fix(async-processor): concurrent exports actually serialised (#3028)

alexbrt · web-flow · commit 063107068cb8 · 2025-07-14T10:37:21.000-07:00
diff --git a/opentelemetry-sdk/CHANGELOG.md b/opentelemetry-sdk/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 - TODO: Placeholder for Span processor related things
   - *Fix* SpanProcessor::on_start is no longer called on non recording spans
+- **Fix**: Restore true parallel exports in the async-native `BatchSpanProcessor` by honoring `OTEL_BSP_MAX_CONCURRENT_EXPORTS` ([#2959](https://github.com/open-telemetry/opentelemetry-rust/pull/3028)). A regression in [#2685](https://github.com/open-telemetry/opentelemetry-rust/pull/2685) inadvertently awaited the `export()` future directly in `opentelemetry-sdk/src/trace/span_processor_with_async_runtime.rs` instead of spawning it on the runtime, forcing all exports to run sequentially.
 
 ## 0.30.0
 
diff --git a/opentelemetry-sdk/Cargo.toml b/opentelemetry-sdk/Cargo.toml
@@ -22,7 +22,7 @@ serde = { workspace = true, features = ["derive", "rc"], optional = true }
 serde_json = { workspace = true, optional = true }
 thiserror = { workspace = true }
 url = { workspace = true, optional = true }
-tokio = { workspace = true, features = ["rt", "time"], optional = true }
+tokio = { workspace = true, default-features = false, optional = true }
 tokio-stream = { workspace = true, optional = true }
 http = { workspace = true, optional = true }
 
@@ -47,15 +47,15 @@ spec_unstable_logs_enabled = ["logs", "opentelemetry/spec_unstable_logs_enabled"
 metrics = ["opentelemetry/metrics"]
 testing = ["opentelemetry/testing", "trace", "metrics", "logs", "rt-tokio", "rt-tokio-current-thread", "tokio/macros", "tokio/rt-multi-thread"]
 experimental_async_runtime = []
-rt-tokio = ["tokio", "tokio-stream", "experimental_async_runtime"]
-rt-tokio-current-thread = ["tokio", "tokio-stream", "experimental_async_runtime"]
+rt-tokio = ["tokio/rt", "tokio/time", "tokio-stream", "experimental_async_runtime"]
+rt-tokio-current-thread = ["tokio/rt", "tokio/time", "tokio-stream", "experimental_async_runtime"]
 internal-logs = ["opentelemetry/internal-logs"]
 experimental_metrics_periodicreader_with_async_runtime = ["metrics", "experimental_async_runtime"]
 spec_unstable_metrics_views = ["metrics"]
 experimental_metrics_custom_reader = ["metrics"]
 experimental_logs_batch_log_processor_with_async_runtime = ["logs", "experimental_async_runtime"]
 experimental_logs_concurrent_log_processor = ["logs"]
-experimental_trace_batch_span_processor_with_async_runtime = ["trace", "experimental_async_runtime"]
+experimental_trace_batch_span_processor_with_async_runtime = ["tokio/sync", "trace", "experimental_async_runtime"]
 experimental_metrics_disable_name_validation = ["metrics"]
 
 [[bench]]
diff --git a/opentelemetry-sdk/src/trace/span_processor_with_async_runtime.rs b/opentelemetry-sdk/src/trace/span_processor_with_async_runtime.rs
@@ -6,19 +6,21 @@ use crate::trace::Span;
 use crate::trace::SpanProcessor;
 use crate::trace::{SpanData, SpanExporter};
 use futures_channel::oneshot;
-use futures_util::pin_mut;
 use futures_util::{
     future::{self, BoxFuture, Either},
-    select,
+    pin_mut, select,
     stream::{self, FusedStream, FuturesUnordered},
     StreamExt as _,
 };
 use opentelemetry::Context;
 use opentelemetry::{otel_debug, otel_error, otel_warn};
 use std::fmt;
-use std::sync::atomic::{AtomicUsize, Ordering};
-use std::sync::Arc;
+use std::sync::{
+    atomic::{AtomicUsize, Ordering},
+    Arc,
+};
 use std::time::Duration;
+use tokio::sync::RwLock;
 
 /// A [`SpanProcessor`] that asynchronously buffers finished spans and reports
 /// them at a preconfigured interval.
@@ -188,13 +190,22 @@ struct BatchSpanProcessorInternal<E, R> {
     spans: Vec<SpanData>,
     export_tasks: FuturesUnordered<BoxFuture<'static, OTelSdkResult>>,
     runtime: R,
-    exporter: E,
     config: BatchConfig,
+    // TODO: Redesign the `SpanExporter` trait to use immutable references (`&self`)
+    // for all methods. This would allow us to remove the `RwLock` and just use `Arc<E>`,
+    // similar to how `crate::logs::LogExporter` is implemented.
+    exporter: Arc<RwLock<E>>,
 }
 
