When setting a head_series_limit to unlimited (0) for a given tenant, it fails to treat this as unlimited.

The following example is currently not possible:

write:
  global:
    max_concurrency: 30
    meta_monitoring_url: "http://localhost:9090"
    meta_monitoring_limit_query: "sum(prometheus_tsdb_head_series) by (tenant)"
  default:
    request:
      size_bytes_limit: 1024
      series_limit: 1000
      samples_limit: 10
    head_series_limit: 1000
  tenants:
    acme:
      request:
        size_bytes_limit: 0
        series_limit: 0
        samples_limit: 0
      head_series_limit: 0

It will result in for example:

"receive-limiter","currentSeries":46496,"level":"error","limit":0,"msg":"tenant above limit",

Looking at the function here

It only checks if the limitsPerTenant length is 0 and if the default limit is 0 (unlimited).

if len(h.limitsPerTenant) == 0 && h.defaultLimit == 0 {
 return true, nil
}

However if the tenant config is not empty, len > 0, we continue to evaluate the value agains the current value from the metamonitoring, which if 0 will always fail.

It should also check if the configured limit for the tenant is 0.

// If tenant limit is 0 we treat it as unlimited.
if limit == 0 {
  return true, nil
}

This helps where you would want to have lower set default limits to prevent abuse, but allow a specific tenant to have an unlimited value.