dagger/dagger · error

failed to create temp file

Error message

failed to create temp file

What it means

When the local mounter wraps a single file (isFile), it writes an empty placeholder file inside the freshly created temp dir so the file can be bind-mounted; failure to create that file aborts Mount and cleans up the temp dir. The cause is the wrapped os.WriteFile error.

Source

Thrown at engine/snapshots/localmounter_linux.go:70

		}
		if !fi.IsDir() {
			isFile = true
		}
	}

	lm.overlayIncompatDirs = fsdiff.VolatileIncompatDirs(lm.mounts)

	dest, err := os.MkdirTemp("", "dagger-mount")
	if err != nil {
		return "", errors.Wrap(err, "failed to create temp dir")
	}
	lm.tmpDir = dest

	if isFile {
		dest = filepath.Join(dest, "file")
		if err := os.WriteFile(dest, []byte{}, 0o644); err != nil {
			os.RemoveAll(lm.tmpDir)
			return "", errors.Wrap(err, "failed to create temp file")
		}
	}

	if err := mount.All(lm.mounts, dest); err != nil {
		os.RemoveAll(lm.tmpDir)
		return "", errors.Wrapf(err, "failed to mount %s: %+v", dest, lm.mounts)
	}
	lm.target = dest
	return dest, nil
}

func (lm *localMounter) Unmount() error {
	lm.mu.Lock()
	defer lm.mu.Unlock()

	if lm.target != "" {
		if err := mount.Unmount(lm.target, 0); err != nil {
			return err

View on GitHub (pinned to 82ba2681db)

Solutions

1. Free space on the filesystem backing TMPDIR
2. Ensure TMPDIR is writable and not read-only mounted
3. Check LSM/permission policies for the process user
4. Inspect the wrapped os.WriteFile error for the exact errno

Example fix

// before
TMPDIR=/readonly-tmp dagger ...
// after
export TMPDIR=$(mktemp -d)  # writable location
dagger ...

probe, err := os.CreateTemp(os.TempDir(), "dagger-probe")
if err != nil {
    return fmt.Errorf("cannot write to temp dir: %w", err)
}
probe.Close(); os.Remove(probe.Name())

dir, err := lm.Mount()
if err != nil && strings.Contains(err.Error(), "failed to create temp file") {
    return fmt.Errorf("temp filesystem not writable or full: %w", err)
}

When it happens

Trigger: Mounting a single host file where the temp dir's filesystem rejects the write: ENOSPC, read-only tmpfs, or permission/ACL restrictions on the dagger-mount temp directory.

Common situations: Disk-full /tmp; containers with read-only rootfs; security policies (SELinux) denying file creation in /tmp.

Related errors

• remount container: %w
• failed to create stable ID temp file: %w
• create checkout pack spool: %w
• finish checkout pack spool: %w
• create uncommitted pack spool: %w

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/1f8b520e5413ae2a. Report an issue: GitHub.