Properly handle grace across multiple tiers

multi_tier_grace.vcl

vcl 4.0;

# Include this VCL on all tiers

# Call this once on the Edge tier
sub dvcl_init_edge
{
  unset req.http.X-DVCL-skip-grace;
}

sub vcl_recv
{
  unset req.http.X-DVCL-graced;
}

sub vcl_hit
{
  if (obj.ttl < 0s) {
    set req.http.X-DVCL-graced = "true";
    if (req.http.X-DVCL-skip-grace == "true") {
      return (fetch);
    }
  }
  
  # Fall thru to vcl_hit and possibly hit conditional grace/miss logic
}

sub vcl_backend_fetch
{
  # If we come here, we are either a bgfetch or a condition grace/miss
  # In either case, we want something fresh, so disable grace on all following tiers
  if (bereq.http.X-DVCL-graced == "true") {
    set bereq.http.X-DVCL-skip-grace = "true";
  }
}