chore(spanner): unflake unit tests

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

@@ -797,7 +797,6 @@ CloseableIterator<PartialResultSet> startStream(
                     isRouteToLeader());
             session.markUsed(clock.instant());
             stream.setCall(call, request.getTransaction().hasBegin());
-            call.request(prefetchChunks);
             return stream;
           }
 
@@ -992,7 +991,6 @@ CloseableIterator<PartialResultSet> startStream(
                     isRouteToLeader());
             session.markUsed(clock.instant());
             stream.setCall(call, /* withBeginTransaction = */ builder.getTransaction().hasBegin());
-            call.request(prefetchChunks);
             return stream;
           }
 

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

@@ -158,6 +158,9 @@ interface CloseableIterator<T> extends Iterator<T> {
     default boolean initiateStreaming(AsyncResultSet.StreamMessageListener streamMessageListener) {
       return false;
     }
+
+    /** it requests the initial prefetch chunks from gRPC stream */
+    default void requestPrefetchChunks() {};
   }
 
   static double valueProtoToFloat64(com.google.protobuf.Value proto) {

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

@@ -16,6 +16,7 @@
 
 package com.google.cloud.spanner;
 
+import com.google.api.core.InternalApi;
 import com.google.api.gax.rpc.ApiCallContext;
 import com.google.cloud.spanner.AbstractResultSet.CloseableIterator;
 import com.google.cloud.spanner.spi.v1.SpannerRpc;
@@ -39,6 +40,7 @@ class GrpcStreamIterator extends AbstractIterator<PartialResultSet>
     implements CloseableIterator<PartialResultSet> {
   private static final Logger logger = Logger.getLogger(GrpcStreamIterator.class.getName());
   static final PartialResultSet END_OF_STREAM = PartialResultSet.newBuilder().build();
+  private final int prefetchChunks;
   private AsyncResultSet.StreamMessageListener streamMessageListener;
 
   private final ConsumerImpl consumer;
@@ -60,6 +62,7 @@ class GrpcStreamIterator extends AbstractIterator<PartialResultSet>
   GrpcStreamIterator(
       Statement statement, int prefetchChunks, boolean cancelQueryWhenClientIsClosed) {
     this.statement = statement;
+    this.prefetchChunks = prefetchChunks;
     this.consumer = new ConsumerImpl(cancelQueryWhenClientIsClosed);
     // One extra to allow for END_OF_STREAM message.
     this.stream = new LinkedBlockingQueue<>(prefetchChunks + 1);
@@ -102,6 +105,13 @@ public void close(@Nullable String message) {
     }
   }
 
+  @Override
+  @InternalApi
+  public void requestPrefetchChunks() {
+    Preconditions.checkState(call != null, "The StreamingCall object is not initialized");
+    call.request(prefetchChunks);
+  }
+
   @Override
   public boolean isWithBeginTransaction() {
     return withBeginTransaction;

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

@@ -326,6 +326,7 @@ private void startGrpcStreaming() {
         // When start a new stream set the Span as current to make the gRPC Span a child of
         // this Span.
         stream = checkNotNull(startStream(resumeToken, streamMessageListener));
+        stream.requestPrefetchChunks();
       }
     }
   }