil
+			},
+		},
 		{
 			Name:  "sync",
 			Usage: "sync all resources from state file (repos && charts)",

state/state.go

@@ -115,6 +115,44 @@ func (state *HelmState) SyncCharts(helm helmexec.Interface, additonalValues []st
 	return nil
 }
 
+func (state *HelmState) DiffCharts(helm helmexec.Interface, additonalValues []string) []error {
+	var wg sync.WaitGroup
+	errs := []error{}
+
+	for _, chart := range state.Charts {
+		wg.Add(1)
+		go func(wg *sync.WaitGroup, chart ChartSpec) {
+			// Plugin command doesn't support explicit namespace
+			chart.Namespace = ""
+			flags, flagsErr := flagsForChart(&chart)
+			if flagsErr != nil {
+				errs = append(errs, flagsErr)
+			}
+			for _, value := range additonalValues {
+				wd, wdErr := os.Getwd()
+				if wdErr != nil {
+					errs = append(errs, wdErr)
+				}
+				valfile := filepath.Join(wd, value)
+				flags = append(flags, "--values", valfile)
+			}
+			if len(errs) == 0 {
+				if err := helm.DiffChart(chart.Name, chart.Chart, flags...); err != nil {
+					errs = append(errs, err)
+				}
+			}
+			wg.Done()
+		}(&wg, chart)
+	}
+	wg.Wait()
+
+	if len(errs) != 0 {
+		return errs
+	}
+
+	return nil
+}
+
 func (state *HelmState) DeleteCharts(helm helmexec.Interface) []error {
 	var wg sync.WaitGroup
 	errs := []error{}