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Preserve string form if the original value can be fully represented #133238

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Preserve string form if the original value can be fully represented #133238

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@tamalsaha tamalsaha commented Jul 26, 2025 •
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What type of PR is this?

/kind feature

What this PR does / why we need it:

resource.Quantity does not preserve the input string format in case of internal decimal presentation. For example,

func main() {
	q := resource.MustParse("1.1Gi")
	fmt.Println(q.String())
}

will print 1181116006400m. This pr will print the original input string "1.1Gi", as it can be fully represented internally.

One of the side effect of this change is that it will not automatically "canonicalize" the input string format. For example, ".001Ki" will not convert into "1024m" anymore. You can see the changes done in tests for more examples.

IMHO, this change is ok, because in handwritten YAMLs, "1.1Gi" are lot more common than ".001Ki".

Which issue(s) this PR is related to:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Preserve string form if the original resource.Quantity value can be fully represented

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: tamalsaha
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@k8s-ci-robot k8s-ci-robot requested review from saad-ali and thockin July 26, 2025 22:15
@k8s-ci-robot k8s-ci-robot added release-note Denotes a PR that will be considered when it comes time to generate release notes. and removed do-not-merge/release-note-label-needed Indicates that a PR should not merge because it's missing one of the release note labels. labels Jul 26, 2025
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ylink-lfs commented Jul 27, 2025 •
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I think it's a won't-fix feature. See the referenced link for detail:

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cc @liggitt

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liggitt commented Jul 29, 2025 •
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One of the side effect of this change is that it will not automatically "canonicalize" the input string format

that's actually sort of a problem, since ~all typed go clients and ~all historical servers will canonicalize it currently

changing mid-stream means version-skewed servers in the same cluster will persist different things into etcd for the same API request, which isn't great, and round-tripping non-canonical API responses through typed clients will canonicalize them on update... I'm not sure if that will trip up immutable-on-update field validations

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liggitt reviewed Jul 29, 2025
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staging/src/k8s.io/apimachinery/pkg/api/resource/quantity.go
@@ -365,6 +365,13 @@ func ParseQuantity(str string) (Quantity, error) {
amount.Neg(amount)
}

copyAmount := new(inf.Dec)
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@liggitt liggitt Jul 29, 2025 •
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this is secondary to the bigger question of whether we even can/should change the canonicalize-on-serialize behavior, but making a second object and doing a comparison seems like a roundabout (no pun intended) and possible expensive way of determining if we lost precision ... do we really not know if we lost precision in the parsing otherwise?

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do we really not know if we lost precision in the parsing otherwise?

I don't know if there is a better / more performant way. Looking at the signature of the .Round() methods, I don't see anything that says if the value was actually rounded or not.

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liggitt commented Jul 29, 2025

If Quantity were just now being introduced, preserving the input string as-is when it is fully representable would seem like a good idea to me. However, since the canonicalize-on-write behavior has existed in the wild for a decade now, both in clients and servers, I think removing that behavior is likely to cause problems. Thanks for the PR, but I don't think we should proceed with this change.

cc @deads2k

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tamalsaha commented Jul 29, 2025 •
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If Quantity were just now being introduced, preserving the input string as-is when it is fully representable would seem like a good idea to me.

The input string format is preserved in some scenarios where it can be fully represented by the integer part. See L 335, L339
https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/apimachinery/pkg/api/resource/quantity.go#L335-L339

It is not preserved in when the decimal representation is used as the value might have been truncated.

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liggitt commented Jul 30, 2025

Right, I mean that changing when canonicalization is applied is difficult to reason about the affect it will have interoperating with existing clients and servers that apply canonicalization differently.

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