Add ExchangeReaderWriter.getResult()

java/flight/flight-core/src/main/java/org/apache/arrow/flight/FlightClient.java

@@ -412,6 +412,16 @@ public ClientStreamListener getWriter() {
       return writer;
     }
 
+    /**
+     * Make sure stream is drained. You must call this to be notified of any errors that may have
+     * happened after the exchange is complete. This should be called after `getWriter().completed()`
+     * instead of `getWriter().getResult`.
+     */
+    public void getResult() {
+      // After exchange is complete, make sure stream is drained to propagate errors through reader
+      reader.next();
+    }
+
     /** Shut down the streams in this call. */
     @Override
     public void close() throws Exception {

java/flight/flight-core/src/test/java/org/apache/arrow/flight/TestDoExchange.java

@@ -392,7 +392,7 @@ public void testDoExchangeError() throws Exception {
       stream.getWriter().completed();
 
       // Must call reader.next() to get any errors after exchange, will return false if no error
-      final FlightRuntimeException fre = assertThrows(FlightRuntimeException.class, reader::next);
+      final FlightRuntimeException fre = assertThrows(FlightRuntimeException.class, stream::getResult);
       assertEquals("error completing exchange", fre.status().description());
     }
   }
@@ -438,7 +438,7 @@ public void testCloseWithMetadata() throws Exception {
 
       // Close our write channel and ensure the server also closes theirs
       stream.getWriter().completed();
-      assertFalse(reader.next());
+      stream.getResult();
 
       // Not necessary to close reader here, but check closing twice doesn't lead to negative refcnt from metadata
       stream.getReader().close();