What causes "image layer data source path is not a file - %s" in slimtoolkit/slim?

Set Type to imagebuilder.DirSource when Source is a directory

image layer data source path is not a file - %s

Error message

image layer data source path is not a file - %s

What it means

For layers whose Type is imagebuilder.TarSource, Build requires the Source path to be a regular file (not a directory, symlink target, socket, etc.). fsutil.IsRegularFile fails for directories or special files, producing this error that names the path.

Source

Thrown at pkg/imagebuilder/internalbuilder/engine.go:167

	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 {
				return nil, err
			}

			layersToAdd = append(layersToAdd, layer)
		case imagebuilder.DirSource:
			if !fsutil.IsDir(layerInfo.Source) {
				return nil, fmt.Errorf("image layer data source path is not a directory - %s", layerInfo.Source)
			}

			layer, err := layerFromDir(layerInfo)

View on GitHub (pinned to 81940d17fa)

Solutions

Set Type to imagebuilder.DirSource when Source is a directory
Point TarSource at a regular .tar file, creating one first if needed
Check the path with os.Stat/Mode().IsRegular() before building
Fix symlink handling: resolve the path to what it actually points to

Example fix

// before
layer := imagebuilder.LayerDataInfo{Type: imagebuilder.TarSource, Source: "/opt/rootfs"} // a directory
// after
layer := imagebuilder.LayerDataInfo{Type: imagebuilder.DirSource, Source: "/opt/rootfs"}

Defensive patterns

Strategy: validation

Validate before calling

for i, l := range opts.Layers {
    if l.Type == imagebuilder.TarSource {
        info, err := os.Stat(l.Source)
        if err != nil || !info.Mode().IsRegular() {
            return fmt.Errorf("layer %d tar source is not a regular file: %s", i, l.Source)
        }
    }
}

Prevention

Match Type to what the path actually is (file vs directory)
Resolve symlinks before classifying layer sources
Choose DirSource for directories instead of forcing TarSource

When it happens

Trigger: Setting LayerDataInfo.Type to TarSource while Source points to a directory (or a non-regular file) and calling Engine.Build.

Common situations: Mixing up TarSource and DirSource: the artifact is a directory but the type was left as TarSource; paths pointing at device files or sockets; symlinked paths resolving to directories.

Related errors

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/5d1e85740548fb1f. Report an issue: GitHub.