Bugfix: Allow aws_elb subnets update that would remove all current subnets

[brittandeyoung]

Description

This PR enables the aws_elb resource update flow to allow for removing all current subnets and setting new subnets. This is accomplished by forcing a new resource when this condition occurs. As the api only supports remove and add of subnets, not a single update (like the new elbv2 api), this is the best way to accomplish this. Currently the apply will simply fail stating that you cannot remove all subnets associated with the load balancer and manually deleting the resource is required in order to perform the update. This update will handle all of that in the resource.

A test step has been added to validate the resolved functionality and test outputs before the issue was resolved show the error. After the fix, the tests are now passing.

Relations

Closes #727.
Closes #8180.

References

Output from Acceptance Testing

Test Added To replicate bug

$  make testacc TESTARGS='-run=TestAccELBLoadBalancer_Swap_subnets' PKG=elb
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/elb/... -v -count 1 -parallel 20  -run=TestAccELBLoadBalancer_Swap_subnets -timeout 180m
=== RUN   TestAccELBLoadBalancer_Swap_subnets
=== PAUSE TestAccELBLoadBalancer_Swap_subnets
=== CONT  TestAccELBLoadBalancer_Swap_subnets
    load_balancer_test.go:383: Step 3/3 error: Error running apply: exit status 1
        
        Error: Failure removing ELB subnets: InvalidConfigurationRequest: Requested configuration change for LoadBalancer "tf-lb-20230613210207413600000001" is invalid because you attempted to detach all the subnets for this LoadBalancer and a LoadBalancer cannot be attached to zero subnets in VPC.
                status code: 409, request id: 675889ea-9c10-46b9-96f6-ad5a79baa2b8
        
          with aws_elb.test,
          on terraform_plugin_test.tf line 60, in resource "aws_elb" "test":
          60: resource "aws_elb" "test" {
        
--- FAIL: TestAccELBLoadBalancer_Swap_subnets (87.71s)
FAIL
FAIL    github.com/hashicorp/terraform-provider-aws/internal/service/elb        92.669s
FAIL
make: *** [testacc] Error 1
...

Test output after bugfix

make testacc TESTARGS='-run=TestAccELBLoadBalancer_Swap_subnets' PKG=elb
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/elb/... -v -count 1 -parallel 20  -run=TestAccELBLoadBalancer_Swap_subnets -timeout 180m
=== RUN   TestAccELBLoadBalancer_Swap_subnets
=== PAUSE TestAccELBLoadBalancer_Swap_subnets
=== CONT  TestAccELBLoadBalancer_Swap_subnets
--- PASS: TestAccELBLoadBalancer_Swap_subnets (82.24s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/elb        85.604s

[github-actions]

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[ewbankkit]

LGTM 🚀.

