Fix updates of unchanged records for Azure Private DNS

[jasper-d]

Due to the missing / the record type of existing records had / as a prefix (i.e. /A). Condition in

external-dns/plan/plan.go

Lines 236 to 238 in f400ded

	switch record.RecordType {
	case endpoint.RecordTypeA, endpoint.RecordTypeCNAME:
	filtered = append(filtered, record)

was therefore never true.

Fixes #1373

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[jasper-d]

I signed it

[jasper-d]

/assign @hjacobs

[saidst]

Hi Jasper,
thanks for your investigations. I also see this behaviour with the latest version.

I verify with a late 2019-version of the master-branch.

Regards

[saidst]

I think we have this behaviour since bf4b176

maybee because of bf4b176#diff-d47c04ae82fc597ab54c2b7998616f1fL109-L110:

-			recordType = strings.TrimLeft(*recordSet.Type, "Microsoft.Network/privateDnsZones")
+			recordType = strings.TrimPrefix(*recordSet.Type, "Microsoft.Network/privateDnsZones")

[jasper-d]

@saidst Looks like it. I wasn't sure why it worked before, had to look up what TrimLeft actually does :/

[saidst]

I will also investigate.
We need to figure whether we can apply your fix or whether to revert to TrimLeft.

[jasper-d]

@saidst Do you think that the Azure API could return something other than "Microsoft.Network/privateDnsZones/<record-type>" as the type property? The API docs aren't really specific, they just define type as "The type of the record set." but I would be suprised if it was anything else.

I think the only other information that could be used to determine the record type is the actual property used for the targets (i.e. properties.aRecords, properties.txtRecords etc.).

[saidst]

I'll debug the concerned piece of code hopefully this afternoon.
After that, I should be able to answer your question.

In any case, I want to complete a fix this week. No matter of which type it is.

[saidst]

Your changes look good. Thanks!

No, Azure's API is OK.

The reason is that TrimLeft cannot simply be substituted by TrimPrefix.
For this case, TrimPrefix would have been more appropriate. But TrimLeft worked with the original pattern too.

@linki: This PR shows room for improvement in the unit-tests of this provider but also that late large-scale "code improvements" without practical checks are risky (bf4b176).

[jasper-d]

The reason is that TrimLeft cannot simply be substituted by TrimPrefix.
For this case, TrimPrefix would have been more appropriate. But TrimLeft worked with the original pattern too.

Agree:

I wasn't sure why it worked before, had to look up what TrimLeft actually does :/

Just my 2 cents: How about adding some debug level logging to plan.go? Currently it is kinda a black box. It most certainly works as expected but we don't know the inputs (from logs) and therefore can't tell why it actually behaves the way it does.

[saidst]

@njuettner, thanks for releasing 0.6.0!

This one is really important. Is it feasible to bundle this in a 0.6.1?

[njuettner]

/lgtm
/approve

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