subscriber: implement FIlter for reload::Subscriber (#2159)

## Motivation

The `reload` subscriber doesn't (and can't) implement downcasting correctly,
which breaks certain subscribers like the opentelemetry one.

## Solution

Most uses of the `reload` module (including mine) are just to change the
filter. Therefore, this PR implements `Filter` for `reload::Subscriber`
to allow users to not need to wrap the whole layer trait. Another
advantage of this is that the common-case critical sections are
shortened.

tracing-subscriber/src/reload.rs

@@ -1,15 +1,25 @@
 //! Wrapper for a `Collect` or `Subscribe` to allow it to be dynamically reloaded.
 //!
-//! This module provides a type implementing [`Subscribe`] which wraps another type implementing
-//! the [`Subscribe`] trait, allowing the wrapped type to be replaced with another
+//! This module provides a type implementing [`Subscribe`] and [`Filter`]
+//! which wraps another type implementing the corresponding trait. This
+//! allows the wrapped type to be replaced with another
 //! instance of that type at runtime.
 //!
-//! This can be used in cases where a subset of `Collect` functionality
+//! This can be used in cases where a subset of `Collect` or `Filter` functionality
 //! should be dynamically reconfigured, such as when filtering directives may
 //! change at runtime. Note that this subscriber introduces a (relatively small)
 //! amount of overhead, and should thus only be used as needed.
 //!
+//! ## Note
+//!
+//! //! The [`Subscribe`] implementation is unable to implement downcasting functionality,
+//! so certain `Subscribers` will fail to reload if wrapped in a `reload::Subscriber`.
+//!
+//! If you only want to be able to dynamically change the
+//! `Filter` on your layer, prefer wrapping that `Filter` in the `reload::Subscriber`.
+//!
 //! [`Subscribe`]: crate::Subscribe
+//! [`Filter`]: crate::subscribe::Filter
 use crate::subscribe;
 use crate::sync::RwLock;
 
@@ -23,7 +33,10 @@ use tracing_core::{
     span, Event, Metadata,
 };
 
-/// Wraps a `Collect` or `Subscribe`, allowing it to be reloaded dynamically at runtime.
+/// Wraps a `Filter` or `Subscribe`, allowing it to be reloaded dynamically at runtime.
+///
+/// [`Filter`]: crate::subscribe::Filter
+/// [`Subscribe`]: crate::Subscribe
 #[derive(Debug)]
 pub struct Subscriber<S> {
     // TODO(eliza): this once used a `crossbeam_util::ShardedRwLock`. We may
@@ -119,10 +132,67 @@ where
     }
 }
 
-impl<S> Subscriber<S> {
-    /// Wraps the given `Subscribe`, returning a subscriber and a `Handle` that allows
+#[cfg(all(feature = "registry", feature = "std"))]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "registry", feature = "std"))))]
+impl<S, C> crate::subscribe::Filter<C> for Subscriber<S>
+where
+    S: crate::subscribe::Filter<C> + 'static,
+    C: Collect,
+{
+    #[inline]
+    fn callsite_enabled(&self, metadata: &'static Metadata<'static>) -> Interest {
+        try_lock!(self.inner.read(), else return Interest::sometimes()).callsite_enabled(metadata)
+    }
+
+    #[inline]
+    fn enabled(&self, metadata: &Metadata<'_>, ctx: &subscribe::Context<'_, C>) -> bool {
+        try_lock!(self.inner.read(), else return false).enabled(metadata, ctx)
+    }
+
+    #[inline]
+    fn on_new_span(
+        &self,
+        attrs: &span::Attributes<'_>,
+        id: &span::Id,
+        ctx: subscribe::Context<'_, C>,
+    ) {
+        try_lock!(self.inner.read()).on_new_span(attrs, id, ctx)
+    }
+
+    #[inline]
+    fn on_record(
+        &self,
+        span: &span::Id,
+        values: &span::Record<'_>,
+        ctx: subscribe::Context<'_, C>,
+    ) {
+        try_lock!(self.inner.read()).on_record(span, values, ctx)
+    }
+
+    #[inline]
+    fn on_enter(&self, id: &span::Id, ctx: subscribe::Context<'_, C>) {
+        try_lock!(self.inner.read()).on_enter(id, ctx)
+    }
+
+    #[inline]
+    fn on_exit(&self, id: &span::Id, ctx: subscribe::Context<'_, C>) {
+        try_lock!(self.inner.read()).on_exit(id, ctx)
+    }
+
+    #[inline]
+    fn on_close(&self, id: span::Id, ctx: subscribe::Context<'_, C>) {
+        try_lock!(self.inner.read()).on_close(id, ctx)
+    }
+}
+
+impl<T> Subscriber<T> {
+    /// Wraps the given `Subscribe` or `Filter`,
+    /// returning a subscriber or filter and a `Handle` that allows
     /// the inner type to be modified at runtime.
-    pub fn new(inner: S) -> (Self, Handle<S>) {
+    ///
+    /// [`Filter`]: crate::subscribe::Filter
+    /// [`Subscribe`]: crate::Subscribe
+    pub fn new(inner: T) -> (Self, Handle<T>) {
         let this = Self {
             inner: Arc::new(RwLock::new(inner)),
         };
@@ -131,7 +201,7 @@ impl<S> Subscriber<S> {
     }
 
     /// Returns a `Handle` that can be used to reload the wrapped `Subscribe`.
-    pub fn handle(&self) -> Handle<S> {
+    pub fn handle(&self) -> Handle<T> {
         Handle {
             inner: Arc::downgrade(&self.inner),
         }
@@ -140,22 +210,25 @@ impl<S> Subscriber<S> {
 
