Be more defensive on waiting for queue.

Update the handling of the force flush span so we only wait on the
channel if we were able to enqueue the span to the queue.

sdk/trace/batch_span_processor.go

@@ -168,13 +168,13 @@ func (bsp *batchSpanProcessor) ForceFlush(ctx context.Context) error {
 	var err error
 	if bsp.e != nil {
 		flushCh := make(chan struct{})
-		bsp.enqueueBlockOnQueueFull(ctx, forceFlushSpan{flushed: flushCh}, true)
-
-		select {
-		case <-flushCh:
-			// Processed any items in queue prior to ForceFlush being called
-		case <-ctx.Done():
-			return ctx.Err()
+		if bsp.enqueueBlockOnQueueFull(ctx, forceFlushSpan{flushed: flushCh}, true) {
+			select {
+			case <-flushCh:
+				// Processed any items in queue prior to ForceFlush being called
+			case <-ctx.Done():
+				return ctx.Err()
+			}
 		}
 
 		wait := make(chan error)
@@ -323,9 +323,9 @@ func (bsp *batchSpanProcessor) enqueue(sd ReadOnlySpan) {
 	bsp.enqueueBlockOnQueueFull(context.TODO(), sd, bsp.o.BlockOnQueueFull)
 }
 
-func (bsp *batchSpanProcessor) enqueueBlockOnQueueFull(ctx context.Context, sd ReadOnlySpan, block bool) {
+func (bsp *batchSpanProcessor) enqueueBlockOnQueueFull(ctx context.Context, sd ReadOnlySpan, block bool) bool {
 	if !sd.SpanContext().IsSampled() {
-		return
+		return false
 	}
 
 	// This ensures the bsp.queue<- below does not panic as the
@@ -345,22 +345,24 @@ func (bsp *batchSpanProcessor) enqueueBlockOnQueueFull(ctx context.Context, sd R
 
 	select {
 	case <-bsp.stopCh:
-		return
+		return false
 	default:
 	}
 
 	if block {
 		select {
 		case bsp.queue <- sd:
-			return
+			return true
 		case <-ctx.Done():
-			return
+			return false
 		}
 	}
 
 	select {
 	case bsp.queue <- sd:
+		return true
 	default:
 		atomic.AddUint32(&bsp.dropped, 1)
 	}
+	return false
 }