feat(template): support repeatable --fqdn-template, --target-template, --fqdn-target-template flags

[ivankatliarchuk]

What does it do ?

• FQDNTemplate, TargetTemplate, and FQDNTargetTemplate on Config changed from string to []string; CLI flags switched from StringVar to StringsVar so each can be specified multiple times
• fqdn-templating.md updated to show repeated-flag syntax as the canonical multi-template form, with FAQ and tips refreshed
• unstructured.md configuration table marks all three flags as repeatable; EndpointSlice example comment fixed and the double-flag pattern explained as intentional and now correct

Motivation

• --fqdn-template, --target-template, and --fqdn-target-template each accepted only a single string; multiple templates had to be crammed into one value as comma-separated text
• The unstructured source docs already showed --fqdn-target-template specified twice (EndpointSlice example), but with StringVar only the last value was used - the first was silently discarded

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

[coveralls]

Coverage Report for CI Build 25665899121

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Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage decreased (-0.01%) to 80.572%

Details

• Coverage decreased (-0.01%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 49 coverage regressions across 4 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

49 previously-covered lines in 4 files lost coverage.

File	Lines Losing Coverage	Coverage
istio_virtualservice.go	26	87.83%
istio_gateway.go	12	90.3%
template/engine.go	8	92.79%
pod.go	3	97.19%

---

Coverage Stats

Relevant Lines:	21397
Covered Lines:	17240
Line Coverage:	80.57%
Coverage Strength:	1451.99 hits per line

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💛 - Coveralls

[vflaux]

One caveat concerns the use of variables or define blocks in templates. Because flags are concatenated into a single, unified template, these variables are shared across all flags. This can lead to unexpected behavior for users.
This should at least be clearly documented.

[ivankatliarchuk]

Could you clarify those unexpected behaviours? I might missing them

[vflaux]

Variable should actually not cause issues as you can redefine them, but define block will :

• {{ define "foo" }}bar{{ end }}{{ template "foo" }}
• {{ define "foo" }}foobar{{ end }}{{ template "foo" }}

Should results in multiple definition of template "foo".
Also any invalid template will make other templates invalid too.

[ivankatliarchuk]

Variable should actually not cause issues as you can redefine them, but define block will :

* `{{ define "foo" }}bar{{ end }}{{ template "foo" }}`

* `{{ define "foo" }}foobar{{ end }}{{ template "foo" }}`

Should results in multiple definition of template "foo". Also any invalid template will make other templates invalid too.

I'm missing the point here, why we care about edge cases, there are way too many things that could go wrong, user input is responsibility of the user, unless we could automate checks? How supporting multiple templates flags relates to all of this? Currently user could define multiple templates in single line, this is just to allow to define them in multiple flags. Template engine is supported for a while, we are using golang template engine. If the user define incorrect template, or missplace variable (could not be compiled or whatever) high chances the external-dns template engine will catch that and it will fail fast

external-dns/source/store.go

Line 131 in 26b72c1

tmpls, err := template.NewEngine(cfg.FQDNTemplate, cfg.TargetTemplate, cfg.FQDNTargetTemplate, cfg.CombineFQDNAndAnnotation)

.

There are two states resulting template either parsed successfully or not.

Could you specify what need to be added and where, I could copy paste it then.

[ivankatliarchuk]

I've removed sort for flags, to make order clear and added logging for resulting templates. The right location for logging is execute.go but it is hard to test it there

[ivankatliarchuk]

I've added validation and debug logging to make things clear in case of config misconfiguration.

[u-kai]

/lgtm

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[u-kai]

Since logs are now emitted, I can't claim there are zero edge cases, but the pre-fix behavior was already broken (specifying --fqdn-target-template multiple times silently discarded all but the last value). IMO, given that, this fix is the right call.