test(server): fix unit tests that sometimes capture no log events

crates/switchyard-server/src/lib.rs

@@ -13,6 +13,8 @@ mod routing_log;
 mod shutdown;
 mod sse;
 mod stats;
+#[cfg(test)]
+mod testing;
 mod usage_metrics;
 
 use std::collections::BTreeMap;
@@ -1943,7 +1945,7 @@ mod tests {
 
         let captured = CapturedEvents::default();
         let subscriber = tracing_subscriber::registry().with(captured.clone());
-        tracing::subscriber::with_default(subscriber, run);
+        crate::testing::with_subscriber(subscriber, run);
         captured.0.lock().clone()
     }
 

crates/switchyard-server/src/observability.rs

@@ -185,7 +185,7 @@ mod tests {
             HeaderValue::from_static("vendor=opaque-value"),
         );
 
-        tracing::subscriber::with_default(subscriber, || {
+        crate::testing::with_subscriber(subscriber, || {
             let span = request_span(&headers);
             let context = span.context();
             let current = context.span();

crates/switchyard-server/src/sse.rs

@@ -231,7 +231,7 @@ mod tests {
             .enable_all()
             .build()?;
 
-        let body = tracing::subscriber::with_default(subscriber, || {
+        let body = crate::testing::with_subscriber(subscriber, || {
             runtime.block_on(chat_body(vec![Err(LlmStreamError::Client(
                 LlmClientError::UpstreamHttp {
                     status: axum::http::StatusCode::BAD_GATEWAY,

crates/switchyard-server/src/testing.rs

@@ -0,0 +1,31 @@
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+//! Helpers shared by this crate's unit tests.
+
+use std::sync::Once;
+
+/// Runs `run` with `subscriber` as this thread's default subscriber, so the test
+/// sees the events and spans that `run` creates.
+///
+/// For each call site, such as one `warn!` line, `tracing` caches whether any
+/// subscriber wants its events. While `subscriber` is the only registered
+/// subscriber, `tracing` fills that cache by asking the default subscriber of
+/// the first thread that reaches the call site. If that thread belongs to a
+/// parallel test with no subscriber, the cache says "never", and this test
+/// captures nothing from that call site. To prevent this, the first call to
+/// this helper in a test binary also sets a global `Registry` with no layers.
+/// That `Registry` wants every call site and writes no output, so the cache can
+/// no longer say "never".
+pub(crate) fn with_subscriber<T>(
+    subscriber: impl tracing::Subscriber + Send + Sync + 'static,
+    run: impl FnOnce() -> T,
+) -> T {
+    static GLOBAL_SUBSCRIBER: Once = Once::new();
+    GLOBAL_SUBSCRIBER.call_once(|| {
+        // An error means that another global subscriber is already set. That
+        // subscriber also keeps the cache from saying "never".
+        let _ = tracing::subscriber::set_global_default(tracing_subscriber::registry());
+    });
+    tracing::subscriber::with_default(subscriber, run)
+}