feat(util): new helper type for recovering recorder after installing it

.github/workflows/ci.yml

@@ -40,7 +40,7 @@ jobs:
     - name: Install Rust ${{ matrix.rust_version }}
       run: rustup default ${{ matrix.rust_version }}
     - name: Run Tests
-      run: cargo test --all-features --workspace --exclude=metrics-observer
+      run: cargo test --all-features --workspace --exclude=metrics-observer -- --test-threads=1
   docs:
     runs-on: ubuntu-latest
     env:

metrics-util/CHANGELOG.md

@@ -9,6 +9,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased] - ReleaseDate
 
+### Added
+
+- Added a new helper type, `RecoverableRecorder`, that allows installing a recorder and then
+  recovering it later.
+
 ## [0.15.0] - 2023-04-16
 
 ### Changed

metrics-util/src/lib.rs

@@ -31,6 +31,9 @@ pub use kind::{MetricKind, MetricKindMask};
 mod histogram;
 pub use histogram::Histogram;
 
+mod recoverable;
+pub use recoverable::RecoverableRecorder;
+
 #[cfg(feature = "summary")]
 mod summary;
 #[cfg(feature = "summary")]

metrics-util/src/recoverable.rs

@@ -0,0 +1,320 @@
+use std::sync::{Arc, Weak};
+
+use metrics::{
+    Counter, Gauge, Histogram, Key, KeyName, Recorder, SetRecorderError, SharedString, Unit,
+};
+
+/// Wraps a recorder to allow for recovering it after being installed.
+///
+/// Installing a recorder generally involves providing an owned value, which means that it is not
+/// possible to recover the recorder after it has been installed. For some recorder implementations,
+/// it can be important to perform finalization before the application exits, which is not possible
+/// if the application cannot consume the recorder.
+///
+/// `RecoverableRecorder` allows wrapping a recorder such that a weak reference to it is installed
+/// globally, while the recorder itself is held by `RecoverableRecorder`. This allows for recovering
+/// the recorder whenever the application chooses.
+///
+/// ## As a drop guard
+///
+/// While `RecoverableRecorder` provides a method to manually recover the recorder directly, one
+/// particular benefit is that due to how the recorder is wrapped, when `RecoverableRecorder` is
+/// dropped, and the last active reference to it is dropped, the recorder itself will be dropped.
+///
+/// This allows using `RecoverableRecorder` as a drop guard, ensuring that by dropping it, the
+/// recorder itself will be dropped, and any finalization logic implemented for the recorder will be
+/// run.
+pub struct RecoverableRecorder<R> {
+    recorder: Arc<R>,
+}
+
+impl<R: Recorder + 'static> RecoverableRecorder<R> {
+    /// Creates a new `RecoverableRecorder`, wrapping the given recorder.
+    ///
+    /// A weakly-referenced version of the recorder is installed globally, while the original
+    /// recorder is held within `RecoverableRecorder`, and can be recovered by calling `into_inner`.
+    ///
+    /// # Errors
+    ///
+    /// If a recorder is already installed, an error is returned.
+    pub fn from_recorder(recorder: R) -> Result<Self, SetRecorderError> {
+        let recorder = Arc::new(recorder);
+
+        let wrapped = WeakRecorder::from_arc(&recorder);
+        metrics::set_boxed_recorder(Box::new(wrapped))?;
+
+        Ok(Self { recorder })
+    }
+
+    /// Consumes this wrapper, returning the original recorder.
+    ///
+    /// This method will loop until there are no active weak references to the recorder. It is not
+    /// advised to call this method under heavy load, as doing so is not deterministic or ordered
+    /// and may block for an indefinite amount of time.
+    pub fn into_inner(mut self) -> R {
+        loop {
+            match Arc::try_unwrap(self.recorder) {
+                Ok(recorder) => break recorder,
+                Err(recorder) => {
+                    self.recorder = recorder;
+                }
+            }
+        }
+    }
+}
+
+struct WeakRecorder<R> {
+    recorder: Weak<R>,
+}
+
+impl<R> WeakRecorder<R> {
+    fn from_arc(recorder: &Arc<R>) -> Self {
+        Self { recorder: Arc::downgrade(recorder) }
+    }
+}
+
+impl<R: Recorder> Recorder for WeakRecorder<R> {
+    fn describe_counter(&self, key: KeyName, unit: Option<Unit>, description: SharedString) {
+        if let Some(recorder) = self.recorder.upgrade() {
