Function channel_progress_frontiers_by_source 

pub fn channel_progress_frontiers_by_source(
    channels: &[ChannelProgress],
) -> Result<BTreeMap<&str, Option<i64>>, String>

Numeric decision frontier retained for each source in an exact channel cut.

Active channels contribute their minimum. An active uninitialized channel withholds only its own source, while an all-idle source retains the greatest initialized channel watermark. This is the source-keyed counterpart of channel_progress_frontier; callers must not use the pipeline-wide minimum when advancing a source-specific ordered operator.

Errors

Returns an error when a channel uses the reserved uninitialized watermark value.