add more logs

google-cloud-spanner/src/main/java/com/google/cloud/spanner/ResumableStreamIterator.java

@@ -71,6 +71,7 @@ abstract class ResumableStreamIterator extends AbstractIterator<PartialResultSet
   private CloseableIterator<PartialResultSet> stream;
   private ByteString resumeToken;
   private boolean finished;
+  private final Object monitor = new Object();
   /**
    * Indicates whether it is currently safe to retry RPCs. This will be {@code false} if we have
    * reached the maximum buffer size without seeing a restart token; in this case, we will drain the
@@ -318,17 +319,19 @@ && prepareIteratorForRetryOnDifferentGrpcChannel()) {
   }
 
   private void startGrpcStreaming() {
-    System.out.println("Starting Stream " + stream + " " + streamMessageListener);
-    if (stream == null) {
-      span.addAnnotation(
-          "Starting/Resuming stream",
-          "ResumeToken",
-          resumeToken == null ? "null" : resumeToken.toStringUtf8());
-      try (IScope scope = tracer.withSpan(span)) {
-        // When start a new stream set the Span as current to make the gRPC Span a child of
-        // this Span.
-        System.out.println("Creating Stream " + stream + " " + streamMessageListener);
-        stream = checkNotNull(startStream(resumeToken, streamMessageListener));
+    synchronized (monitor) {
+      System.out.println("Starting Stream " + stream + " " + streamMessageListener);
+      if (stream == null) {
+        span.addAnnotation(
+            "Starting/Resuming stream",
+            "ResumeToken",
+            resumeToken == null ? "null" : resumeToken.toStringUtf8());
+        try (IScope scope = tracer.withSpan(span)) {
+          // When start a new stream set the Span as current to make the gRPC Span a child of
+          // this Span.
+          System.out.println("Creating Stream " + stream + " " + streamMessageListener);
+          stream = checkNotNull(startStream(resumeToken, streamMessageListener));
+        }
       }
     }
   }