fix(java-client): guard against NPE in FutureGroup.waitAllComplete when future cause is null

java-client/src/main/java/org/apache/pegasus/client/FutureGroup.java

@@ -112,10 +112,15 @@ public void waitAllComplete(List<Pair<PException, Result>> results, int timeoutM
       if (fu.isSuccess()) {
         results.add(Pair.of(null, fu.getNow()));
       } else {
+        // fu.cause() is null when the future never completed (e.g. await timed out),
+        // in which case dereferencing it throws a NullPointerException. Guard against
+        // that so a failing-but-incomplete task is reported instead of crashing the
+        // caller. See https://github.com/apache/incubator-pegasus/issues/2152.
+        Throwable cause = fu.cause();
+        String causeMsg =
+            cause != null ? cause.getMessage() : "unknown cause (future not completed)";
         results.add(
-            Pair.of(
-                new PException("async task #[" + i + "] await failed: " + fu.cause().getMessage()),
-                null));
+            Pair.of(new PException("async task #[" + i + "] await failed: " + causeMsg), null));
       }
     }
   }

java-client/src/test/java/org/apache/pegasus/client/TestFutureGroup.java

@@ -18,13 +18,23 @@
  */
 package org.apache.pegasus.client;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.fail;
+import static org.mockito.ArgumentMatchers.anyLong;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
+import io.netty.util.concurrent.Future;
 import io.netty.util.concurrent.Promise;
 import io.netty.util.concurrent.SingleThreadEventExecutor;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.concurrent.Executors;
 import java.util.concurrent.atomic.AtomicBoolean;
+import org.apache.commons.lang3.tuple.Pair;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.TestInfo;
 
@@ -100,4 +110,28 @@ public void testFutureWaitTimeout(TestInfo testInfo) throws Exception {
     }
     fail();
   }
+
+  @Test
+  public void testWaitAllCompleteHandlesNullCause(TestInfo testInfo) throws Exception {
+    // Reproduces #2152: a Future that is neither completed nor successful (e.g. its
+    // await timed out before it ever resolved) reports isSuccess()==false and
+    // cause()==null. waitAllComplete must not dereference the null cause.
+    @SuppressWarnings("unchecked")
+    Future<String> unfinished = mock(Future.class);
+    when(unfinished.await(anyLong())).thenReturn(false); // timed out, still not done
+    when(unfinished.isSuccess()).thenReturn(false);
+    when(unfinished.cause()).thenReturn(null);
+
+    FutureGroup<String> group = new FutureGroup<>(1);
+    group.add(unfinished);
+
+    List<Pair<PException, String>> results = new ArrayList<>();
+    // Before the fix this threw NullPointerException inside waitAllComplete.
+    group.waitAllComplete(results, 5000);
+
+    assertEquals(1, results.size());
+    assertNotNull(results.get(0).getLeft()); // a PException is reported
+    assertNull(results.get(0).getRight()); // no result
+    System.err.println(testInfo.getDisplayName() + ": " + results.get(0).getLeft().toString());
+  }
 }