check elb type in compare_subnets for elbv2

[ezmac]

SUMMARY

Fixes #49558
Add a check for LB type before accessing property that may not be there.

NLB and ALB have different structures for AvailabilityZone property

https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/elbv2.html#ElasticLoadBalancingv2.Client.create_load_balancer

For NLB:

'AvailabilityZones': [
  {
    'ZoneName': 'string',
    'SubnetId': 'string',
    'LoadBalancerAddresses': [
        {
          'IpAddress': 'string',
          'AllocationId': 'string'
        },
      ]
  },
],

For ALB

'AvailabilityZones': [
  {
    'SubnetId': 'subnet-8360a9e7',
    'ZoneName': 'us-west-2a',
  },
  {
    'SubnetId': 'subnet-b7d581c0',
    'ZoneName': 'us-west-2b',
  },
],

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

elbv2.py

[ezmac]

Someone might want to add
+label aws

[ezmac]

ready_for_review

[briantist]

Hello @ezmac ! I wrote that part of the code in elbv2.py that does the comparison. Although I wrote it with NLB in mind, it was tested and has worked with ALB as well.

In our own environment, we've quite suddenly been having the missing key problem, even with NLBs that don't have a static IP. This previously, absolutely, worked, because the key was always present, with an array containing an empty dict.

Something has changed, either in boto or in the AWS API return, that's excluding the "empty" entry. I think it may be the API, based on the fact that this seems to have affected different people at different times, and though I may have missed it I don't see anything in the boto project that would have cleared this key out.

To that end, the code in your PR will "fix" ALBs but will still cause NLBs with no static IP to fail.

I recommend removing the conditional about ELB type and replacing it with this smaller fix instead. From the original file's line 172, which was this:

for address in subnet['LoadBalancerAddresses']:

Change it to this:

for address in subnet.get('LoadBalancerAddresses', []):

So far this is working well in my local environment. I could create a separate PR if you want but it may be easier for you to just make the change here.

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[ezmac]

Thanks for the input; your solution is cleaner and looks like it still works for me.

[briantist]

Thanks for the quick update @ezmac !

[flowerysong]

shipit

[willthames]

Thanks @ezmac for the fix, @briantist for the improvements and @flowerysong for the review.