aws-cloudwatch: Imported composite alarm ARN incorrect

[nakedible-p]

Describe the bug

It seems that the ARN used for CompositeAlarms is incorrectly generated when using CompositeAlarm.fromCompositeAlarmName. The resulting ARN has / as separator where as the actual alarm has :.

The whole thing is a bit wonky too:

• In reality, ARNs for Alarm and CompositeAlarm instances are equivalent, yet in CDK they are different
• There is no Alarm.fromAlarmName, even though CompositeAlarm.fromCompositeAlarmName exists
• The resulting type for either is IAlarm anyway, so the distinction between Alarm and CompositeAlarm is lost

Fix is trivial, but might require more rework to fix this all.

Expected Behavior

I expected an ARN of the form:

arn:aws:cloudwatch:eu-central-1:219189227975:alarm:NpayGatewayRegionalSyteCentral-Stack-RollbackAlarm

Current Behavior

I got an ARN of the form:

arn:aws:cloudwatch:eu-central-1:219189227975:alarm/NpayGatewayRegionalSyteCentral-Stack-RollbackAlarm

I tried using this as a rollback trigger, and got the error:

RelativeId of a Rollback Trigger's ARN is incorrect

Reproduction Steps

Call code as such:

const testAlarm = CompositeAlarm.fromCompositeAlarmName(this, 'alarm', `TestAlarm`);
console.log(testAlarm.alarmArn);

Possible Solution

https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/aws-cloudwatch/lib/composite-alarm.ts#L76-L80

Pass in arnFormat: ArnFormat.COLON_RESOURCE_NAME.

Also below:

https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/aws-cloudwatch/lib/composite-alarm.ts#L93

Use ArnFormat.COLON_RESOURCE_NAME.

Suggest aligning these methods with Alarm class as well.

Additional Information/Context

No response

CDK CLI Version

2.68.0 (build 25fda51)

Framework Version

No response

Node.js Version

v19.7.0

OS

Linux codespaces-3481fc 5.4.0-1104-azure #110~18.04.1-Ubuntu SMP Sat Feb 11 17:41:21 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

Language

Typescript

Language Version

Version 4.9.5

Other information

No response

[pahud]

Thank you for the bug report. I'm making it p1 though small effort. Any PR submission would be welcome and appreciated.

[nakedible-p]

Added a quick PR, most likely still needs work to be complete.

[github-actions]

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