Backport: CompletableContext class to avoid throwable

libs/core/src/main/java/org/elasticsearch/common/concurrent/CompletableContext.java

@@ -0,0 +1,63 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.common.concurrent;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.function.BiConsumer;
+
+/**
+ * A thread-safe completable context that allows listeners to be attached. This class relies on the
+ * {@link CompletableFuture} for the concurrency logic. However, it does not accept {@link Throwable} as
+ * an exceptional result. This allows attaching listeners that only handle {@link Exception}.
+ *
+ * @param <T> the result type
+ */
+public class CompletableContext<T> {
+
+    private final CompletableFuture<T> completableFuture = new CompletableFuture<>();
+
+    public void addListener(BiConsumer<T, ? super Exception> listener) {
+        BiConsumer<T, Throwable> castThrowable = (v, t) -> {
+            if (t == null) {
+                listener.accept(v, null);
+            } else {
+                assert !(t instanceof Error) : "Cannot be error";
+                listener.accept(v, (Exception) t);
+            }
+        };
+        completableFuture.whenComplete(castThrowable);
+    }
+
+    public boolean isDone() {
+        return completableFuture.isDone();
+    }
+
+    public boolean isCompletedExceptionally() {
+        return completableFuture.isCompletedExceptionally();
+    }
+
+    public boolean completeExceptionally(Exception ex) {
+        return completableFuture.completeExceptionally(ex);
+    }
+
+    public boolean complete(T value) {
+        return completableFuture.complete(value);
+    }
+}