Perpetual diff on treat_missing_data for imported aws_cloudwatch_metric_alarm

[a13xb]

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Summary

I believe this is due to a quirk of the CloudWatch API.

I imported a CloudWatch alarm created with CloudFormation into Terraform (but reproducible with manually-created alarm as well). When running terraform plan there is always a diff on treat_missing_data attribute:

      + treat_missing_data        = "missing"

It's always there regardless of what it's set to in the configuration. In the state file it's set to empty string. Even if I edit the state file to set it to "missing" (the default value and matching what's in the web console), when terraform refresh is run it's reverted back to the empty string.

And here is the CloudWatch API quirk: if I call aws cloudwatch describe-alarms for the region, TreatMissingData attribute for all the alarms is missing in the response, which would explain reverting to the empty string. If I manually change it in the web console, the attribute will start showing up in the response and the diff disappears, even if I change it back to "missing".

Terraform Version

Terraform v0.12.0
+ provider.aws v1.60.0-dev20190216h00-dev

Affected Resource(s)

aws_cloudwatch_metric_alarm

Expected Behavior

No diff.

Actual Behavior

There is always a diff until "Treat Missing Data" attribute is edited manually.

Steps to Reproduce

1. Create a CloudWatch alarm outside of Terraform.
2. Import it into Terraform.
3. Run terraform plan

[a13xb]

Same with 2.18.0, and also happens to the DatapointsToAlarm attribute.

[nullscan]

It also happens with Terraform v0.12.18 and aws provider v2.43.0

[github-actions]

This functionality has been released in v3.64.0 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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