% make testacc TESTARGS='-run=TestAccELBLoadBalancer_' PKG=elb ACCTEST_PARALLELISM=3
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/elb/... -v -count 1 -parallel 3  -run=TestAccELBLoadBalancer_ -timeout 180m
=== RUN   TestAccELBLoadBalancer_basic
=== PAUSE TestAccELBLoadBalancer_basic
=== RUN   TestAccELBLoadBalancer_disappears
=== PAUSE TestAccELBLoadBalancer_disappears
=== RUN   TestAccELBLoadBalancer_namePrefix
=== PAUSE TestAccELBLoadBalancer_namePrefix
=== RUN   TestAccELBLoadBalancer_nameGenerated
=== PAUSE TestAccELBLoadBalancer_nameGenerated
=== RUN   TestAccELBLoadBalancer_tags
=== PAUSE TestAccELBLoadBalancer_tags
=== RUN   TestAccELBLoadBalancer_fullCharacterRange
=== PAUSE TestAccELBLoadBalancer_fullCharacterRange
=== RUN   TestAccELBLoadBalancer_AccessLogs_enabled
=== PAUSE TestAccELBLoadBalancer_AccessLogs_enabled
=== RUN   TestAccELBLoadBalancer_AccessLogs_disabled
=== PAUSE TestAccELBLoadBalancer_AccessLogs_disabled
=== RUN   TestAccELBLoadBalancer_generatesNameForZeroValue
=== PAUSE TestAccELBLoadBalancer_generatesNameForZeroValue
=== RUN   TestAccELBLoadBalancer_availabilityZones
=== PAUSE TestAccELBLoadBalancer_availabilityZones
=== RUN   TestAccELBLoadBalancer_ListenerSSLCertificateID_iamServerCertificate
=== PAUSE TestAccELBLoadBalancer_ListenerSSLCertificateID_iamServerCertificate
=== RUN   TestAccELBLoadBalancer_Swap_subnets
=== PAUSE TestAccELBLoadBalancer_Swap_subnets
=== RUN   TestAccELBLoadBalancer_instanceAttaching
=== PAUSE TestAccELBLoadBalancer_instanceAttaching
=== RUN   TestAccELBLoadBalancer_listener
=== PAUSE TestAccELBLoadBalancer_listener
=== RUN   TestAccELBLoadBalancer_healthCheck
=== PAUSE TestAccELBLoadBalancer_healthCheck
=== RUN   TestAccELBLoadBalancer_timeout
=== PAUSE TestAccELBLoadBalancer_timeout
=== RUN   TestAccELBLoadBalancer_connectionDraining
=== PAUSE TestAccELBLoadBalancer_connectionDraining
=== RUN   TestAccELBLoadBalancer_securityGroups
=== PAUSE TestAccELBLoadBalancer_securityGroups
=== RUN   TestAccELBLoadBalancer_desyncMitigationMode
=== PAUSE TestAccELBLoadBalancer_desyncMitigationMode
=== RUN   TestAccELBLoadBalancer_desyncMitigationMode_update
=== PAUSE TestAccELBLoadBalancer_desyncMitigationMode_update
=== CONT  TestAccELBLoadBalancer_basic
=== CONT  TestAccELBLoadBalancer_ListenerSSLCertificateID_iamServerCertificate
=== CONT  TestAccELBLoadBalancer_fullCharacterRange
=== CONT  TestAccELBLoadBalancer_nameGenerated
--- PASS: TestAccELBLoadBalancer_fullCharacterRange (34.72s)
--- PASS: TestAccELBLoadBalancer_basic (39.70s)
=== CONT  TestAccELBLoadBalancer_tags
--- PASS: TestAccELBLoadBalancer_ListenerSSLCertificateID_iamServerCertificate (51.56s)
=== CONT  TestAccELBLoadBalancer_namePrefix
--- PASS: TestAccELBLoadBalancer_nameGenerated (33.58s)
=== CONT  TestAccELBLoadBalancer_timeout
--- PASS: TestAccELBLoadBalancer_namePrefix (35.37s)
=== CONT  TestAccELBLoadBalancer_desyncMitigationMode_update
--- PASS: TestAccELBLoadBalancer_timeout (57.21s)
=== CONT  TestAccELBLoadBalancer_desyncMitigationMode
--- PASS: TestAccELBLoadBalancer_tags (89.02s)
=== CONT  TestAccELBLoadBalancer_securityGroups
--- PASS: TestAccELBLoadBalancer_desyncMitigationMode (37.25s)
=== CONT  TestAccELBLoadBalancer_connectionDraining
--- PASS: TestAccELBLoadBalancer_desyncMitigationMode_update (90.59s)
=== CONT  TestAccELBLoadBalancer_generatesNameForZeroValue
--- PASS: TestAccELBLoadBalancer_securityGroups (62.26s)
=== CONT  TestAccELBLoadBalancer_availabilityZones
--- PASS: TestAccELBLoadBalancer_generatesNameForZeroValue (32.49s)
=== CONT  TestAccELBLoadBalancer_AccessLogs_disabled
--- PASS: TestAccELBLoadBalancer_connectionDraining (79.81s)
=== CONT  TestAccELBLoadBalancer_AccessLogs_enabled
--- PASS: TestAccELBLoadBalancer_availabilityZones (55.97s)
=== CONT  TestAccELBLoadBalancer_disappears
--- PASS: TestAccELBLoadBalancer_disappears (26.07s)
=== CONT  TestAccELBLoadBalancer_listener
--- PASS: TestAccELBLoadBalancer_AccessLogs_disabled (81.01s)
=== CONT  TestAccELBLoadBalancer_healthCheck
--- PASS: TestAccELBLoadBalancer_AccessLogs_enabled (81.08s)
=== CONT  TestAccELBLoadBalancer_instanceAttaching
--- PASS: TestAccELBLoadBalancer_healthCheck (53.73s)
=== CONT  TestAccELBLoadBalancer_Swap_subnets
--- PASS: TestAccELBLoadBalancer_Swap_subnets (90.01s)
--- PASS: TestAccELBLoadBalancer_listener (136.53s)
--- PASS: TestAccELBLoadBalancer_instanceAttaching (121.77s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/elb	451.558s

[ewbankkit]

@brittandeyoung Thanks for the contribution 🎉 👏.

[github-actions]

This functionality has been released in v5.4.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.