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

Set Type to imagebuilder.TarSource when Source is a file

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

Error message

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

What it means

For layers whose Type is imagebuilder.DirSource, Build requires Source to be a directory. If the path is a regular file or other non-directory entry, fsutil.IsDir fails and the build returns this error naming the path.

Source

Thrown at pkg/imagebuilder/internalbuilder/engine.go:182

		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)
			if err != nil {
				return nil, err
			}

			layersToAdd = append(layersToAdd, layer)
		default:
			return nil, fmt.Errorf("unknown image data source - %v", layerInfo.Source)
		}
	}

	log.Debug("DefaultSimpleBuilder.Build: adding layers to image")
	newImg, err := mutate.AppendLayers(img, layersToAdd...)
	if err != nil {
		return nil, err
	}

View on GitHub (pinned to 81940d17fa)

Solutions

Set Type to imagebuilder.TarSource when Source is a file
Point DirSource at the actual directory containing the layer contents
Create the directory if it was expected but missing
Validate with os.Stat and info.IsDir() before calling Build

Example fix

// before
layer := imagebuilder.LayerDataInfo{Type: imagebuilder.DirSource, Source: "/tmp/app.tar"}
// after
layer := imagebuilder.LayerDataInfo{Type: imagebuilder.TarSource, Source: "/tmp/app.tar"}
// or use the dir: imagebuilder.LayerDataInfo{Type: imagebuilder.DirSource, Source: "/tmp/app"}

Defensive patterns

Strategy: validation

Validate before calling

for i, l := range opts.Layers {
    if l.Type == imagebuilder.DirSource {
        info, err := os.Stat(l.Source)
        if err != nil || !info.IsDir() {
            return fmt.Errorf("layer %d dir source is not a directory: %s", i, l.Source)
        }
    }
}

Prevention

Use TarSource for files, DirSource for directories — keep them consistent
Create expected directories before building
Validate layer entries with os.Stat in a pre-build step

When it happens

Trigger: Setting LayerDataInfo.Type to DirSource while Source points to a regular file, e.g. pointing at app.tar instead of the unpacked directory.

Common situations: Swapped TarSource/DirSource types; the directory was replaced by a file (or never created and a file of the same name exists); config where the source root points at a single artifact file.

Related errors

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/a086b3fbc4c715d5. Report an issue: GitHub.