Fix panic, deadlock and race in BatchSpanProcessor

CHANGELOG.md

@@ -43,6 +43,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Fix `resource.WithHostID()` to not set an empty `host.id`. (#4317)
 - Use the instrument identifying fields to cache aggregators and determine duplicate instrument registrations in `go.opentelemetry.io/otel/sdk/metric`. (#4337)
 - Detect duplicate instruments for case-insensitive names in `go.opentelemetry.io/otel/sdk/metric`. (#4338)
+- Fix possible deadlock and race condition in batch span processor in `go.opentelemetry.io/otel/sdk/trace`. (#4353)
 
 ## [1.16.0/0.39.0] 2023-05-18
 

sdk/trace/batch_span_processor.go

@@ -16,7 +16,6 @@
 
 import (
 	"context"
-	"runtime"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -84,6 +83,7 @@
 	stopWait   sync.WaitGroup
 	stopOnce   sync.Once
 	stopCh     chan struct{}
+	stopped    atomic.Bool
 }
 
 var _ SpanProcessor = (*batchSpanProcessor)(nil)
@@ -137,6 +137,11 @@
 
 // OnEnd method enqueues a ReadOnlySpan for later processing.
 func (bsp *batchSpanProcessor) OnEnd(s ReadOnlySpan) {
+	// Do not enqueue spans ater Shutdown.
+	if bsp.stopped.Load() {
+		return
+	}
+
 	// Do not enqueue spans if we are just going to drop them.
 	if bsp.e == nil {
 		return
@@ -149,6 +154,7 @@
 func (bsp *batchSpanProcessor) Shutdown(ctx context.Context) error {
 	var err error
 	bsp.stopOnce.Do(func() {
+		bsp.stopped.Store(true)
 		wait := make(chan struct{})
 		go func() {
 			close(bsp.stopCh)
@@ -181,11 +187,19 @@
 
 // ForceFlush exports all ended spans that have not yet been exported.
 func (bsp *batchSpanProcessor) ForceFlush(ctx context.Context) error {
+	// Do nothing after Shutdown.
+	if bsp.stopped.Load() {
+		return nil
+	}
+
 	var err error
 	if bsp.e != nil {
 		flushCh := make(chan struct{})
 		if bsp.enqueueBlockOnQueueFull(ctx, forceFlushSpan{flushed: flushCh}) {
 			select {
+			case <-bsp.stopCh:
+				// The batchSpanProcessor is Shutdown.
+				return nil
 			case <-flushCh:
 				// Processed any items in queue prior to ForceFlush being called
 			case <-ctx.Done():
@@ -326,13 +340,6 @@
 	for {
 		select {
 		case sd := <-bsp.queue:
-			if sd == nil {
-				if err := bsp.exportSpans(ctx); err != nil {
-					otel.Handle(err)
-				}
-				return
-			}
-
 			bsp.batchMutex.Lock()
 			bsp.batch = append(bsp.batch, sd)
 			shouldExport := len(bsp.batch) == bsp.o.MaxExportBatchSize
@@ -344,7 +351,11 @@
 				}
 			}
 		default:
-			close(bsp.queue)
+			// There are no more enqueued spans. Make final export.
+			if err := bsp.exportSpans(ctx); err != nil {
+				otel.Handle(err)
+			}
+			return
 		}
 	}
 }
@@ -358,34 +369,11 @@
 	}
 }
 
-func recoverSendOnClosedChan() {
-	x := recover()
-	switch err := x.(type) {
-	case nil:
-		return
-	case runtime.Error:
-		if err.Error() == "send on closed channel" {
-			return
-		}
-	}
-	panic(x)
-}
-
 func (bsp *batchSpanProcessor) enqueueBlockOnQueueFull(ctx context.Context, sd ReadOnlySpan) bool {
 	if !sd.SpanContext().IsSampled() {
 		return false
 	}
 
-	// This ensures the bsp.queue<- below does not panic as the
-	// processor shuts down.
-	defer recoverSendOnClosedChan()
-
-	select {
-	case <-bsp.stopCh:
-		return false
-	default:
-	}
-
 	select {
 	case bsp.queue <- sd:
 		return true
@@ -399,16 +387,6 @@
 		return false
 	}
 
-	// This ensures the bsp.queue<- below does not panic as the
-	// processor shuts down.
-	defer recoverSendOnClosedChan()
-
-	select {
-	case <-bsp.stopCh:
-		return false
-	default:
-	}
-
 	select {
 	case bsp.queue <- sd:
 		return true

sdk/trace/batch_span_processor_test.go

@@ -594,6 +594,49 @@ func TestBatchSpanProcessorForceFlushQueuedSpans(t *testing.T) {
 	}
 }
 
+func TestBatchSpanProcessorConcurrentSafe(t *testing.T) {
+	ctx := context.Background()
+	var bp testBatchExporter
+	bsp := sdktrace.NewBatchSpanProcessor(&bp)
+	tp := basicTracerProvider(t)
+	tp.RegisterSpanProcessor(bsp)
+	tr := tp.Tracer(t.Name())
+
+	var wg sync.WaitGroup
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		generateSpan(t, tr, testOption{genNumSpans: 1})
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		_ = bsp.ForceFlush(ctx)
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		_ = bsp.Shutdown(ctx)
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		_ = tp.ForceFlush(ctx)
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		_ = tp.Shutdown(ctx)
+	}()
+
+	wg.Wait()
+}
+
 func BenchmarkSpanProcessor(b *testing.B) {
 	tp := sdktrace.NewTracerProvider(
 		sdktrace.WithBatcher(