DigitalOcean: support multiple targets per endpoint

[tdyas]

This PR teaches the DigialOcean provider how to support multiple targets per endpoint. (The DigitalOcean provider currently only supports one endpoint per target which other providers do support.)

The Problem

I am running nginx-ingress in DaemonSet mode with host networking enabled on my Kubernetes cluster. I want the IPs for every node running nginx-ingress to appear as A records in DNS. First, because the DigitalOcean provider only supports one endpoint per ingress currently, only one of the A records is created which means the additional nginx-ingress pods will not receive traffic.

Moreover, because external-dns believes that there should be multiple A records for the endpoint in question, the reconciliation loop never converges and external-dns continually sends update API calls every minute. This is not how external-dns seems intended to operate. (Indeed, with this PR, the reconciliation loop does converge and avoids sending an update every minute.)

Solution

If the DigitalOcean API were like the AWS or GCP APIs, then the API representation of a DNS record would have an attribute that is a list of RR datas, and supporting multiple targets per endpoint would be as simple as setting that attribute to the list of targets. The DigitalOcean API is not like that though because it represents multiple A records as multiple "domain record" instances.

This representation is actually quite similar to the Linode API. The Linode provider models updates by expanding an endpoint with multiple targets into multiple domain records to be sent to the Linode API. I have taken that approach with this PR: The DigitalOcean provider now expands an endpoint with multiple targets into multiple godo.DomainRecord instances.

Overview of the changes:

1. The provider's Records method uses the new mergeEndpointsByNameType function to combine multiple godo.DomainRecords for the same DNS name into a single endpoint.Endpoint with multiple targets. This maps the DigitalOcean representation to the external-dns data model

2. The provider's ApplyChanges method has been rewritten to compute a set of digitalOceanChangeCreate, digitalOceanChangeUpdate, and digitalOceanChangeDelete instances representing how to modify the godo.DomainRecord instances associated with a godo.Domain. Computation of each type of change has been put into the processCreateActions, processUpdateActions, and processDeleteActions functions.

3. Tests: Lots more testing of individual functions. I added go-cmp to the project so I could use its Equal and Diff functions. These handle structs with nested pointers to other structs much better than the testify package. I did that so I could actually compare the *godo.DomainRecordEditRequest struct contained in the change structs. (Added a TODO to try and see if testify would be willing to allow custom matching functions.)

4. Upgraded godo to the latest just to keep it current. go bumped the version of two other packages in the go.mod as well, not sure why it did that.

This is currently in production for my cluster. Seems to be working nicely. I did some basic tests against production by deleting and adding some records and external-dns figures out what changed and fixes it. And the reconciliation loop actually converges.

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[tdyas]

/assign @hjacobs
(the k8s-ci-robot said to assign)

[tdyas]

/assign @Raffo

[tdyas]

cc @andrewsomething: Just a heads up in case anyone at DigitalOcean has an interest in external-dns or Kubernetes stuff.

[Raffo]

Thank you for this PR @tdyas especially for the great explanation of what this is doing, I really enjoyed reading it.
I added a few minor comments, but overall the code looks okay.

[Raffo]

Should we have this done before merging this PR or do you want to keep it like this? What are the consequences?

[tdyas]

Not implementing this TODO would leave the status quo in place which is that we would just retrieve all of the records that Records retrieves, just as the existing version of the provider does. (--domain-filter would still be applied to limit the provider to only those zones where external-dns is managing records.)

My concern here was for avoiding having to retrieve all of the records if the zone had potentially many records. #1429 was on my mind as it flagged too many API calls as a potential hazard in such cases.

This TODO is a nice to have but should not block landing this PR.

[Raffo]

@andrewsomething should you be willing to maintain the DO provider, feel free to ping me here or on slack, we are trying to scale how we maintain providers and we would definitely be happy to have someone from DO maintain the DO provider 😄

[Raffo]

/approve
/lgtm

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This pull-request has been approved by: Raffo, tdyas

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