Fix panic when deleting an ALB with no default SG in the VPC

It still fails to delete the ALB because ALBs require at least one SG, but at least it wont panic.

Subnets that are shared across accounts using AWS Resource Access Manager do not have a `default` security group by default.

Author: rifelpet

internal/alb/sg/lb_attachment.go

@@ -2,6 +2,7 @@ package sg
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
 	"github.com/aws/aws-sdk-go/service/elbv2"
@@ -62,5 +63,8 @@ func (controller *lbAttachmentController) getDefaultSecurityGroupID() (string, e
 	if err != nil {
 		return "", err
 	}
+	if defaultSG == nil {
+		return "", errors.New("default security group not found")
+	}
 	return aws.StringValue(defaultSG.GroupId), nil
 }

internal/alb/sg/lb_attachment_test.go

@@ -142,6 +142,18 @@ func Test_LBAttachmentDelete(t *testing.T) {
 			},
 			ExpectedError: errors.New("failed to get default securityGroup for current vpc due to GetSecurityGroupByNameCall"),
 		},
+		{
+			Name: "delete failed when no default securityGroup",
+			Instance: elbv2.LoadBalancer{
+				LoadBalancerArn: aws.String("arn"),
+				SecurityGroups:  []*string{aws.String("sg-abcd")},
+			},
+			GetSecurityGroupByNameCall: &GetSecurityGroupByNameCall{
+				GroupName: "default",
+				Instance:  nil,
+			},
+			ExpectedError: errors.New("failed to get default securityGroup for current vpc due to default security group not found"),
+		},
 		{
 			Name: "delete failed when modify SG to default one",
 			Instance: elbv2.LoadBalancer{