RDSInstance cannot be deleted about provider-aws

[hasheddan]

Ref TODO: https://github.com/crossplane/stack-aws/blob/12b4b63972249afa4e098311e48ff3c9440360bd/pkg/controller/database/rdsinstance.go#L228

[muvaf]

I think my TODO was more related to having deletionProtection: true because in that case even if the user changes it to false, it won't matter since Update is never called when the deletion timestamp exists. See crossplane/crossplane-runtime#59

[hasheddan]

This was solved by updating the config as follows:

apiVersion: database.aws.crossplane.io/v1beta1
kind: RDSInstance
metadata:
  name: rdsmysql
  labels:
    example: "true"
spec:
  forProvider:
    dbInstanceClass: db.t2.small
    masterUsername: masteruser
    allocatedStorage: 20
    engine: mysql
    applyModificationsImmediately: true # this was added
    skipFinalSnapshotBeforeDeletion: true # this was added
  writeConnectionSecretsToNamespace: crossplane-system
  providerRef:
    name: aws-provider
  reclaimPolicy: Delete

This works because:

1. applyModificationsImmediately: true makes it such that RDS will not wait for the next maintenance window to apply changes. This was the source of the error described above.
2. When applying changes immediately, we must specify skipFinalSnapshotBeforeDeletion. If we do not, you will observe the following error from AWS:

- lastTransitionTime: "2020-02-25T17:44:02Z"     
    message: "delete failed: cannot delete RDS instance: InvalidParameterCombination:
      FinalDBSnapshotIdentifier is required unless SkipFinalSnapshot is specified.\n\tstatus
      code: 400, request id: 589d07eb-73e3-443b-9229-6700d363b5cc"
    reason: Encountered an error during resource reconciliation         
    status: "False"                   
    type: Synced