Cache lookahead refactoring

[mzihlmann]

Description

To enable cache lookahead we need to redefine some of the datastructures we use during build. The idea is to move transformations forwards and optimizations backwards, s.t. ultimately we can do cache elimination over all stages prior to build. Here we only do the refactoring, nothing should change on the functional level.

The core-idea is to make KanikoStage more potent. Currently it starts out as a dockerfile stage (parser result) and gradually gets modified to become more useful and finally gets converted into a stagebuilder struct. The problem I have with this that at different points in the build flow, the fields in kanikostage serve different functions. The most gross violation I think is that at some point the COPY --from=base get replaced with the numeric COPY --from=0. Which, don't get me wrong, I think is a good transformation, but I think it should happen directly when we convert to kanikoStage not at some point later, s.t. the programmer has to be aware which transformations were already done and which weren't.

Another transformation is that on-build commands get merged into the regular commands, this just happens at some point, let's all move it into the makeKanikoStages function, s.t. it is a clear cut transformation.

Optimizations on the other hand, which currently are in that function, should imo. happen outside and afterwards.

Submitter Checklist

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• Includes unit tests
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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- kaniko adds a new flag `--registry-repo` to override registry

[Copilot]

Pull Request Overview

This PR refactors the KanikoStage data structure to better separate concerns between parsing, transformation, and optimization phases. The key change is removing the embedded instructions.Stage from KanikoStage and replacing it with explicit fields (Name, BaseName, Commands). This makes the structure more explicit about what data it contains and when transformations (like resolving cross-stage references) are applied.

Key Changes:

• Refactored KanikoStage to contain explicit fields instead of embedding instructions.Stage
• Moved cross-stage command resolution into MakeKanikoStages function for clearer transformation boundaries
• Changed stage index mapping from map[string]string to map[string]int throughout the codebase

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/config/stage.go	Modified KanikoStage struct to use explicit fields instead of embedding instructions.Stage
pkg/dockerfile/dockerfile.go	Moved cross-stage resolution into MakeKanikoStages, changed stage mapping to use int instead of string, updated squash function
pkg/executor/build.go	Updated stageBuilder and related functions to work with new KanikoStage structure, changed stage index maps from string to int
pkg/executor/build_test.go	Updated test code to construct KanikoStage with explicit fields, fixed field references in tests
pkg/image/image_util_test.go	Updated test instantiation of KanikoStage to use explicit fields
pkg/dockerfile/dockerfile_test.go	Updated tests to use MakeKanikoStages instead of manual construction, fixed case sensitivity in test strings

Comments suppressed due to low confidence (1)

pkg/executor/build.go:1

• This line removes a call to ResolveCrossStageCommands that was previously executed in ResolveCrossStageInstructions. Since cross-stage resolution has been moved to MakeKanikoStages (called earlier in DoBuild), this removal is correct. However, the function ResolveCrossStageInstructions now only builds the name-to-index map without actually resolving commands, which contradicts its name and could confuse future maintainers.

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