Implement initial 'wdcli backup_instance' command

This commit performs an initial implementation of the 'backup_instance'
command.

env/backup.go

@@ -0,0 +1,52 @@
+package env
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync/atomic"
+	"time"
+)
+
+func (dis Distillery) BackupDir() string {
+	return filepath.Join(dis.Config.DeployRoot, "backups")
+}
+
+func (dis Distillery) InprogressBackupPath() string {
+	return filepath.Join(dis.BackupDir(), "inprogress")
+}
+
+func (dis Distillery) FinalBackupPath() string {
+	return filepath.Join(dis.BackupDir(), "final")
+}
+
+// NewFinalBackupFile returns the path to a new final backup file.
+func (dis Distillery) FinalBackupArchive(prefix string) string {
+	counter := atomic.AddUint64(&globalBackupCounter, 1)
+
+	// generate a new name with the current time, a global counter, and the prefix
+	name := fmt.Sprintf("%s-%d-%d.tar.gz", prefix, time.Now().Unix(), counter)
+	path := filepath.Join(dis.FinalBackupPath(), name)
+
+	return path
+}
+
+var globalBackupCounter uint64
+
+// NewInprogressBackupPath returns the path to a new inprogress backup directory.
+// The directory is guaranteed to have been freshly created.
+func (dis Distillery) NewInprogressBackupPath(prefix string) (string, error) {
+	counter := atomic.AddUint64(&globalBackupCounter, 1)
+
+	// generate a new name with the current time, a global counter, and the prefix
+	name := fmt.Sprintf("%s-%d-%d", prefix, time.Now().Unix(), counter)
+	path := filepath.Join(dis.InprogressBackupPath(), name)
+
+	// create the directory
+	if err := os.Mkdir(path, os.ModeDir); err != nil {
+		return "", err
+	}
+
+	// and it is here!
+	return path, nil
+}