//! Traces, for ipx and the directory both (#213): the exporter, and W3C `traceparent` in from a
//! caller and out to a callee, so one trace follows a request from the browser through ipx-web
//! and ipx into the directory.

use axum::http::HeaderMap;

/// Traces over OTLP, to Tempo for one, when OTEL_EXPORTER_OTLP_ENDPOINT names a collector
/// (`http://host:4318`: the exporter speaks OTLP over HTTP and adds `/v1/traces`). The exporter
/// reads that and the other `OTEL_` variables itself.
pub fn provider(service: &'static str) -> anyhow::Result<Option<opentelemetry_sdk::trace::SdkTracerProvider>> {
    if std::env::var("OTEL_EXPORTER_OTLP_ENDPOINT").unwrap_or_default().is_empty() {
        return Ok(None);
    }
    let exporter = opentelemetry_otlp::SpanExporter::builder().with_http().build()?;
    let resource = opentelemetry_sdk::Resource::builder().with_service_name(service).build();
    Ok(Some(
        opentelemetry_sdk::trace::SdkTracerProvider::builder()
            .with_batch_exporter(exporter)
            .with_resource(resource)
            .build(),
    ))
}

/// The trace a request from the page belongs to, from its `traceparent` (#190), so the requests
/// one action sets off are one trace. None for a missing or malformed one. In the query where no
/// header can be set: a position saved by sendBeacon, and the playlist Safari's player fetches
/// itself (#192).
#[must_use]
pub fn remote_parent(headers: &HeaderMap, query: Option<&str>) -> Option<opentelemetry::Context> {
    use opentelemetry::trace::{SpanContext, SpanId, TraceContextExt, TraceFlags, TraceId, TraceState};
    let v = match headers.get("traceparent") {
        Some(h) => h.to_str().ok()?,
        None => query?.split('&').find_map(|kv| kv.strip_prefix("traceparent="))?,
    };
    let mut parts = v.split('-');
    let (version, trace, span) = (parts.next()?, parts.next()?, parts.next()?);
    if version != "00" || trace.len() != 32 || span.len() != 16 {
        return None;
    }
    let sc = SpanContext::new(
        TraceId::from_hex(trace).ok()?,
        SpanId::from_hex(span).ok()?,
        TraceFlags::SAMPLED,
        true,
        TraceState::default(),
    );
    sc.is_valid().then(|| opentelemetry::Context::new().with_remote_span_context(sc))
}

/// The current span as a `traceparent`, for a request to another service, so its spans join
/// this trace. None outside a traced span.
#[must_use]
pub fn traceparent() -> Option<String> {
    use opentelemetry::trace::TraceContextExt;
    use tracing_opentelemetry::OpenTelemetrySpanExt;
    let cx = tracing::Span::current().context();
    let sc = cx.span().span_context().clone();
    sc.is_valid().then(|| format!("00-{}-{}-{:02x}", sc.trace_id(), sc.span_id(), sc.trace_flags().to_u8()))
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn a_requests_trace_is_the_pages_from_its_traceparent() {
        use opentelemetry::trace::TraceContextExt;
        let with = |v: &str| {
            let mut h = axum::http::HeaderMap::new();
            h.insert("traceparent", v.parse().unwrap());
            remote_parent(&h, None)
        };
        let cx = with("00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01").expect("parsed");
        assert_eq!(cx.span().span_context().trace_id().to_string(), "4bf92f3577b34da6a3ce929d0e0e4736");
        assert!(cx.span().span_context().is_remote());
        // Anything else from the browser starts a trace of its own.
        for bad in [
            "",
            "00-xyz-00f067aa0ba902b7-01",
            "01-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01",
            "00-00000000000000000000000000000000-00f067aa0ba902b7-01",
            "00-4bf92f3577b34da6a3ce929d0e0e4736",
        ] {
            assert!(with(bad).is_none(), "{bad}");
        }
        assert!(remote_parent(&axum::http::HeaderMap::new(), None).is_none());
        // From the query, where a beacon or Safari's player cannot send a header (#192).
        let q = remote_parent(
            &axum::http::HeaderMap::new(),
            Some("x=1&traceparent=00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01"),
        );
        assert_eq!(q.unwrap().span().span_context().trace_id().to_string(), "4bf92f3577b34da6a3ce929d0e0e4736");
        assert!(remote_parent(&axum::http::HeaderMap::new(), Some("traceparent=nonsense")).is_none());
    }
}