chore: make state field volatile in AsyncResultSetImpl (googleapis#3550)

* chore: make state field volatile in AsyncResultSetImpl

Mark the `state` field in `AsyncResultSetImpl` volatile, as it is
inspected by different threads.

* fix: protect writes with monitor

---------

Co-authored-by: rahul2393 <[email protected]>

Author: olavloite

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

@@ -112,9 +112,9 @@ private enum State {
    * Listeners that will be called when the {@link AsyncResultSetImpl} has finished fetching all
    * rows and any underlying transaction or session can be closed.
    */
-  private Collection<Runnable> listeners = new LinkedList<>();
+  private final Collection<Runnable> listeners = new LinkedList<>();
 
-  private State state = State.INITIALIZED;
+  private volatile State state = State.INITIALIZED;
 
   /** This variable indicates that produce rows thread is initiated */
   private volatile boolean produceRowsInitiated;
@@ -498,10 +498,12 @@ public ApiFuture<Void> setCallback(Executor exec, ReadyCallback cb) {
   }
 
   private void initiateProduceRows() {
-    if (this.state == State.STREAMING_INITIALIZED) {
-      this.state = State.RUNNING;
+    synchronized (monitor) {
+      if (this.state == State.STREAMING_INITIALIZED) {
+        this.state = State.RUNNING;
+      }
+      produceRowsInitiated = true;
     }
-    produceRowsInitiated = true;
     this.service.execute(new ProduceRowsRunnable());
   }