A play's trace holds the playlist, audio and saved place it fetched (#192)

hls.js sends traceparent on its own requests (xhrSetup); a beacon, which
cannot set a header, and Safari's player, fetching the playlist itself,
carry it in the query, which the server reads where there is no header.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/web.rs

@@ -1329,7 +1329,7 @@ mod tests {
         let with = |v: &str| {
             let mut h = HeaderMap::new();
             h.insert("traceparent", v.parse().unwrap());
-            remote_parent(&h)
+            remote_parent(&h, None)
         };
         let cx = with("00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01").expect("parsed");
         assert_eq!(cx.span().span_context().trace_id().to_string(), "4bf92f3577b34da6a3ce929d0e0e4736");
@@ -1339,7 +1339,11 @@ mod tests {
                     "00-00000000000000000000000000000000-00f067aa0ba902b7-01", "00-4bf92f3577b34da6a3ce929d0e0e4736"] {
             assert!(with(bad).is_none(), "{bad}");
         }
-        assert!(remote_parent(&HeaderMap::new()).is_none());
+        assert!(remote_parent(&HeaderMap::new(), None).is_none());
+        // From the query, where a beacon or Safari's player cannot send a header (#192).
+        let q = remote_parent(&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(&HeaderMap::new(), Some("traceparent=nonsense")).is_none());
     }
 
     #[tokio::test]
@@ -2456,10 +2460,15 @@ async fn client_event(user: crate::db::User, Json(e): Json<ClientEvent>) -> Stat
 }
 
 /// 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.
-fn remote_parent(headers: &HeaderMap) -> Option<opentelemetry::Context> {
+/// 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).
+fn remote_parent(headers: &HeaderMap, query: Option<&str>) -> Option<opentelemetry::Context> {
     use opentelemetry::trace::{SpanContext, SpanId, TraceContextExt, TraceFlags, TraceId, TraceState};
-    let v = headers.get("traceparent")?.to_str().ok()?;
+    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 {
@@ -2514,7 +2523,7 @@ async fn access_log(req: Request, next: Next) -> Response {
             http.response.status_code = tracing::field::Empty,
         )
     };
-    if let Some(parent) = remote_parent(req.headers()) {
+    if let Some(parent) = remote_parent(req.headers(), req.uri().query()) {
         use tracing_opentelemetry::OpenTelemetrySpanExt;
         let _ = span.set_parent(parent);
     }