[FAB-9124] Fix race in nextBlock

Send result over chan instead of using shared, named return value.

Change-Id: I3c5edbdbd792f7787a21f6de7f2fde2df5ffd9b5
Signed-off-by: Matthew Sykes <sykesmat@us.ibm.com>

Author: sykesm

common/deliver/deliver.go

@@ -306,16 +306,21 @@ func (h *Handler) validateChannelHeader(ctx context.Context, chdr *cb.ChannelHea
 	return nil
 }
 
-func nextBlock(cursor blockledger.Iterator, cancel <-chan struct{}) (block *cb.Block, status cb.Status) {
-	done := make(chan struct{})
+func nextBlock(cursor blockledger.Iterator, cancel <-chan struct{}) (*cb.Block, cb.Status) {
+	type result struct {
+		block  *cb.Block
+		status cb.Status
+	}
+
+	resultCh := make(chan *result, 1)
 	go func() {
-		defer close(done)
-		block, status = cursor.Next()
+		block, status := cursor.Next()
+		resultCh <- &result{block, status}
 	}()
 
 	select {
-	case <-done:
-		return
+	case r := <-resultCh:
+		return r.block, r.status
 	case <-cancel:
 		logger.Warningf("Aborting deliver for request because of background error")
 		return nil, cb.Status_SERVICE_UNAVAILABLE