JAVA-3167: CompletableFutures.allSuccessful() may return never completed future

patch by Lukasz Antoniak; reviewed by Andy Tolbert, and Bret McGuire for JAVA-3167

Author: lukasz-antoniak

core/src/main/java/com/datastax/oss/driver/internal/core/util/concurrent/CompletableFutures.java

@@ -100,7 +100,10 @@ public static <T> CompletionStage<Void> allSuccessful(List<CompletionStage<T>> i
                 } else {
                   Throwable finalError = errors.get(0);
                   for (int i = 1; i < errors.size(); i++) {
-                    finalError.addSuppressed(errors.get(i));
+                    Throwable suppressedError = errors.get(i);
+                    if (finalError != suppressedError) {
+                      finalError.addSuppressed(suppressedError);
+                    }
                   }
                   result.completeExceptionally(finalError);
                 }

core/src/test/java/com/datastax/oss/driver/internal/core/util/concurrent/CompletableFuturesTest.java

@@ -0,0 +1,29 @@
+package com.datastax.oss.driver.internal.core.util.concurrent;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Arrays;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.TimeUnit;
+import org.junit.Test;
+
+public class CompletableFuturesTest {
+  @Test
+  public void should_not_suppress_identical_exceptions() throws Exception {
+    RuntimeException error = new RuntimeException();
+    CompletableFuture<Void> future1 = new CompletableFuture<>();
+    future1.completeExceptionally(error);
+    CompletableFuture<Void> future2 = new CompletableFuture<>();
+    future2.completeExceptionally(error);
+    try {
+      // if timeout exception is thrown, it indicates that CompletableFutures.allSuccessful()
+      // did not complete the returned future and potentially caller will wait infinitely
+      CompletableFutures.allSuccessful(Arrays.asList(future1, future2))
+          .toCompletableFuture()
+          .get(1, TimeUnit.SECONDS);
+    } catch (ExecutionException e) {
+      assertThat(e.getCause()).isEqualTo(error);
+    }
+  }
+}