+            recorder.describe_counter(key, unit, description);
+        }
+    }
+
+    fn describe_gauge(&self, key: KeyName, unit: Option<Unit>, description: SharedString) {
+        if let Some(recorder) = self.recorder.upgrade() {
+            recorder.describe_gauge(key, unit, description);
+        }
+    }
+
+    fn describe_histogram(&self, key: KeyName, unit: Option<Unit>, description: SharedString) {
+        if let Some(recorder) = self.recorder.upgrade() {
+            recorder.describe_histogram(key, unit, description);
+        }
+    }
+
+    fn register_counter(&self, key: &Key) -> Counter {
+        if let Some(recorder) = self.recorder.upgrade() {
+            recorder.register_counter(key)
+        } else {
+            Counter::noop()
+        }
+    }
+
+    fn register_gauge(&self, key: &Key) -> Gauge {
+        if let Some(recorder) = self.recorder.upgrade() {
+            recorder.register_gauge(key)
+        } else {
+            Gauge::noop()
+        }
+    }
+
+    fn register_histogram(&self, key: &Key) -> Histogram {
+        if let Some(recorder) = self.recorder.upgrade() {
+            recorder.register_histogram(key)
+        } else {
+            Histogram::noop()
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::atomic::{AtomicBool, Ordering};
+
+    use super::*;
+    use metrics::{atomics::AtomicU64, CounterFn, GaugeFn, HistogramFn, Key, Recorder};
+
+    struct CounterWrapper(AtomicU64);
+    struct GaugeWrapper(AtomicU64);
+    struct HistogramWrapper(AtomicU64);
+
+    impl CounterWrapper {
+        fn get(&self) -> u64 {
+            self.0.load(Ordering::Acquire)
+        }
+    }
+
+    impl GaugeWrapper {
+        fn get(&self) -> u64 {
+            self.0.load(Ordering::Acquire)
+        }
+    }
+
+    impl HistogramWrapper {
+        fn get(&self) -> u64 {
+            self.0.load(Ordering::Acquire)
+        }
+    }
+
+    impl CounterFn for CounterWrapper {
+        fn increment(&self, value: u64) {
+            self.0.fetch_add(value, Ordering::Release);
+        }
+
+        fn absolute(&self, value: u64) {
+            self.0.store(value, Ordering::Release);
+        }
+    }
+
+    impl GaugeFn for GaugeWrapper {
+        fn increment(&self, value: f64) {
+            self.0.fetch_add(value as u64, Ordering::Release);
+        }
+
+        fn decrement(&self, value: f64) {
+            self.0.fetch_sub(value as u64, Ordering::Release);
+        }
+
+        fn set(&self, value: f64) {
+            self.0.store(value as u64, Ordering::Release);
+        }
+    }
+
+    impl HistogramFn for HistogramWrapper {
+        fn record(&self, value: f64) {
+            self.0.fetch_add(value as u64, Ordering::Release);
+        }
+    }
+
+    struct TestRecorder {
+        dropped: Arc<AtomicBool>,
+        counter: Arc<CounterWrapper>,
+        gauge: Arc<GaugeWrapper>,
+        histogram: Arc<HistogramWrapper>,
+    }
+
+    impl TestRecorder {
+        fn new() -> (Self, Arc<CounterWrapper>, Arc<GaugeWrapper>, Arc<HistogramWrapper>) {
+            let (recorder, _, counter, gauge, histogram) = Self::new_with_drop();
+            (recorder, counter, gauge, histogram)
+        }
+
+        fn new_with_drop(
+        ) -> (Self, Arc<AtomicBool>, Arc<CounterWrapper>, Arc<GaugeWrapper>, Arc<HistogramWrapper>)
+        {
+            let dropped = Arc::new(AtomicBool::new(false));
+            let counter = Arc::new(CounterWrapper(AtomicU64::new(0)));
+            let gauge = Arc::new(GaugeWrapper(AtomicU64::new(0)));
+            let histogram = Arc::new(HistogramWrapper(AtomicU64::new(0)));
+
+            let recorder = Self {
+                dropped: Arc::clone(&dropped),
+                counter: Arc::clone(&counter),
+                gauge: Arc::clone(&gauge),
+                histogram: Arc::clone(&histogram),
+            };
+
+            (recorder, dropped, counter, gauge, histogram)
+        }
+    }
+
+    impl Recorder for TestRecorder {
+        fn describe_counter(&self, _key: KeyName, _unit: Option<Unit>, _description: SharedString) {
+            todo!()
+        }
+
+        fn describe_gauge(&self, _key: KeyName, _unit: Option<Unit>, _description: SharedString) {
+            todo!()
+        }
