fix: Fix stack-overflow inducing infinite feedback loop when calling tracing::event! after SdkLoggerProvider::shutdown

Author: lucamuscat

opentelemetry-appender-tracing/tests/test_stack_overflow_after_shutdown.rs

@@ -0,0 +1,20 @@
+use opentelemetry_appender_tracing::layer;
+use opentelemetry_sdk::logs::SdkLoggerProvider;
+use tracing::info;
+use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
+
+#[test]
+fn test_no_stack_overflow_when_event_is_emitted_after_shutdown() {
+    let exporter = opentelemetry_stdout::LogExporter::default();
+    let provider: SdkLoggerProvider = SdkLoggerProvider::builder()
+        .with_batch_exporter(exporter)
+        .build();
+
+    let otel_layer = layer::OpenTelemetryTracingBridge::new(&provider);
+
+    tracing_subscriber::registry().with(otel_layer).init();
+
+    provider.shutdown().unwrap();
+
+    info!("Don't crash")
+}

opentelemetry-sdk/src/logs/batch_log_processor.rs

@@ -207,12 +207,20 @@ impl LogProcessor for BatchLogProcessor {
                 }
             }
             Err(mpsc::TrySendError::Disconnected(_)) => {
-                // Given background thread is the only receiver, and it's
-                // disconnected, it indicates the thread is shutdown
-                otel_warn!(
-                    name: "BatchLogProcessor.Emit.AfterShutdown",
-                    message = "Logs are being emitted even after Shutdown. This indicates incorrect lifecycle management of OTelLoggerProvider in application. Logs will not be exported."
-                );
+                // Re-emitting the following warning here may lead to a stack overflow due to the
+                // warning produced by this branch being reconsumed by this same branch, producing a
+                // cycle. This may occur when integrating with the `tracing` crate due to
+                // `otel_warn!` possibly calling `tracing::warn!` under the hood, which will emit
+                // the warning back into the otel log processor.
+                if matches!(record.event_name, Some(event_name) if event_name != Self::AFTER_SHUTDOWN_WARNING_NAME)
+                {
+                    // Given background thread is the only receiver, and it's
+                    // disconnected, it indicates the thread is shutdown
+                    otel_warn!(
+                        name: BatchLogProcessor::AFTER_SHUTDOWN_WARNING_NAME,
+                        message = "Logs are being emitted even after Shutdown. This indicates incorrect lifecycle management of OTelLoggerProvider in application. Logs will not be exported."
+                    );
+                };
             }
         }
     }
@@ -325,6 +333,8 @@ impl LogProcessor for BatchLogProcessor {
 }
 
 impl BatchLogProcessor {
+    const AFTER_SHUTDOWN_WARNING_NAME: &str = "BatchLogProcessor.Emit.AfterShutdown";
+
     pub(crate) fn new<E>(mut exporter: E, config: BatchConfig) -> Self
     where
         E: LogExporter + Send + Sync + 'static,