slimtoolkit/slim · error

failed to finish tar: %w

Error message

failed to finish tar: %w

What it means

After the walk completes, tw.Close() flushes the tar footer; failure here is wrapped as "failed to finish tar". With an in-memory writer over a bytes.Buffer this is rare and usually signals an internal inconsistency (e.g. Close called after a prior write error left the writer in a bad state).

Source

Thrown at pkg/imagebuilder/internalbuilder/engine.go:327

		}
		if !info.IsDir() {
			f, err := os.Open(fp)
			if err != nil {
				return err
			}
			if _, err := io.Copy(tw, f); err != nil {
				return fmt.Errorf("failed to read file into the tar: %w", err)
			}
			f.Close()
		}
		return nil
	})

	if err != nil {
		return nil, fmt.Errorf("failed to scan files: %w", err)
	}
	if err := tw.Close(); err != nil {
		return nil, fmt.Errorf("failed to finish tar: %w", err)
	}

	return tarball.LayerFromReader(&b)
}

View on GitHub (pinned to 81940d17fa)

Solutions

1. Check the wrapped error (%w) — fix the underlying earlier failure first.
2. Ensure the machine has sufficient memory for the in-memory layer archive (the whole layer is buffered in RAM).
3. Re-run the build; if persistent, report with the unwrapped cause.

Example fix

// before
// layer of many GB tar-ed fully in memory -> buffer pressure
layerFromDir(LayerDataInfo{Source: "/huge/rootfs"})
// after
// use a disk-backed tar instead of bytes.Buffer for large layers
tw := tar.NewWriter(diskFile) // or stream via tarball.LayerFromOpener

info, _ := os.Stat(src)
// estimate layer size; prefer disk-backed archiving above ~1GB
if info != nil && dirSize(src) > 1<<30 { useDiskBackedTar = true }

if err != nil && strings.Contains(err.Error(), "failed to finish tar") {
    // retry once; if it persists, check memory and underlying write errors
}

When it happens

Trigger: tar.Writer.Close fails while finalizing the archive in layerFromDir — practically only when a previous write to the writer failed or the buffer write errored.

Common situations: Rarely hit directly; usually a downstream symptom of an earlier tar write problem, or OOM/allocations failing while the buffer grows.

Related errors

• failed to calculate relative path: %w
• not implemented archiving file type %s (%s)
• failed to write tar header: %w
• failed to read file into the tar: %w
• failed to scan files: %w

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/e0597c9f21279cd7. Report an issue: GitHub.