Function report_varlink_failure 

pub fn report_varlink_failure(
    no_fallback: bool,
    collector: &str,
    fallback: &str,
    source: Error,
) -> Result<(), NoFallbackError>

Record a varlink failure and decide whether the caller may fall back.

Returns Ok(()) (after logging the falling back warning) by default, so the caller proceeds to its fallback. Returns Err when no_fallback is true, turning the failure into the collector’s error — which stat_collector logs per-collector and counts in collector_timings success, so a no-fallback CI run fails visibly rather than earning a quiet varlink_usage.* = 0 gauge.

source is taken as anyhow::Error because every varlink call site already produces one (sockets, protocol and version-gate failures all funnel through anyhow).