+
+        fn describe_histogram(
+            &self,
+            _key: KeyName,
+            _unit: Option<Unit>,
+            _description: SharedString,
+        ) {
+            todo!()
+        }
+
+        fn register_counter(&self, _: &Key) -> Counter {
+            Counter::from_arc(Arc::clone(&self.counter))
+        }
+
+        fn register_gauge(&self, _: &Key) -> Gauge {
+            Gauge::from_arc(Arc::clone(&self.gauge))
+        }
+
+        fn register_histogram(&self, _: &Key) -> Histogram {
+            Histogram::from_arc(Arc::clone(&self.histogram))
+        }
+    }
+
+    impl Drop for TestRecorder {
+        fn drop(&mut self) {
+            self.dropped.store(true, Ordering::Release);
+        }
+    }
+
+    #[test]
+    fn basic() {
+        // Create and install the recorder.
+        let (recorder, counter, gauge, histogram) = TestRecorder::new();
+        unsafe {
+            metrics::clear_recorder();
+        }
+        let recoverable =
+            RecoverableRecorder::from_recorder(recorder).expect("failed to install recorder");
+
+        // Record some metrics, and make sure the atomics for each metric type are
+        // incremented as we would expect them to be.
+        metrics::counter!("counter", 5);
+        metrics::increment_gauge!("gauge", 5.0);
+        metrics::increment_gauge!("gauge", 5.0);
+        metrics::histogram!("histogram", 5.0);
+        metrics::histogram!("histogram", 5.0);
+        metrics::histogram!("histogram", 5.0);
+
+        let _recorder = recoverable.into_inner();
+        assert_eq!(counter.get(), 5);
+        assert_eq!(gauge.get(), 10);
+        assert_eq!(histogram.get(), 15);
+
+        // Now that we've recovered the recorder, incrementing the same metrics should
+        // not actually increment the value of the atomics for each metric type.
+        metrics::counter!("counter", 7);
+        metrics::increment_gauge!("gauge", 7.0);
+        metrics::histogram!("histogram", 7.0);
+
+        assert_eq!(counter.get(), 5);
+        assert_eq!(gauge.get(), 10);
+        assert_eq!(histogram.get(), 15);
+    }
+
+    #[test]
+    fn on_drop() {
+        // Create and install the recorder.
+        let (recorder, dropped, counter, gauge, histogram) = TestRecorder::new_with_drop();
+        unsafe {
+            metrics::clear_recorder();
+        }
+        let recoverable =
+            RecoverableRecorder::from_recorder(recorder).expect("failed to install recorder");
+
+        // Record some metrics, and make sure the atomics for each metric type are
+        // incremented as we would expect them to be.
+        metrics::counter!("counter", 5);
+        metrics::increment_gauge!("gauge", 5.0);
+        metrics::increment_gauge!("gauge", 5.0);
+        metrics::histogram!("histogram", 5.0);
+        metrics::histogram!("histogram", 5.0);
+        metrics::histogram!("histogram", 5.0);
+
+        drop(recoverable.into_inner());
+        assert_eq!(counter.get(), 5);
+        assert_eq!(gauge.get(), 10);
+        assert_eq!(histogram.get(), 15);
+
+        // Now that we've recovered the recorder, incrementing the same metrics should
+        // not actually increment the value of the atomics for each metric type.
+        metrics::counter!("counter", 7);
+        metrics::increment_gauge!("gauge", 7.0);
+        metrics::histogram!("histogram", 7.0);
+
+        assert_eq!(counter.get(), 5);
+        assert_eq!(gauge.get(), 10);
+        assert_eq!(histogram.get(), 15);
+
+        // And we should be able to check that the recorder was indeed dropped.
+        assert!(dropped.load(Ordering::Acquire));
+    }
+}

metrics/Cargo.toml

@@ -16,8 +16,6 @@ readme = "README.md"
 categories = ["development-tools::debugging"]
 keywords = ["metrics", "facade"]
 
-build = "build.rs"
-
 [lib]
 bench = false
 

metrics/benches/macros.rs

@@ -24,8 +24,10 @@ impl Recorder for TestRecorder {
 }
 
 fn reset_recorder() {
-    let recorder = Box::leak(Box::new(TestRecorder::default()));
-    unsafe { metrics::set_recorder_racy(recorder).unwrap() }
+    unsafe {
+        metrics::clear_recorder();
+    }
+    metrics::set_boxed_recorder(Box::new(TestRecorder::default())).unwrap()
 }
 
 fn macro_benchmark(c: &mut Criterion) {