slimtoolkit/slim · error

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

Error message

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

What it means

For TarSource layers, after confirming the path is a regular file, Build verifies the file is a valid tar archive using fsutil.IsTarFile. A regular file that is not a tar (e.g. plain binary, gzip-only stream mislabeled, truncated tar) triggers this error.

Source

Thrown at pkg/imagebuilder/internalbuilder/engine.go:171

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

View on GitHub (pinned to 81940d17fa)

Solutions

1. Supply an uncompressed, valid tar archive (tar -cf layer.tar dir/)
2. If the data is .tar.gz, decompress it to a plain .tar first or use DirSource on the unpacked directory
3. Verify with tar -tf layer.tar before building
4. Regenerate a corrupted tar from the original files

Example fix

// before
layer := imagebuilder.LayerDataInfo{Type: imagebuilder.TarSource, Source: "/tmp/app.tar.gz"}
// after
// gunzip -k /tmp/app.tar.gz  -> /tmp/app.tar
layer := imagebuilder.LayerDataInfo{Type: imagebuilder.TarSource, Source: "/tmp/app.tar"}

for i, l := range opts.Layers {
    if l.Type == imagebuilder.TarSource {
        f, err := os.Open(l.Source)
        if err != nil { return err }
        tr := tar.NewReader(f)
        if _, err := tr.Next(); err != nil {
            f.Close()
            return fmt.Errorf("layer %d is not a valid tar: %s", i, l.Source)
        }
        f.Close()
    }
}

When it happens

Trigger: Passing a compressed archive (.tar.gz) or arbitrary file as a TarSource layer; a corrupted/truncated tar file produced by a failed packaging step.

Common situations: Pointing Source at app.tar.gz or a binary instead of an uncompressed tar; packaging script crashed mid-write leaving a partial tar; confusing docker-save output with layer tars.

Related errors

• too many layers
• empty image layer data source
• unknown image data source - %v
• no layers
• bad architecture value

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/5e55da454f837c8d. Report an issue: GitHub.