Add Filter::event_enabled

tracing-subscriber/src/filter/subscriber_filters/combinator.rs

@@ -137,6 +137,11 @@ where
         cmp::min(self.a.max_level_hint(), self.b.max_level_hint())
     }
 
+    #[inline]
+    fn event_enabled(&self, event: &tracing_core::Event<'_>, cx: &Context<'_, S>) -> bool {
+        self.a.event_enabled(event, cx) && self.b.event_enabled(event, cx)
+    }
+
     #[inline]
     fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
         self.a.on_new_span(attrs, id, ctx.clone());
@@ -324,6 +329,11 @@ where
         Some(cmp::max(self.a.max_level_hint()?, self.b.max_level_hint()?))
     }
 
+    #[inline]
+    fn event_enabled(&self, event: &tracing_core::Event<'_>, cx: &Context<'_, S>) -> bool {
+        self.a.event_enabled(event, cx) || self.b.event_enabled(event, cx)
+    }
+
     #[inline]
     fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
         self.a.on_new_span(attrs, id, ctx.clone());
@@ -421,6 +431,14 @@ where
         None
     }
 
+    #[inline]
+    fn event_enabled(&self, event: &tracing_core::Event<'_>, cx: &Context<'_, S>) -> bool {
+        // Never disable based on event_enabled; we "disabled" it in `enabled`,
+        // so the `not` has already been applied and filtered this not out.
+        let _ = (event, cx);
+        true
+    }
+
     #[inline]
     fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
         self.a.on_new_span(attrs, id, ctx);

tracing-subscriber/src/filter/subscriber_filters/mod.rs

@@ -639,6 +639,22 @@ where
         }
     }
 
+    fn event_enabled(&self, event: &Event<'_>, cx: Context<'_, C>) -> bool {
+        let cx = cx.with_filter(self.id());
+        let enabled = FILTERING
+            .with(|filtering| filtering.and(self.id(), || self.filter.event_enabled(event, &cx)));
+
+        if enabled {
+            // If the filter enabled this event, ask the wrapped subscriber if
+            // _it_ wants it --- it might have a global filter.
+            self.subscriber.event_enabled(event, cx)
+        } else {
+            // Otherwise, return `true`. See the comment in `enabled` for why this
+            // is necessary.
+            true
+        }
+    }
+
     fn on_event(&self, event: &Event<'_>, cx: Context<'_, C>) {
         self.did_enable(|| {
             self.subscriber.on_event(event, cx.with_filter(self.id()));
@@ -1006,6 +1022,14 @@ impl FilterState {
         }
     }
 
+    /// Run a second filtering pass, e.g. for Subscribe::event_enabled.
+    fn and(&self, filter: FilterId, f: impl FnOnce() -> bool) -> bool {
+        let map = self.enabled.get();
+        let enabled = map.is_enabled(filter) && f();
+        self.enabled.set(map.set(filter, enabled));
+        enabled
+    }
+
     /// Clears the current in-progress filter state.
     ///
     /// This resets the [`FilterMap`] and current [`Interest`] as well as

tracing-subscriber/src/registry/sharded.rs

@@ -275,6 +275,13 @@ impl Collect for Registry {
 
     fn record_follows_from(&self, _span: &span::Id, _follows: &span::Id) {}
 
+    fn event_enabled(&self, _event: &Event<'_>) -> bool {
+        if self.has_per_subscriber_filters() {
+            return FilterState::event_enabled();
+        }
+        true
+    }
+
     /// This is intentionally not implemented, as recording events
     /// is the responsibility of subscribers atop of this registry.
     fn event(&self, _: &Event<'_>) {}

tracing-subscriber/src/subscribe/mod.rs

@@ -1382,6 +1382,18 @@ pub trait Filter<S> {
         None
     }
 
+    /// Called before the filtered subscribers' [`on_event`], to determine if
+    /// `on_event` should be called.
+    ///
+    /// This gives a chance to filter events based on their fields.
+    ///
+    /// [`on_event`]: crate::subscribe::Subscribe::on_event
+    #[inline] // collapse this to a constant please mrs optimizer
+    fn event_enabled(&self, event: &Event<'_>, cx: &Context<'_, S>) -> bool {
+        let _ = (event, cx);
+        true
+    }
+
     /// Notifies this filter that a new span was constructed with the given
     /// `Attributes` and `Id`.
     ///

tracing-subscriber/tests/subscriber_filters/main.rs

@@ -3,6 +3,7 @@
 mod support;
 use self::support::*;
 mod filter_scopes;
+mod per_event;
 mod targets;
 mod trees;
 mod vec;

tracing-subscriber/tests/subscriber_filters/per_event.rs

@@ -0,0 +1,61 @@
+use crate::support::*;
+use tracing::Level;
+use tracing_subscriber::{field::Visit, prelude::*, subscribe::Filter};
+
+struct FilterEvent;
+
+impl<S> Filter<S> for FilterEvent {
+    fn enabled(
+        &self,
+        _meta: &tracing::Metadata<'_>,
+        _cx: &tracing_subscriber::subscribe::Context<'_, S>,
+    ) -> bool {
+        true
+    }
+
+    fn event_enabled(
+        &self,
+        event: &tracing::Event<'_>,
+        _cx: &tracing_subscriber::subscribe::Context<'_, S>,
+    ) -> bool {
+        struct ShouldEnable(bool);
+        impl Visit for ShouldEnable {
+            fn record_bool(&mut self, field: &tracing_core::Field, value: bool) {
+                if field.name() == "enable" {
+                    self.0 = value;
+                }
+            }
+
+            fn record_debug(
+                &mut self,
+                _field: &tracing_core::Field,
+                _value: &dyn core::fmt::Debug,
+            ) {
+            }
+        }
+        let mut should_enable = ShouldEnable(false);
+        event.record(&mut should_enable);
+        should_enable.0
+    }
+}
+
+#[test]
+fn per_subscriber_event_field_filtering() {
+    let (expect, handle) = subscriber::mock()
+        .event(event::mock().at_level(Level::TRACE))
+        .event(event::mock().at_level(Level::INFO))
+        .done()
+        .run_with_handle();
+
+    let _subscriber = tracing_subscriber::registry()
+        .with(expect.with_filter(FilterEvent))
+        .set_default();
+
+    tracing::trace!(enable = true, "hello trace");
+    tracing::debug!("hello debug");
+    tracing::info!(enable = true, "hello info");
+    tracing::warn!(enable = false, "hello warn");
+    tracing::error!("hello error");
+
+    handle.assert_finished();
+}