subscriber: pass through max_level_hint in reload (#2204)

## Motivation

When using a `reload` layer, the fast-path current level check doesn't
work, as the `max_level_hint` is just `None`, which `rebuild_interest`
interprets as `TRACE`

## Solution

Pass through to the underlying layer/filter. On poisons, when already
panicking, just return `None`

tracing-subscriber/src/reload.rs

@@ -30,7 +30,7 @@ use std::{
 use tracing_core::{
     callsite,
     collect::{Collect, Interest},
-    span, Event, Metadata,
+    span, Event, LevelFilter, Metadata,
 };
 
 /// Wraps a `Filter` or `Subscribe`, allowing it to be reloaded dynamically at runtime.
@@ -135,6 +135,11 @@ where
     fn on_id_change(&self, old: &span::Id, new: &span::Id, ctx: subscribe::Context<'_, C>) {
         try_lock!(self.inner.read()).on_id_change(old, new, ctx)
     }
+
+    #[inline]
+    fn max_level_hint(&self) -> Option<LevelFilter> {
+        try_lock!(self.inner.read(), else return None).max_level_hint()
+    }
 }
 
 #[cfg(all(feature = "registry", feature = "std"))]
@@ -188,6 +193,11 @@ where
     fn on_close(&self, id: span::Id, ctx: subscribe::Context<'_, C>) {
         try_lock!(self.inner.read()).on_close(id, ctx)
     }
+
+    #[inline]
+    fn max_level_hint(&self) -> Option<LevelFilter> {
+        try_lock!(self.inner.read(), else return None).max_level_hint()
+    }
 }
 
 impl<T> Subscriber<T> {

tracing-subscriber/tests/reload.rs

@@ -1,12 +1,16 @@
-#![cfg(feature = "reload")]
+#![cfg(feature = "registry")]
 use std::sync::atomic::{AtomicUsize, Ordering};
 use tracing_core::{
     collect::Interest,
     span::{Attributes, Id, Record},
-    Collect, Event, Metadata,
+    Collect, Event, LevelFilter, Metadata,
 };
 use tracing_subscriber::{prelude::*, reload::*, subscribe};
 
+fn event() {
+    tracing::info!("my event");
+}
+
 pub struct NopCollector;
 
 impl Collect for NopCollector {
@@ -67,9 +71,13 @@ fn reload_handle() {
             };
             true
         }
-    }
-    fn event() {
-        tracing::trace!("my event");
+
+        fn max_level_hint(&self) -> Option<LevelFilter> {
+            match self {
+                Filter::One => Some(LevelFilter::INFO),
+                Filter::Two => Some(LevelFilter::DEBUG),
+            }
+        }
     }
 
     let (subscriber, handle) = Subscriber::new(Filter::One);
@@ -85,7 +93,9 @@ fn reload_handle() {
         assert_eq!(FILTER1_CALLS.load(Ordering::SeqCst), 1);
         assert_eq!(FILTER2_CALLS.load(Ordering::SeqCst), 0);
 
+        assert_eq!(LevelFilter::current(), LevelFilter::INFO);
         handle.reload(Filter::Two).expect("should reload");
+        assert_eq!(LevelFilter::current(), LevelFilter::DEBUG);
 
         event();
 
@@ -113,9 +123,13 @@ fn reload_filter() {
             };
             true
         }
-    }
-    fn event() {
-        tracing::trace!("my event");
+
+        fn max_level_hint(&self) -> Option<LevelFilter> {
+            match self {
+                Filter::One => Some(LevelFilter::INFO),
+                Filter::Two => Some(LevelFilter::DEBUG),
+            }
+        }
     }
 
     let (filter, handle) = Subscriber::new(Filter::One);
@@ -133,7 +147,9 @@ fn reload_filter() {
         assert_eq!(FILTER1_CALLS.load(Ordering::SeqCst), 1);
         assert_eq!(FILTER2_CALLS.load(Ordering::SeqCst), 0);
 
+        assert_eq!(LevelFilter::current(), LevelFilter::INFO);
         handle.reload(Filter::Two).expect("should reload");
+        assert_eq!(LevelFilter::current(), LevelFilter::DEBUG);
 
         event();