fix: pass nil tags when task definition has no tags

AWS rejects RegisterTaskDefinition with an empty Tags slice ("Tags can
not be empty"). #43 passed describeResult.Tags directly, which is an
empty slice for untagged task definitions rather than nil. The SDK
treats nil (omit the field) and []Tag{} (empty list) differently.

Add nilIfEmpty() helper that converts an empty slice to nil so untagged
task definitions continue to work, while tagged task definitions still
propagate their tags correctly.

Author: teaguru

lib/deploy.go

@@ -108,7 +108,7 @@ func deployService(ctx log.Interface, cluster, imageTag string, imageTags []stri
 		RequiresCompatibilities: taskDefinition.Compatibilities,
 		TaskRoleArn:             taskDefinition.TaskRoleArn,
 		Volumes:                 taskDefinition.Volumes,
-		Tags:                    describeTaskResult.Tags,
+		Tags:                    nilIfEmpty(describeTaskResult.Tags),
 	})
 	if err != nil {
 		ctx.WithError(err).Error("Can't register task definition")

lib/run.go

@@ -11,14 +11,14 @@ import (
 // RunTask runs the specified one-off task in the cluster using the task definition
 func RunTask(profile, cluster, service, taskDefinitionName, imageTag string, imageTags []string, workDir, containerName, awslogGroup, launchType string, args []string) (exitCode int, err error) {
 	ctx := log.WithFields(log.Fields{
-        "task_definition": taskDefinitionName,
-        "launch_type":     launchType,
-    })
+		"task_definition": taskDefinitionName,
+		"launch_type":     launchType,
+	})
 	err = makeSession(profile)
 	if err != nil {
 		return 1, err
 	}
-	
+
 	svc := ecs.New(localSession)
 
 	describeResult, err := svc.DescribeTaskDefinition(&ecs.DescribeTaskDefinitionInput{
@@ -68,7 +68,7 @@ func RunTask(profile, cluster, service, taskDefinitionName, imageTag string, ima
 		RequiresCompatibilities: taskDefinition.Compatibilities,
 		TaskRoleArn:             taskDefinition.TaskRoleArn,
 		Volumes:                 taskDefinition.Volumes,
-		Tags:                    describeResult.Tags,
+		Tags:                    nilIfEmpty(describeResult.Tags),
 	})
 	if err != nil {
 		ctx.WithError(err).Error("Can't register task definition")

lib/runFargate.go

@@ -112,7 +112,7 @@ func RunFargate(profile, cluster, service, taskDefinitionName, imageTag string,
 		RequiresCompatibilities: taskDefinition.Compatibilities,
 		TaskRoleArn:             taskDefinition.TaskRoleArn,
 		Volumes:                 taskDefinition.Volumes,
-		Tags:                    describeResult.Tags,
+		Tags:                    nilIfEmpty(describeResult.Tags),
 	})
 	if err != nil {
 		ctx.WithError(err).Error("Can't register task definition")

lib/util.go

@@ -179,3 +179,13 @@ func fetchSecurityGroupsByName(svc *ec2.EC2, securityGroupFilter string) ([]*str
 	// Return the filtered list of security group IDs
 	return securityGroups, nil
 }
+
+// nilIfEmpty returns nil when tags is empty so AWS doesn't reject the call with
+// "Tags can not be empty" — the AWS API rejects an empty Tags list at the wire level;
+// passing nil omits the field entirely.
+func nilIfEmpty(tags []*ecs.Tag) []*ecs.Tag {
+	if len(tags) == 0 {
+		return nil
+	}
+	return tags
+}

lib/util_test.go

@@ -4,13 +4,28 @@ import (
 	"testing"
 
 	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/service/ecs"
 )
 
 var testdata = map[string]string{
 	"arn:aws:ecs:us-west-1:433844053624:task/scratchpower-staging/334083dd41c04b429cbf99ea7aeeef19": "334083dd41c04b429cbf99ea7aeeef19",     // new longer
 	"arn:aws:ecs:ap-southeast-2:208168611618:task/0c5027c6-7bbd-476b-a534-adaf56760ca9":             "0c5027c6-7bbd-476b-a534-adaf56760ca9", // old format
 }
 
+func TestNilIfEmpty(t *testing.T) {
+	if nilIfEmpty(nil) != nil {
+		t.Fatal("nil input should return nil")
+	}
+	if nilIfEmpty([]*ecs.Tag{}) != nil {
+		t.Fatal("empty slice should return nil")
+	}
+	tags := []*ecs.Tag{{Key: aws.String("env"), Value: aws.String("prod")}}
+	result := nilIfEmpty(tags)
+	if len(result) != 1 || aws.StringValue(result[0].Key) != "env" || aws.StringValue(result[0].Value) != "prod" {
+		t.Fatalf("non-empty slice should be returned unchanged, got %v", result)
+	}
+}
+
 func TestParseTaskUUID(t *testing.T) {
 	for testArn, uuid := range testdata {
 		if parsedUUID, err := parseTaskUUID(aws.String(testArn)); err != nil {