core: fix missed register_callsite error

There are 2 triggers which will cause a subscriber to receive a call to
`Subscriber::register_callsite` for a specific callsite.
1. The first time the event or span at that callsite is executed.
2. When a new subscriber is added or removed (for example, calls to
   `set_default` or `with_default`)

It is trigger (2) that will cause a new subscriber to receive
`Subscriber::register_callsite` for all the callsites which had already
been registered before it became active.

When a callsite is registered for trigger (1), the callsite starts in
state `UNREGISTERED`.

The first thread to encounter the callsite will transition it to
`REGISTERING` and determine the overall interest for the callsite by
registering with all known dispatchers (which will call into
`Subscriber::register_callsite`).

Once that is complete, the callsite is added to the list of all known
callsites and its state is transitioned to `REGISTERED`.

is (re)built for all known dispatchers. The callsite starts in state
`UNREGISTERED`. The  This calls down into
`Subscriber::register_callsite` for each subscriber. Once that is
complete, the callsite is added to the global list of known callsites.

While the callsite interest is being rebuilt, other threads that
encounter the callsite will be given `Interest::sometimes()` until the
registration is complete. However, if a new subscriber is added during
this window, all the interest for all callsites will be rebuilt, but
because the new callsite (in state `REGISTERING`) won't be included
because it isn't yet in the global list of callsites.

This can cause a case where that new subscriber being added won't
receive `Subscriber::register_callsite` before it receives the subsequent
call to `Subscriber::event` or `Subscriber::new_span`.

The documentation on [Registering Callsites] is not very explicit on
this point, but it does suggest that `Subscriber::register_callsite`
will be called before the call to either `Subscriber::event` or
`Subscriber::new_span`, and the current behavior can break this implicit
contract.

[Registering Callsites]: https://docs.rs/tracing-core/0.1.32/tracing_core/callsite/index.html#registering-callsites

This change swaps the order of rebuilding the callsite interest and
adding the callsite to the global list so that the callsite gets pushed
first, avoiding this window in which a subscriber won't get a call to
`register_callsite`.

As such, a callsite may have its interest read before it is set. In this
case, the existing implementation will return `Interest::sometimes()`
for the `DefaultCallsite` implementation. Other implementations (outside
of the `tracing` project) may perform this differently, but in this
case, there is no documented guarantee regarding the ordering
This is the same

tracing-core/src/callsite.rs

@@ -235,8 +235,6 @@ pub fn rebuild_interest_cache() {
 /// [`Callsite`]: crate::callsite::Callsite
 /// [reg-docs]: crate::callsite#registering-callsites
 pub fn register(callsite: &'static dyn Callsite) {
-    rebuild_callsite_interest(callsite, &DISPATCHERS.rebuilder());
-
     // Is this a `DefaultCallsite`? If so, use the fancy linked list!
     if callsite.private_type_id(private::Private(())).0 == TypeId::of::<DefaultCallsite>() {
         let callsite = unsafe {
@@ -248,10 +246,11 @@ pub fn register(callsite: &'static dyn Callsite) {
             &*(callsite as *const dyn Callsite as *const DefaultCallsite)
         };
         CALLSITES.push_default(callsite);
-        return;
+    } else {
+        CALLSITES.push_dyn(callsite);
     }
 
-    CALLSITES.push_dyn(callsite);
+    rebuild_callsite_interest(callsite, &DISPATCHERS.rebuilder());
 }
 
 static CALLSITES: Callsites = Callsites {
@@ -317,8 +316,8 @@ impl DefaultCallsite {
         ) {
             Ok(_) => {
                 // Okay, we advanced the state, try to register the callsite.
-                rebuild_callsite_interest(self, &DISPATCHERS.rebuilder());
                 CALLSITES.push_default(self);
+                rebuild_callsite_interest(self, &DISPATCHERS.rebuilder());
                 self.registration.store(Self::REGISTERED, Ordering::Release);
             }
             // Great, the callsite is already registered! Just load its