-impl<E: SpanExporter, R: RuntimeChannel> BatchSpanProcessorInternal<E, R> {
+impl<E: SpanExporter + 'static, R: RuntimeChannel> BatchSpanProcessorInternal<E, R> {
     async fn flush(&mut self, res_channel: Option<oneshot::Sender<OTelSdkResult>>) {
-        let export_result = self.export().await;
+        let export_result = Self::export(
+            self.spans.split_off(0),
+            self.exporter.clone(),
+            self.runtime.clone(),
+            self.config.max_export_timeout,
+        )
+        .await;
         let task = Box::pin(async move {
             if let Some(channel) = res_channel {
                 // If a response channel is provided, attempt to send the export result through it.
@@ -243,9 +254,15 @@ impl<E: SpanExporter, R: RuntimeChannel> BatchSpanProcessorInternal<E, R> {
                         self.export_tasks.next().await;
                     }
 
-                    let export_result = self.export().await;
+                    let batch = self.spans.split_off(0);
+                    let exporter = self.exporter.clone();
+                    let runtime = self.runtime.clone();
+                    let max_export_timeout = self.config.max_export_timeout;
+
                     let task = async move {
-                        if let Err(err) = export_result {
+                        if let Err(err) =
+                            Self::export(batch, exporter, runtime, max_export_timeout).await
+                        {
                             otel_error!(
                                 name: "BatchSpanProcessor.Export.Error",
                                 reason = format!("{}", err)
@@ -254,6 +271,7 @@ impl<E: SpanExporter, R: RuntimeChannel> BatchSpanProcessorInternal<E, R> {
 
                         Ok(())
                     };
+
                     // Special case when not using concurrent exports
                     if self.config.max_concurrent_exports == 1 {
                         let _ = task.await;
@@ -288,34 +306,39 @@ impl<E: SpanExporter, R: RuntimeChannel> BatchSpanProcessorInternal<E, R> {
             // Stream has terminated or processor is shutdown, return to finish execution.
             BatchMessage::Shutdown(ch) => {
                 self.flush(Some(ch)).await;
-                let _ = self.exporter.shutdown();
+                let _ = self.exporter.write().await.shutdown();
                 return false;
             }
             // propagate the resource
             BatchMessage::SetResource(resource) => {
-                self.exporter.set_resource(&resource);
+                self.exporter.write().await.set_resource(&resource);
             }
         }
         true
     }
 
-    async fn export(&mut self) -> OTelSdkResult {
+    async fn export(
+        batch: Vec<SpanData>,
+        exporter: Arc<RwLock<E>>,
+        runtime: R,
+        max_export_timeout: Duration,
+    ) -> OTelSdkResult {
         // Batch size check for flush / shutdown. Those methods may be called
         // when there's no work to do.
-        if self.spans.is_empty() {
+        if batch.is_empty() {
             return Ok(());
         }
 
-        let export = self.exporter.export(self.spans.split_off(0));
-        let timeout = self.runtime.delay(self.config.max_export_timeout);
-        let time_out = self.config.max_export_timeout;
+        let exporter_guard = exporter.read().await;
+        let export = exporter_guard.export(batch);
+        let timeout = runtime.delay(max_export_timeout);
 
         pin_mut!(export);
         pin_mut!(timeout);
 
         match future::select(export, timeout).await {
             Either::Left((export_res, _)) => export_res,
-            Either::Right((_, _)) => Err(OTelSdkError::Timeout(time_out)),
+            Either::Right((_, _)) => Err(OTelSdkError::Timeout(max_export_timeout)),
         }
     }
 
@@ -368,7 +391,7 @@ impl<R: RuntimeChannel> BatchSpanProcessor<R> {
                 export_tasks: FuturesUnordered::new(),
                 runtime: timeout_runtime,
                 config,
-                exporter,
+                exporter: Arc::new(RwLock::new(exporter)),
             };
 
             processor.run(messages).await
@@ -435,6 +458,8 @@ mod tests {
     use crate::trace::{SpanData, SpanExporter};
     use futures_util::Future;
     use std::fmt::Debug;
+    use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+    use std::sync::Arc;
     use std::time::Duration;
 
     struct BlockingExporter<D> {
@@ -463,6 +488,39 @@ mod tests {
         }
     }
 
+    /// Exporter that records whether two exports overlap in time.
+    struct TrackingExporter {
+        /// Artificial delay to keep each export alive for a while.
+        delay: Duration,
+        /// Current number of in-flight exports.
+        active: Arc<AtomicUsize>,
+        /// Set to true the first time we see overlap.
+        concurrent_seen: Arc<AtomicBool>,
+    }
+
+    impl Debug for TrackingExporter {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            f.write_str("tracking exporter")
+        }
+    }
+
+    impl SpanExporter for TrackingExporter {
+        async fn export(&self, _batch: Vec<SpanData>) -> crate::error::OTelSdkResult {
+            // Increment in-flight counter and note any overlap.
+            let inflight = self.active.fetch_add(1, Ordering::SeqCst) + 1;
+            if inflight > 1 {
+                self.concurrent_seen.store(true, Ordering::SeqCst);
+            }
+
+            // Keep the export "busy" for a bit.
+            tokio::time::sleep(self.delay).await;
+
+            // Decrement counter.
+            self.active.fetch_sub(1, Ordering::SeqCst);
+            Ok(())
+        }
+    }
+
     #[test]
     fn test_build_batch_span_processor_builder() {
         let mut env_vars = vec![
@@ -532,8 +590,8 @@ mod tests {
         );
     }
 
