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/prefix.go

@@ -0,0 +1,77 @@
+package cmd
+
+import (
+	"io/fs"
+	"os"
+
+	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 UpdatePrefixConfig wisski_distillery.Command = updateprefixconfig{}
+
+type updateprefixconfig struct{}
+
+func (updateprefixconfig) Description() wisski_distillery.Description {
+	return wisski_distillery.Description{
+		Requirements: env.Requirements{
+			NeedsConfig: true,
+		},
+		Command:     "update_prefix_config",
+		Description: "Updates the prefix configuration",
+	}
+}
+
+var errPrefixUpdateFailed = exit.Error{
+	Message:  "Failed to update the prefix configuration: %s",
+	ExitCode: exit.ExitGeneric,
+}
+
+func (upc updateprefixconfig) Run(context wisski_distillery.Context) error {
+	dis := context.Environment
+
+	instances, err := dis.AllInstances()
+	if err != nil {
+		return errPrefixUpdateFailed.WithMessageF(err)
+	}
+
+	target := dis.ResolverPrefixConfig()
+
+	// print the configuration
+	config, err := os.OpenFile(target, os.O_WRONLY, fs.ModePerm)
+	if err != nil {
+		return errPrefixUpdateFailed.WithMessageF(err)
+	}
+
+	// iterate over the instances and store the last value of error
+	for _, instance := range instances {
+		if err := logging.LogOperation(func() error {
+			// read the prefix config
+			data, err := instance.PrefixConfig()
+			if err != nil {
+				return err
+			}
+			context.IOStream.Printf("%s", data)
+
+			// and write it out!
+			if _, err := config.WriteString(data); err != nil {
+				return err
+			}
+
+			return nil
+		}, context.IOStream, "reading prefix config %s", instance.Slug); err != nil {
+			return errPrefixUpdateFailed.WithMessageF(err)
+		}
+	}
+
+	// and restart the resolver to apply the config!
+	logging.LogMessage(context.IOStream, "restarting resolver stack")
+	if err := dis.ResolverStack().Restart(context.IOStream); err != nil {
+		return errPrefixUpdateFailed.WithMessageF(err)
+	}
+
+	return err
+}