What causes "failed to read file into the tar: %w" in slimtoolkit/slim?

Re-run the build; ensure no concurrent process mutates the layer directory.

failed to read file into the tar: %w

Error message

failed to read file into the tar: %w

What it means

After writing the tar header for a regular file, io.Copy(tw, f) failed while streaming the file contents into the archive. This wraps read/write errors — the file could not be read from disk or the tar writer rejected the data mid-copy. The raw os.Open error is returned unwrapped separately, so this specifically covers copy-stage failures.

Source

Thrown at pkg/imagebuilder/internalbuilder/engine.go:316

		if info.Mode().IsDir() {
			hdr.Typeflag = tar.TypeDir
		} else if info.Mode().IsRegular() {
			hdr.Typeflag = tar.TypeReg
		} else {
			return fmt.Errorf("not implemented archiving file type %s (%s)", info.Mode(), rel)
		}

		if err := tw.WriteHeader(hdr); err != nil {
			return fmt.Errorf("failed to write tar header: %w", err)
		}
		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

Re-run the build; ensure no concurrent process mutates the layer directory.
Check disk health and free space (dmesg / filesystem errors).
If the layer is on a network mount, copy it to local disk before building.

Example fix

// before
// build reads /mnt/nfs/rootfs directly
layerFromDir(LayerDataInfo{Source: "/mnt/nfs/rootfs"})
// after
os.CopyFS("/tmp/rootfs-snapshot", os.DirFS("/mnt/nfs/rootfs"))
layerFromDir(LayerDataInfo{Source: "/tmp/rootfs-snapshot"})

Defensive patterns

Strategy: retry

Validate before calling

f, err := os.Open(fp)
if err != nil { return err }
st, err := f.Stat()
if err != nil || !st.Mode().IsRegular() { f.Close(); return fmt.Errorf("file changed during archive: %s", fp) }

Try / catch

if err != nil && strings.Contains(err.Error(), "failed to read file into the tar") {
    // retry the build after ensuring no concurrent writes to the layer dir
}

Prevention

Build from a stable snapshot of the layer directory
Keep layers on reliable local storage, not network mounts
Check disk health/free space if failures persist

When it happens

Trigger: A file that opened successfully becomes unreadable during copy — deleted/truncated concurrently, disk I/O error, or permission change between open and copy.

Common situations: Another process deleting or truncating files in the layer directory during the build, failing disk/IO errors, or files on flaky network mounts.

Understand the failure class

Background: "failed to read file", EACCES, ENOENT and "could not read <path>" errors: when a program can't read a file from disk — this error's family across 49 libraries.

Related errors

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/c07a7b3ec1c9da6d. Report an issue: GitHub.