What causes "empty image layer data source" in slimtoolkit/slim?

Set LayerDataInfo.Source to a valid absolute path for every layer entry

empty image layer data source

Error message

empty image layer data source

What it means

Every layer entry must point at a data source path on disk. Before creating any layer, Build checks that LayerDataInfo.Source is non-empty; an empty Source string means there is no tar file or directory to turn into an image layer, so the build aborts with this error.

Source

Thrown at pkg/imagebuilder/internalbuilder/engine.go:157

	if !options.ImageConfig.Created.IsZero() {
		imgConfig.Created = v1.Time{Time: options.ImageConfig.Created}
	}

	log.Debug("DefaultSimpleBuilder.Build: config image")

	img, err := mutate.ConfigFile(img, imgConfig)
	if err != nil {
		return nil, err
	}

	var layersToAdd []v1.Layer

	for i, layerInfo := range options.Layers {
		log.Debugf("DefaultSimpleBuilder.Build: [%d] create image layer (type=%v source=%s)",
			i, layerInfo.Type, layerInfo.Source)

		if layerInfo.Source == "" {
			return nil, fmt.Errorf("empty image layer data source")
		}

		if !fsutil.Exists(layerInfo.Source) {
			return nil, fmt.Errorf("image layer data source path doesnt exist - %s", layerInfo.Source)
		}

		switch layerInfo.Type {
		case imagebuilder.TarSource:
			if !fsutil.IsRegularFile(layerInfo.Source) {
				return nil, fmt.Errorf("image layer data source path is not a file - %s", layerInfo.Source)
			}

			if !fsutil.IsTarFile(layerInfo.Source) {
				return nil, fmt.Errorf("image layer data source path is not a tar file - %s", layerInfo.Source)
			}

			layer, err := layerFromTar(layerInfo)
			if err != nil {

View on GitHub (pinned to 81940d17fa)

Solutions

Set LayerDataInfo.Source to a valid absolute path for every layer entry
Validate all layer entries before calling Build (skip or error on empty sources)
Check the code that produces the layer list for empty path variables
Log/dump the Layers slice when building to catch empty entries early

Example fix

// before
layers = append(layers, imagebuilder.LayerDataInfo{Type: imagebuilder.DirSource}) // Source missing
// after
layers = append(layers, imagebuilder.LayerDataInfo{Type: imagebuilder.DirSource, Source: "/path/to/rootfs-dir"})

Defensive patterns

Strategy: validation

Validate before calling

for i, l := range opts.Layers {
    if l.Source == "" {
        return fmt.Errorf("layer %d has empty Source", i)
    }
}

Prevention

Validate every layer entry before calling Build
Use named constructor helpers that require a Source argument
Check dynamic path variables for empty values before building

When it happens

Trigger: Appending imagebuilder.LayerDataInfo to SimpleBuildOptions.Layers without setting the Source field (Type may be set, but Source is ""), then calling Engine.Build.

Common situations: Struct-literal construction where the Source field name was misspelled or simply omitted; dynamically populated layer lists where a path variable was empty due to a failed lookup; deserialized config with missing source keys.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/0459d45af2c9bcb5. Report an issue: GitHub.