Add method for deduping images

This is helpful with Nomad because some images end up used as sidecars for many jobs.
Also periodic jobs spawn many tasks with the same image.

internal/model/image.go

@@ -1,7 +1,10 @@
 package model
 
 import (
+	"encoding/json"
+
 	"github.com/crazy-max/diun/v4/pkg/registry"
+	"github.com/pkg/errors"
 )
 
 // Image holds image configuration
@@ -20,6 +23,38 @@ type Image struct {
 	Metadata    map[string]string `yaml:"metadata,omitempty" json:",omitempty"`
 }
 
+func (i Image) hash() (string, error) {
+	// Return json serialized image to use as a hashable key
+	b, err := json.Marshal(i)
+	if err != nil {
+		return "", errors.Errorf("cannot hash image: %v", err)
+	}
+
+	return string(b), nil
+}
+
+type ImageList []Image
+
+// Dedupe removes duplicate images from the list and returns a new list
+func (il ImageList) Dedupe() []Image {
+	keys := make(map[string]bool)
+	list := []Image{}
+	for _, entry := range il {
+		hash, err := entry.hash()
+		if err != nil {
+			// If we couldn't hash the entry, we can't dedupe it so we add it anyway
+			list = append(list, entry)
+		} else {
+			if _, value := keys[hash]; !value {
+				keys[hash] = true
+				list = append(list, entry)
+			}
+		}
+	}
+
+	return list
+}
+
 // ImagePlatform holds image platform configuration
 type ImagePlatform struct {
 	OS      string `yaml:"os,omitempty" json:",omitempty"`

internal/provider/nomad/task.go

@@ -50,7 +50,7 @@ func (c *Client) listTaskImages() []model.Image {
 		c.logger.Error().Err(err).Msg("Cannot list Nomad jobs")
 	}
 
-	var list []model.Image
+	var list model.ImageList
 
 	for _, job := range jobs {
 		jobInfo, _, err := client.Jobs().Info(job.ID, nil)
@@ -129,7 +129,7 @@ func (c *Client) listTaskImages() []model.Image {
 		}
 	}
 
-	return list
+	return list.Dedupe()
 }
 
 func metadata(job *nomad.JobListStub, taskGroup *nomad.TaskGroup, task *nomad.Task) map[string]string {