resource/aws_elb_attachment: Retry ELB attachment on InvalidTarget error

[jbarrick-mesosphere]

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Fixes #7561

Changes proposed in this pull request:

• Retry aws_elb_attachment create when AWS returns InvalidTarget because the instance is not running yet.

[nywilken]

hi @jbarrick-mesosphere thanks for handling this issue, and for your contribution. The approach looks good. I left a couple of suggestions around the practices used for timeouts and AWS error checking to help get this merged.

[nywilken]

There is a pattern we use throughout the code base to help when checking against AWS errors. The non-nested error checks help readability and the helper isAWSErr wraps all the type assertion logic.

err := resource.Retry(10*time.Minute, func() *resource.RetryError {                                                                                                                         
    _, err := elbconn.RegisterInstancesWithLoadBalancer(&registerInstancesOpts)                                                                                                               
                                                                                                                                                                                              
    if isAWSErr(err, "InvalidTarget", "") {                                                                                                                                                   
      return resource.RetryableError(fmt.Errorf("Error attaching instance to ELB, retrying: %s", err))                                                                                        
    }                                                                                                                                                                                         
                                                                                                                                                                                              
    if err != nil {                                                                                                                                                                           
      return resource.NonRetryableError(err)                                                                                                                                                  
    }                                                                                                                                                                                         
                                                                                                                                                                                              
    return nil                                                                                                                                                                                
})   

[jbarrick-mesosphere]

Thanks for review @nywilken ! Addressed your comments.

[nywilken]

LGTM 👍 👍

[ghost]

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