Merge pull request apache#21739: Fix for increased FAILED_PRECONDITIO…

…N errors in BQ Read API.

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryStorageStreamSource.java

@@ -299,6 +299,7 @@ public synchronized BigQueryStorageStreamSource<T> getCurrentSource() {
     }
 
     @Override
+    @SuppressWarnings("ReturnValueIgnored")
     public @Nullable BoundedSource<T> splitAtFraction(double fraction) {
       // Because superclass cannot have preconditions around these variables, cannot use
       // @RequiresNonNull
@@ -355,6 +356,10 @@ public synchronized BigQueryStorageStreamSource<T> getCurrentSource() {
                       .build(),
                   source.readSession.getTable());
           newResponseIterator = newResponseStream.iterator();
+          // The following line is required to trigger the `FailedPreconditionException` on which
+          // the SplitReadStream validation logic depends. Removing it will cause incorrect
+          // split operations to succeed.
+          newResponseIterator.hasNext();
         } catch (FailedPreconditionException e) {
           // The current source has already moved past the split point, so this split attempt
           // is unsuccessful.