feat: Optional detailed exitcodes for `helmfile diff`

Adds the `--detailed-exitcode` to the `helmfile diff` command to return `1` on failure, and `2` when no error but diff is seen.

This feature requires the latest `helm-diff` containing https://github.com/databus23/helm-diff/pull/78, and `helm` containing https://github.com/helm/helm/pull/4367.

This is verified to work by manually running commands like the followings:

```bash
./helmfile --helm-binary helm211dev -f ./examples/helmfile.d diff --detailed-exitcode; echo $?
./helmfile --helm-binary helm211dev -f ./examples/helmfile.d diff; echo $?
```

Note that, in above example commands, `helm211dev` is a custom `helm` binary that is built from helm's master branch containing [the necessary enhancement to allow propagate non-zero plugin exit code](https://github.com/helm/helm/pull/4367).

main.go

@@ -15,6 +15,7 @@ import (
 	"github.com/urfave/cli"
 	"go.uber.org/zap"
 	"go.uber.org/zap/zapcore"
+	"os/exec"
 )
 
 const (
@@ -166,6 +167,10 @@ func main() {
 					Name:  "sync-repos",
 					Usage: "enable a repo sync prior to diffing",
 				},
+				cli.BoolFlag{
+					Name:  "detailed-exitcode",
+					Usage: "return a non-zero exit code when there are changes",
+				},
 				cli.IntFlag{
 					Name:  "concurrency",
 					Value: 0,
@@ -190,8 +195,9 @@ func main() {
 
 					values := c.StringSlice("values")
 					workers := c.Int("concurrency")
+					detailedExitCode := c.Bool("detailed-exitcode")
 
-					return state.DiffReleases(helm, values, workers)
+					return state.DiffReleases(helm, values, workers, detailedExitCode)
 				})
 			},
 		},
@@ -367,8 +373,7 @@ func main() {
 
 	err := app.Run(os.Args)
 	if err != nil {
-		log.Printf("err: %s", err.Error())
+		log.Panicf("[bug] this code path shouldn't be arrived: helmfile is expected to exit from within the `cleanup` func in main.go: %v", err)
-		os.Exit(1)
 	}
 }
 
@@ -517,7 +522,14 @@ func clean(state *state.HelmState, errs []error) error {
 		for _, err := range errs {
 			fmt.Printf("err: %s\n", err.Error())
 		}
-		os.Exit(1)
+		switch e := errs[0].(type) {
+		case *exec.ExitError:
+			// Propagate any non-zero exit status from the external command like `helm` that is failed under the hood
+			status := e.Sys().(syscall.WaitStatus)
+			os.Exit(status.ExitStatus())
+		default:
+			os.Exit(1)
+		}
 	}
 	return nil
 }

state/state.go

@@ -254,7 +254,7 @@ func (state *HelmState) SyncReleases(helm helmexec.Interface, additionalValues [
 }
 
 // DiffReleases wrapper for executing helm diff on the releases
-func (state *HelmState) DiffReleases(helm helmexec.Interface, additionalValues []string, workerLimit int) []error {
+func (state *HelmState) DiffReleases(helm helmexec.Interface, additionalValues []string, workerLimit int, detailedExitCode bool) []error {
 	var wgRelease sync.WaitGroup
 	var wgError sync.WaitGroup
 	errs := []error{}
@@ -289,6 +289,10 @@ func (state *HelmState) DiffReleases(helm helmexec.Interface, additionalValues [
 					flags = append(flags, "--values", valfile)
 				}
 
+				if detailedExitCode {
+					flags = append(flags, "--detailed-exitcode")
+				}
+
 				if len(errs) == 0 {
 					if err := helm.DiffRelease(release.Name, normalizeChart(state.BaseChartPath, release.Chart), flags...); err != nil {
 						errs = append(errs, err)