slimtoolkit/slim · error

unknown image data source - %v

Error message

unknown image data source - %v

What it means

The layer Type switch only accepts imagebuilder.TarSource and imagebuilder.DirSource; any other value hits the default branch and aborts with this error. Notably the message prints layerInfo.Source (not the Type), so the printed value may look odd — the real problem is an unrecognized Type constant.

Source

Thrown at pkg/imagebuilder/internalbuilder/engine.go:192

			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
	}

	if len(options.Tags) == 0 {
		return nil, fmt.Errorf("missing tags")
	}

	tag, err := name.NewTag(options.Tags[0])
	if err != nil {
		return nil, err
	}

View on GitHub (pinned to 81940d17fa)

Solutions

1. Set Type to imagebuilder.TarSource or imagebuilder.DirSource explicitly for every layer
2. Check for zero-value LayerDataInfo structs missing Type initialization
3. Always use the exported constants rather than literal strings
4. If the message value is a valid path, inspect the Type field — the error prints Source, not Type

Example fix

// before
layers = append(layers, imagebuilder.LayerDataInfo{Source: "/tmp/rootfs"}) // Type is zero value
// after
layers = append(layers, imagebuilder.LayerDataInfo{Type: imagebuilder.DirSource, Source: "/tmp/rootfs"})

for i, l := range opts.Layers {
    switch l.Type {
    case imagebuilder.TarSource, imagebuilder.DirSource:
    default:
        return fmt.Errorf("layer %d has unknown Type %q", i, l.Type)
    }
}

if _, err := engine.Build(opts); err != nil {
    if strings.Contains(err.Error(), "unknown image data source") {
        // inspect each layer's Type; note the error prints Source, not Type
    }
    return err
}

When it happens

Trigger: Setting LayerDataInfo.Type to an undefined/zero value, a type from a different package, or a custom string; constructing LayerDataInfo without initializing Type.

Common situations: Zero-value structs (Type "") where Source was set but Type forgotten; renaming or hand-writing type strings instead of using imagebuilder.TarSource/DirSource constants; version drift between producer and builder code.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• too many layers
• empty image layer data source
• image layer data source path is not a tar file - %s
• no layers
• bad architecture value

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/a809caf6e09cbc67. Report an issue: GitHub.