Add optional jittering before any backup actions (#654)

* NEW: Add optional jittering before backing up

* Removed deprecated rand seeding

* Updated BACKUP_JITTER config option doc

* Moved jittering logic from script init to runScript

* Changed formatting of jittering log message

cmd/backup/config.go

@@ -37,6 +37,7 @@ type Config struct {
 	BackupLatestSymlink           string          `split_words:"true"`
 	BackupArchive                 string          `split_words:"true" default:"/archive"`
 	BackupCronExpression          string          `split_words:"true" default:"@daily"`
+	BackupJitter                  time.Duration   `split_words:"true" default:"0s"`
 	BackupRetentionDays           int32           `split_words:"true" default:"-1"`
 	BackupPruningLeeway           time.Duration   `split_words:"true" default:"1m"`
 	BackupPruningPrefix           string          `split_words:"true"`

cmd/backup/run_script.go

@@ -6,7 +6,9 @@ package main
 import (
 	"errors"
 	"fmt"
+	"math/rand"
 	"runtime/debug"
+	"time"
 
 	"github.com/offen/docker-volume-backup/internal/errwrap"
 )
@@ -51,6 +53,15 @@ func runScript(c *Config) (err error) {
 		}
 	}()
 
+	if s.c != nil && s.c.BackupJitter > 0 {
+		max := s.c.BackupJitter
+		delay := time.Duration(rand.Int63n(int64(max) + 1))
+		if delay > 0 {
+			s.logger.Info(fmt.Sprintf("Applying startup jitter of %v", delay))
+			time.Sleep(delay)
+		}
+	}
+
 	if initErr := s.init(); initErr != nil {
 		err = errwrap.Wrap(initErr, "error instantiating script")
 		return