tracing/tests/missed_register_callsite.rs

@@ -0,0 +1,109 @@
+use std::{
+    ptr,
+    sync::atomic::{AtomicPtr, Ordering},
+    thread::{self, JoinHandle},
+    time::Duration,
+};
+
+use tracing::Subscriber;
+use tracing_core::{span, Metadata};
+
+struct TestSubscriber {
+    creator_thread: String,
+    sleep: Duration,
+    callsite: AtomicPtr<Metadata<'static>>,
+}
+
+impl TestSubscriber {
+    fn new(sleep_micros: u64) -> Self {
+        let creator_thread = thread::current()
+            .name()
+            .unwrap_or("<unknown thread>")
+            .to_owned();
+        Self {
+            creator_thread,
+            sleep: Duration::from_micros(sleep_micros),
+            callsite: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
+}
+
+impl Subscriber for TestSubscriber {
+    fn register_callsite(&self, metadata: &'static Metadata<'static>) -> tracing_core::Interest {
+        if !self.sleep.is_zero() {
+            thread::sleep(self.sleep);
+        }
+
+        self.callsite
+            .store(metadata as *const _ as *mut _, Ordering::SeqCst);
+        println!(
+            "{creator} from {thread:?}: register_callsite: {callsite:#?}",
+            creator = self.creator_thread,
+            callsite = metadata as *const _,
+            thread = thread::current().name(),
+        );
+        tracing_core::Interest::always()
+    }
+
+    fn event(&self, event: &tracing_core::Event<'_>) {
+        let stored_callsite = self.callsite.load(Ordering::SeqCst);
+        let event_callsite: *mut Metadata<'static> = event.metadata() as *const _ as *mut _;
+
+        println!(
+            "{creator} from {thread:?}: event (with callsite): {event_callsite:#?} (stored callsite: {stored_callsite:#?})",
+            creator = self.creator_thread,
+            thread = thread::current().name(),
+        );
+
+        // This assert is the actual test.
+        assert_eq!(
+            stored_callsite, event_callsite,
+            "stored callsite: {stored_callsite:#?} does not match event \
+            callsite: {event_callsite:#?}. Was `event` called before \
+            `register_callsite`?"
+        );
+    }
+
+    fn enabled(&self, _metadata: &Metadata<'_>) -> bool {
+        true
+    }
+    fn new_span(&self, _span: &span::Attributes<'_>) -> span::Id {
+        span::Id::from_u64(0)
+    }
+    fn record(&self, _span: &span::Id, _values: &span::Record<'_>) {}
+    fn record_follows_from(&self, _span: &span::Id, _follows: &span::Id) {}
+    fn enter(&self, _span: &tracing_core::span::Id) {}
+    fn exit(&self, _span: &tracing_core::span::Id) {}
+}
+
+fn subscriber_thread(idx: usize, register_sleep_micros: u64) -> JoinHandle<()> {
+    thread::Builder::new()
+        .name(format!("subscriber-{idx}"))
+        .spawn(move || {
+            // We use a sleep to ensure the starting order of the 2 threads.
+            let subscriber = TestSubscriber::new(register_sleep_micros);
+            let _subscriber_guard = tracing::subscriber::set_default(subscriber);
+
+            tracing::info!("event-from-{idx}", idx = idx);
+
+            // Wait a bit for everything to end (we don't want to remove the subscriber
+            // immediately because that will mix up the test).
+            thread::sleep(Duration::from_millis(100));
+        })
+        .expect("failed to spawn thread")
+}
+
+#[test]
+fn event_before_register() {
+    let subscriber_1_register_sleep_micros = 100;
+    let subscriber_2_register_sleep_micros = 0;
+
+    let jh1 = subscriber_thread(1, subscriber_1_register_sleep_micros);
+
+    // This delay ensures that the event!() in the first thread is executed first.
+    thread::sleep(Duration::from_micros(50));
+    let jh2 = subscriber_thread(2, subscriber_2_register_sleep_micros);
+
+    jh1.join().expect("failed to join thread");
+    jh2.join().expect("failed to join thread");
+}