fix concurrent close

Author: xborder

flight/flight-sql-jdbc-core/src/main/java/org/apache/arrow/driver/jdbc/utils/FlightEndpointDataQueue.java

@@ -97,7 +97,8 @@ interface EndpointStreamSupplier {
 
   private CloseableEndpointStreamPair next(final EndpointStreamSupplier endpointStreamSupplier)
       throws SQLException {
-    checkOpen();
+    // No checkOpen here: a concurrent close() is an end-of-stream signal for readers,
+    // not an error. Cancelled in-flight streams still surface via future.get() below.
     while (!futures.isEmpty()) {
       final Future<CloseableEndpointStreamPair> future = endpointStreamSupplier.get();
       futures.remove(future);

flight/flight-sql-jdbc-core/src/test/java/org/apache/arrow/driver/jdbc/utils/FlightEndpointDataQueueTest.java

@@ -48,10 +48,9 @@ public void testNextShouldRetrieveNullIfEmpty() throws Exception {
   }
 
   @Test
-  public void testNextShouldThrowExceptionUponClose() throws Exception {
+  public void testNextShouldReturnNullUponClose() throws Exception {
     queue.close();
-    ThrowableAssertionUtils.simpleAssertThrowableClass(
-        IllegalStateException.class, () -> queue.next());
+    assertThat(queue.next(), is(nullValue()));
   }
 
   @Test