Do a large chunk of the move to go

This commit moves a huge chunk of the code to go. The TODO.md document
indicates what is left to be done.

cmd/cron.go

@@ -0,0 +1,55 @@
+package cmd
+
+import (
+	wisski_distillery "github.com/FAU-CDI/wisski-distillery"
+	"github.com/FAU-CDI/wisski-distillery/env"
+	"github.com/FAU-CDI/wisski-distillery/internal/logging"
+	"github.com/tkw1536/goprogram/exit"
+)
+
+// Cron is the 'cron' command
+var Cron wisski_distillery.Command = cron{}
+
+type cron struct {
+	Positionals struct {
+		Slug []string `positional-arg-name:"SLUG" required:"0" description:"slug of instance(s) to run cron in"`
+	} `positional-args:"true"`
+}
+
+func (cron) Description() wisski_distillery.Description {
+	return wisski_distillery.Description{
+		Requirements: env.Requirements{
+			NeedsConfig: true,
+		},
+		Command:     "cron",
+		Description: "Runs the cron script for several instances",
+	}
+}
+
+var errCronFailed = exit.Error{
+	Message:  "Failed to run cron script for instance %q: exited with code %s",
+	ExitCode: exit.ExitGeneric,
+}
+
+func (cr cron) Run(context wisski_distillery.Context) error {
+	instances, err := context.Environment.Instances(cr.Positionals.Slug...)
+	if err != nil {
+		return err
+	}
+
+	// iterate over the instances and store the last value of error
+	for _, instance := range instances {
+		logging.LogOperation(func() error {
+			code := instance.Shell(context.IOStream, "/utils/cron.sh")
+			if code != 0 {
+				// keep going, because we want to run as many crons as possible
+				err = errBlindUpdateFailed.WithMessageF(instance.Slug, code)
+				context.EPrintln(err)
+			}
+
+			return nil
+		}, context.IOStream, "running cron for instance %s", instance.Slug)
+	}
+
+	return err
+}