Nothing to update in cloudformation should not result in errors

[ewbankkit]

Fixes #5653.

If a CloudFormation stack is updated by Terraform but the template has not changed (for example the template_url attribute's value changes but the contents of the template have not changed), AWS returns an HTTP 400 error with message ValidationError: No updates are to be performed.
This should not be treated as an error by Terraform.

[ewbankkit]

Acceptance test:

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSCloudFormation_withUrl_withParams_noUpdate'

[ewbankkit]

The acceptance test changes the template_url value so required some refactoring of resource_aws_cloudformation_stack_test.go.

[grubernaut]

Hey @ewbankkit, thanks for this!

Would you be able to paste the output of the acceptance test run? Thanks!

[ewbankkit]

All acceptance tests for CloudFormation stack:

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSCloudFormation_'==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/16 18:40:25 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSCloudFormation_ -timeout 120m
=== RUN   TestAccAWSCloudFormation_basic
--- PASS: TestAccAWSCloudFormation_basic (69.10s)
=== RUN   TestAccAWSCloudFormation_yaml
--- PASS: TestAccAWSCloudFormation_yaml (68.37s)
=== RUN   TestAccAWSCloudFormation_defaultParams
--- PASS: TestAccAWSCloudFormation_defaultParams (68.67s)
=== RUN   TestAccAWSCloudFormation_allAttributes
--- PASS: TestAccAWSCloudFormation_allAttributes (67.92s)
=== RUN   TestAccAWSCloudFormation_withParams
--- PASS: TestAccAWSCloudFormation_withParams (128.97s)
=== RUN   TestAccAWSCloudFormation_withUrl_withParams
--- PASS: TestAccAWSCloudFormation_withUrl_withParams (164.96s)
=== RUN   TestAccAWSCloudFormation_withUrl_withParams_withYaml
--- PASS: TestAccAWSCloudFormation_withUrl_withParams_withYaml (92.93s)
=== RUN   TestAccAWSCloudFormation_withUrl_withParams_noUpdate
--- PASS: TestAccAWSCloudFormation_withUrl_withParams_noUpdate (120.14s)
    PASS

The test added for this PR is the final one, TestAccAWSCloudFormation_withUrl_withParams_noUpdate.

[grubernaut]

LGTM, thanks!

[ghost]

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