 // ===== impl Handle =====
 
-impl<S> Handle<S> {
-    /// Replace the current subscriber with the provided `new_subscriber`.
+impl<T> Handle<T> {
+    /// Replace the current subscriber or filter with the provided `new_value`.
+    ///
+    /// [`Handle::reload`] cannot be used with the [`Filtered`](crate::filter::Filtered)
+    /// subscriber; use [`Handle::modify`] instead (see [this issue] for additional details).
+    ///
+    /// However, if the _only_ the [`Filter`](crate::subscribe::Filter) needs to be modified,
+    /// use `reload::Subscriber` to wrap the `Filter` directly.
     ///
-    /// **Warning:** The [`Filtered`](crate::filter::Filtered) type currently can't be changed
-    /// at runtime via the [`Handle::reload`] method.
-    /// Use the [`Handle::modify`] method to change the filter instead.
-    /// (see <https://github.com/tokio-rs/tracing/issues/1629>)
-    pub fn reload(&self, new_subscriber: impl Into<S>) -> Result<(), Error> {
-        self.modify(|subscriber| {
-            *subscriber = new_subscriber.into();
+    /// [this issue]: https://github.com/tokio-rs/tracing/issues/1629
+    pub fn reload(&self, new_value: impl Into<T>) -> Result<(), Error> {
+        self.modify(|object| {
+            *object = new_value.into();
         })
     }
 
     /// Invokes a closure with a mutable reference to the current subscriber,
     /// allowing it to be modified in place.
-    pub fn modify(&self, f: impl FnOnce(&mut S)) -> Result<(), Error> {
+    pub fn modify(&self, f: impl FnOnce(&mut T)) -> Result<(), Error> {
         let inner = self.inner.upgrade().ok_or(Error {
             kind: ErrorKind::CollectorGone,
         })?;
@@ -182,16 +255,16 @@ impl<S> Handle<S> {
 
     /// Returns a clone of the subscriber's current value if it still exists.
     /// Otherwise, if the collector has been dropped, returns `None`.
-    pub fn clone_current(&self) -> Option<S>
+    pub fn clone_current(&self) -> Option<T>
     where
-        S: Clone,
+        T: Clone,
     {
-        self.with_current(S::clone).ok()
+        self.with_current(T::clone).ok()
     }
 
     /// Invokes a closure with a borrowed reference to the current subscriber,
     /// returning the result (or an error if the collector no longer exists).
-    pub fn with_current<T>(&self, f: impl FnOnce(&S) -> T) -> Result<T, Error> {
+    pub fn with_current<T2>(&self, f: impl FnOnce(&T) -> T2) -> Result<T2, Error> {
         let inner = self.inner.upgrade().ok_or(Error {
             kind: ErrorKind::CollectorGone,
         })?;
@@ -200,7 +273,7 @@ impl<S> Handle<S> {
     }
 }
 
-impl<S> Clone for Handle<S> {
+impl<T> Clone for Handle<T> {
     fn clone(&self) -> Self {
         Handle {
             inner: self.inner.clone(),

tracing-subscriber/tests/reload.rs

@@ -32,6 +32,17 @@ impl Collect for NopCollector {
     }
 }
 
+pub struct NopSubscriber;
+impl<S: Collect> tracing_subscriber::Subscribe<S> for NopSubscriber {
+    fn register_callsite(&self, _m: &Metadata<'_>) -> Interest {
+        Interest::sometimes()
+    }
+
+    fn enabled(&self, _m: &Metadata<'_>, _: subscribe::Context<'_, S>) -> bool {
+        true
+    }
+}
+
 #[test]
 fn reload_handle() {
     static FILTER1_CALLS: AtomicUsize = AtomicUsize::new(0);
@@ -82,3 +93,51 @@ fn reload_handle() {
         assert_eq!(FILTER2_CALLS.load(Ordering::SeqCst), 1);
     })
 }
+
+#[test]
+fn reload_filter() {
+    static FILTER1_CALLS: AtomicUsize = AtomicUsize::new(0);
+    static FILTER2_CALLS: AtomicUsize = AtomicUsize::new(0);
+
+    enum Filter {
+        One,
+        Two,
+    }
+
+    impl<S: Collect> tracing_subscriber::subscribe::Filter<S> for Filter {
+        fn enabled(&self, m: &Metadata<'_>, _: &subscribe::Context<'_, S>) -> bool {
+            println!("ENABLED: {:?}", m);
+            match self {
+                Filter::One => FILTER1_CALLS.fetch_add(1, Ordering::SeqCst),
+                Filter::Two => FILTER2_CALLS.fetch_add(1, Ordering::SeqCst),
+            };
+            true
+        }
+    }
+    fn event() {
+        tracing::trace!("my event");
+    }
+
+    let (filter, handle) = Subscriber::new(Filter::One);
+
+    let dispatcher = tracing_core::dispatch::Dispatch::new(
+        tracing_subscriber::registry().with(NopSubscriber.with_filter(filter)),
+    );
+
+    tracing_core::dispatch::with_default(&dispatcher, || {
+        assert_eq!(FILTER1_CALLS.load(Ordering::SeqCst), 0);
+        assert_eq!(FILTER2_CALLS.load(Ordering::SeqCst), 0);
+
+        event();
+
+        assert_eq!(FILTER1_CALLS.load(Ordering::SeqCst), 1);
+        assert_eq!(FILTER2_CALLS.load(Ordering::SeqCst), 0);
+
+        handle.reload(Filter::Two).expect("should reload");
+
+        event();
+
+        assert_eq!(FILTER1_CALLS.load(Ordering::SeqCst), 1);
+        assert_eq!(FILTER2_CALLS.load(Ordering::SeqCst), 1);
+    })
+}