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kyaml merge2: $patch directives on list elements ignored with ListPrepend #6146
Description
/kind bug
Bug
When using merge2.MergeStrings with MergeOptions{ListIncreaseDirection: MergeOptionsListPrepend}, strategic merge patch directives ($patch: replace, $patch: delete) on individual associative list elements are silently ignored.
With default MergeOptions{} (Append), $patch: replace correctly replaces the matched list element. With ListPrepend, the original element remains unchanged and the replacement is dropped.
Root cause
In kyaml/yaml/walk/associative_sequence.go, setAssociativeSequenceElements() assembles the final list differently depending on the merge direction:
- Append (correct):
appendListNode(dest, itemsToBeAdded, ...); merged results fromitemsToBeAddedoverwrite originals indest. - ListPrepend (buggy):
appendListNode(itemsToBeAdded, dest, ...); the arguments are swapped for ordering, but this causesElementSetterto overwrite already-merged items initemsToBeAddedwith the unmerged originals fromdest, undoing$patchdirective effects.
Reproduction
Minimal reproduction project: https://github.com/rm3l/kyaml-listprepend-bug
git clone https://github.com/rm3l/kyaml-listprepend-bug
cd kyaml-listprepend-bug
go test -v ./...TestPatchReplaceWithDefaultMerge: PASSTestPatchReplaceWithListPrepend: FAIL (the$patch: replaceis ignored)
Versions affected
Tested and confirmed on:
sigs.k8s.io/kustomize/kyamlv0.18.1sigs.k8s.io/kustomize/kyamlv0.19.0sigs.k8s.io/kustomize/kyamlv0.21.0sigs.k8s.io/kustomize/kyamlv0.21.1- Latest master commit (313aace, 2025-05-02)
Impact
This bug prevents combining ListPrepend (for controlling list item ordering) with $patch: replace or $patch: delete (for replacing/removing individual items by merge key). The two features are independent and should compose correctly IMHO.
Activity
k8s-ci-robot commentedon May 12, 2026
This issue is currently awaiting triage.
SIG CLI takes a lead on issue triage for this repo, but any Kubernetes member can accept issues by applying the
triage/acceptedlabel.The
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Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.
[-]kyaml merge2: $patch directives on list elements ignored with ListPrepend[/-][+]kyaml merge2: `$patch` directives on list elements ignored with `ListPrepend`[/+]rm3l commentedon May 12, 2026
/kind bug
jang-hs commentedon Sep 11, 2026
This is not limited to direct kyaml users:
api/filters/patchstrategicmergemerges withMergeOptionsListPrepend, so plainkustomize buildhits it too. With this base and patch:kustomize v5.8.x emits the original container untouched — the patch is silently a no-op.
#5255 looks like the same thing reported from the CLI side.
/assign
jang-hs commentedon Sep 11, 2026
Unassigning myself — @rm3l had #6147 open for this well before I picked it up, and I missed
it. #6265 stays open in case the maintainers find it useful, but this one is not mine to hold.
/unassign