Add proper enqueue sync

sdk/trace/batch_span_processor.go

@@ -17,6 +17,7 @@ package trace
 import (
 	"context"
 	"errors"
+	"log"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -70,9 +71,10 @@ type BatchSpanProcessor struct {
 	queue   chan *export.SpanData
 	dropped uint32
 
-	stopWait sync.WaitGroup
+	enqueueWait sync.WaitGroup
-	stopOnce sync.Once
+	stopWait    sync.WaitGroup
-	stopCh   chan struct{}
+	stopOnce    sync.Once
+	stopCh      chan struct{}
 }
 
 var _ SpanProcessor = (*BatchSpanProcessor)(nil)
@@ -192,45 +194,58 @@ loop:
 		}
 	}
 
-	// Consume queue before close to unblock enqueue and prevent
+	go func() {
-	// "panic: send on closed channel".
+		bsp.enqueueWait.Wait()
+		close(bsp.queue)
+	}()
+
 	for {
+		if !timer.Stop() {
+			<-timer.C
+		}
+		const waitTimeout = 30 * time.Second
+		timer.Reset(waitTimeout)
+
 		select {
 		case sd := <-bsp.queue:
 			if sd == nil {
 				exportSpans()
 				return
 			}
+
 			if sd.SpanContext.IsSampled() {
 				batch = append(batch, sd)
 				if len(batch) == bsp.o.MaxExportBatchSize {
 					exportSpans()
 				}
 			}
-		default:
+		case <-timer.C:
-			close(bsp.queue)
+			log.Println("bsp.enqueueWait timeout")
+			exportSpans()
+			return
 		}
 	}
 }
 
 func (bsp *BatchSpanProcessor) enqueue(sd *export.SpanData) {
+	bsp.enqueueWait.Add(1)
+
 	select {
 	case <-bsp.stopCh:
+		bsp.enqueueWait.Done()
 		return
 	default:
 	}
+
 	if bsp.o.BlockOnQueueFull {
 		bsp.queue <- sd
 	} else {
-		var ok bool
 		select {
 		case bsp.queue <- sd:
-			ok = true
 		default:
-			ok = false
-		}
-		if !ok {
 			atomic.AddUint32(&bsp.dropped, 1)
 		}
 	}
+
+	bsp.enqueueWait.Done()
 }