fix: do not merge CNAME with multiple targets

[dtuck9]

Description

While using the Google Cloud DNS provider, we encountered the following error:

{"level":"error","msg":"googleapi: Error 400: Invalid value for 'entity.change.additions[<CNAME DNS Name>][CNAME].rrdata[1]': '<RRData[1].Value>', invalid","time":"2024-11-05T22:21:52Z"}`

Where <CNAME DNS Name> as the DNS Name and <RRData[1].Value> is the targeted hostname without a trailing .. After several hours of troubleshooting this failure, we discovered that it was actually a collision with a pre-existing DNS Name, and that the targets had been merged into the same Endpoint, where only rrdata[0] had the trailing . ensured by external-dns.

We attempted the following on our end to get around this unexpected behavior in order to trigger the expected 409 error:

1. Ensure a trailing . in our hostname definitions -- this way the 2 rrdata values would at least pass the Cloud DNS API validation for the trailing ., and then fail with an error that makes more sense (CNAME not allowing more than one value in the rrdata). This however caused a reconciliation loop where external-dns kept deleting and re-adding the records in every batch.
2. Set the external-dns.alpha.kubernetes.io/set-identifier annotation to prevent the merge in the first place, which would trigger the 409 for the CNAME DNS Name already existing. Unfortunately this also seemed to cause a reconciliation loop where external-dns kept deleting and re-adding the records in every batch.

I considered making a change in the Cloud DNS provider to avoid impacting other providers where perhaps this behavior is expected in order to allow different routing policies that are more tightly coupled with the DNS record. However, by the time the endpoint gets into the provider, the context of the duplicate record's resource label has already been lost due to the merge.

We are certainly open to suggestions on how to get around this behavior, or if you'd rather us file an issue and close this PR, that's fine, too.

No relevant user documentation was found.

Checklist

• Unit tests updated
• End user documentation updated

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[mloiseleur]

/ok-to-test

[mloiseleur]

/retitle fix: do not merge CNAME with multiple targets

[mloiseleur]

/lgtm
/assign @Raffo