Honor explicit parent spans for events (#92)

## Motivation

Currently, when an event is created, no matter what is set as its
parent, `OpenTelemetryLayer` will call `lookup_current` and use that as
the parent span. It should honor the configured parent, if any, or be
ignored, if the parent is explicitly set to `None`.

## Solution

The code is changed so that the parent is correctly resolved. The logic
is somewhat similar to `OpenTelemetryLayer::parent_context`.

src/layer.rs

@@ -996,7 +996,12 @@ where
     /// [`Error`]: opentelemetry::trace::StatusCode::Error
     fn on_event(&self, event: &Event<'_>, ctx: Context<'_, S>) {
         // Ignore events that are not in the context of a span
-        if let Some(span) = ctx.lookup_current() {
+        if let Some(span) = event.parent().and_then(|id| ctx.span(id)).or_else(|| {
+            event
+                .is_contextual()
+                .then(|| ctx.lookup_current())
+                .flatten()
+        }) {
             // Performing read operations before getting a write lock to avoid a deadlock
             // See https://github.com/tokio-rs/tracing/issues/763
             #[cfg(feature = "tracing-log")]

tests/parents.rs

@@ -0,0 +1,98 @@
+use futures_util::future::BoxFuture;
+use opentelemetry::trace::TracerProvider as _;
+use opentelemetry_sdk::{
+    export::trace::{ExportResult, SpanData, SpanExporter},
+    trace::{Tracer, TracerProvider},
+};
+use std::sync::{Arc, Mutex};
+use tracing::level_filters::LevelFilter;
+use tracing::Subscriber;
+use tracing_opentelemetry::layer;
+use tracing_subscriber::prelude::*;
+
+#[derive(Clone, Default, Debug)]
+struct TestExporter(Arc<Mutex<Vec<SpanData>>>);
+
+impl SpanExporter for TestExporter {
+    fn export(&mut self, mut batch: Vec<SpanData>) -> BoxFuture<'static, ExportResult> {
+        let spans = self.0.clone();
+        Box::pin(async move {
+            if let Ok(mut inner) = spans.lock() {
+                inner.append(&mut batch);
+            }
+            Ok(())
+        })
+    }
+}
+
+fn test_tracer() -> (Tracer, TracerProvider, TestExporter, impl Subscriber) {
+    let exporter = TestExporter::default();
+    let provider = TracerProvider::builder()
+        .with_simple_exporter(exporter.clone())
+        .build();
+    let tracer = provider.tracer("test");
+
+    let subscriber = tracing_subscriber::registry()
+        .with(layer().with_tracer(tracer.clone()).with_filter(LevelFilter::DEBUG))
+        .with(tracing_subscriber::fmt::layer().with_filter(LevelFilter::TRACE));
+
+    (tracer, provider, exporter, subscriber)
+}
+
+#[test]
+fn explicit_parents_of_events() {
+    let (_tracer, provider, exporter, subscriber) = test_tracer();
+
+    tracing::subscriber::with_default(subscriber, || {
+        let root = tracing::debug_span!("root").entered();
+
+        tracing::debug!("1");
+        tracing::debug!(parent: &root, "2");
+        tracing::debug!(parent: None, "3");
+
+        let child = tracing::debug_span!(parent: &root, "child");
+        child.in_scope(|| {
+            tracing::debug!("4");
+            tracing::debug!(parent: &root, "5");
+            tracing::debug!(parent: &child, "6");
+            tracing::debug!(parent: None, "7");
+        });
+
+        tracing::debug!("8");
+        tracing::debug!(parent: &root, "9");
+        tracing::debug!(parent: &child, "10");
+        tracing::debug!(parent: None, "11");
+
+        let root = root.exit();
+
+        tracing::debug!("12");
+        tracing::debug!(parent: &root, "13");
+        tracing::debug!(parent: &child, "14");
+        tracing::debug!(parent: None, "15");
+    });
+
+    drop(provider); // flush all spans
+    let spans = exporter.0.lock().unwrap();
+
+    assert_eq!(spans.len(), 2);
+
+    {
+        // Check the root span
+        let expected_root_events = ["1", "2", "5", "8", "9", "13"];
+
+        let root_span = spans.iter().find(|s| s.name == "root").unwrap();
+        let actual_events: Vec<_> = root_span.events.iter().map(|event| event.name.to_string()).collect();
+
+        assert_eq!(&expected_root_events, &actual_events[..]);
+    }
+
+    {
+        // Check the child span
+        let expected_child_events = ["4", "6", "10", "14"];
+
+        let child_span = spans.iter().find(|s| s.name == "child").unwrap();
+        let actual_events: Vec<_> = child_span.events.iter().map(|event| event.name.to_string()).collect();
+
+        assert_eq!(&expected_child_events, &actual_events[..]);
+    }
+}