-    // If the time_out is true, then the result suppose to ended with timeout.
-    // otherwise the exporter should be able to export within time out duration.
+    // If `time_out` is `true`, then the export should fail with a timeout.
+    // Else, the exporter should be able to export within the timeout duration.
     async fn timeout_test_tokio(time_out: bool) {
         let config = BatchConfig {
             max_export_timeout: Duration::from_millis(if time_out { 5 } else { 60 }),
@@ -557,24 +615,92 @@ mod tests {
         assert!(shutdown_res.is_ok());
     }
 
-    #[test]
-    fn test_timeout_tokio_timeout() {
+    #[tokio::test(flavor = "multi_thread")]
+    async fn test_timeout_tokio_timeout() {
         // If time_out is true, then we ask exporter to block for 60s and set timeout to 5s.
         // If time_out is false, then we ask the exporter to block for 5s and set timeout to 60s.
         // Either way, the test should be finished within 5s.
-        let runtime = tokio::runtime::Builder::new_multi_thread()
-            .enable_all()
-            .build()
-            .unwrap();
-        runtime.block_on(timeout_test_tokio(true));
+        timeout_test_tokio(true).await;
     }
 
-    #[test]
-    fn test_timeout_tokio_not_timeout() {
-        let runtime = tokio::runtime::Builder::new_multi_thread()
-            .enable_all()
-            .build()
-            .unwrap();
-        runtime.block_on(timeout_test_tokio(false));
+    #[tokio::test(flavor = "multi_thread")]
+    async fn test_timeout_tokio_not_timeout() {
+        timeout_test_tokio(false).await;
+    }
+
+    #[tokio::test(flavor = "multi_thread")]
+    async fn test_concurrent_exports_expected() {
+        // Shared state for the exporter.
+        let active = Arc::new(AtomicUsize::new(0));
+        let concurrent_seen = Arc::new(AtomicBool::new(false));
+
+        let exporter = TrackingExporter {
+            delay: Duration::from_millis(50),
+            active: active.clone(),
+            concurrent_seen: concurrent_seen.clone(),
+        };
+
+        // Intentionally tiny batch-size so every span forces an export.
+        let config = BatchConfig {
+            max_export_batch_size: 1,
+            max_queue_size: 16,
+            scheduled_delay: Duration::from_secs(3600), // effectively disabled
+            max_export_timeout: Duration::from_secs(5),
+            max_concurrent_exports: 2, // what we want to verify
+        };
+
+        // Spawn the processor.
+        let processor = BatchSpanProcessor::new(exporter, config, runtime::Tokio);
+
+        // Finish three spans in rapid succession.
+        processor.on_end(new_test_export_span_data());
+        processor.on_end(new_test_export_span_data());
+        processor.on_end(new_test_export_span_data());
+
+        // Wait until everything has been exported.
+        processor.force_flush().expect("force flush failed");
+        processor.shutdown().expect("shutdown failed");
+
+        // Expect at least one period with >1 export in flight.
+        assert!(
+            concurrent_seen.load(Ordering::SeqCst),
+            "exports never overlapped, processor is still serialising them"
+        );
+    }
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_exports_serial_when_max_concurrent_exports_1() {
+        let active = Arc::new(AtomicUsize::new(0));
+        let concurrent_seen = Arc::new(AtomicBool::new(false));
+
+        let exporter = TrackingExporter {
+            delay: Duration::from_millis(50),
+            active: active.clone(),
+            concurrent_seen: concurrent_seen.clone(),
+        };
+
+        let config = BatchConfig {
+            max_export_batch_size: 1,
+            max_queue_size: 16,
+            scheduled_delay: Duration::from_secs(3600),
+            max_export_timeout: Duration::from_secs(5),
+            max_concurrent_exports: 1, // what we want to verify
+        };
+
+        let processor = BatchSpanProcessor::new(exporter, config, runtime::Tokio);
+
+        // Finish several spans quickly.
+        processor.on_end(new_test_export_span_data());
+        processor.on_end(new_test_export_span_data());
+        processor.on_end(new_test_export_span_data());
+
+        processor.force_flush().expect("force flush failed");
+        processor.shutdown().expect("shutdown failed");
+
+        // There must never have been more than one export in flight.
+        assert!(
+            !concurrent_seen.load(Ordering::SeqCst),
+            "exports overlapped even though max_concurrent_exports was 1"
+        );
     }
 }
