Add helper for combining multiple completable futures

3 files changed, 138 insertions, 0 deletions

diff --git a/vespajlib/pom.xml b/vespajlib/pom.xml
index 302a3c6f5bf..68639d30ab2 100644
--- a/vespajlib/pom.xml
+++ b/vespajlib/pom.xml
@@ -78,6 +78,11 @@
       <artifactId>jackson-databind</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter</artifactId>
+      <scope>test</scope>
+    </dependency>
 
   </dependencies>
   <build>
diff --git a/vespajlib/src/main/java/com/yahoo/concurrent/CompletableFutures.java b/vespajlib/src/main/java/com/yahoo/concurrent/CompletableFutures.java
new file mode 100644
index 00000000000..b1fa6a9438d
--- /dev/null
+++ b/vespajlib/src/main/java/com/yahoo/concurrent/CompletableFutures.java
@@ -0,0 +1,67 @@
+// Copyright Verizon Media. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.concurrent;
+
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+
+/**
+ * Helper for {@link java.util.concurrent.CompletableFuture} / {@link java.util.concurrent.CompletionStage}.
+ *
+ * @author bjorncs
+ */
+public class CompletableFutures {
+
+    private CompletableFutures() {}
+
+    /**
+     * Returns a new completable future that is either
+     * - completed when any of the provided futures complete without exception
+     * - completed exceptionally once all provided futures complete exceptionally
+     */
+    public static <T> CompletableFuture<T> firstOf(List<CompletableFuture<T>> futures) {
+        class Combiner {
+            final Object monitor = new Object();
+            final CompletableFuture<T> combined = new CompletableFuture<>();
+            final int futuresCount;
+
+            Throwable error = null;
+            int exceptionCount = 0;
+
+            Combiner(int futuresCount) { this.futuresCount = futuresCount; }
+
+            void onCompletion(T value, Throwable error) {
+                if (combined.isDone()) return;
+                T valueToComplete = null;
+                Throwable exceptionToComplete = null;
+
+                synchronized (monitor) {
+                    if (value != null) {
+                        valueToComplete = value;
+                    } else {
+                        if (this.error == null) {
+                            this.error = error;
+                        } else {
+                            this.error.addSuppressed(error);
+                        }
+                        if (++exceptionCount == futuresCount) {
+                            exceptionToComplete = this.error;
+                        }
+                    }
+                }
+                if (valueToComplete != null) {
+                    combined.complete(value);
+                } else if (exceptionToComplete != null) {
+                    combined.completeExceptionally(exceptionToComplete);
+                }
+            }
+        }
+
+        int size = futures.size();
+        if (size == 0) throw new IllegalArgumentException();
+        if (size == 1) return futures.get(0);
+        Combiner combiner = new Combiner(size);
+        futures.forEach(future -> future.whenComplete(combiner::onCompletion));
+        return combiner.combined;
+    }
+
+}
diff --git a/vespajlib/src/test/java/com/yahoo/concurrent/CompletableFuturesTest.java b/vespajlib/src/test/java/com/yahoo/concurrent/CompletableFuturesTest.java
new file mode 100644
index 00000000000..cf9c36537d9
--- /dev/null
+++ b/vespajlib/src/test/java/com/yahoo/concurrent/CompletableFuturesTest.java
@@ -0,0 +1,66 @@
+package com.yahoo.concurrent;// Copyright Verizon Media. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionException;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.fail;
+
+/**
+ * @author bjorncs
+ */
+class CompletableFuturesTest {
+
+    @Test
+    public void firstof_completes_when_first_futures_has_completed() {
+        CompletableFuture<String> f1 = new CompletableFuture<>();
+        CompletableFuture<String> f2 = new CompletableFuture<>();
+        CompletableFuture<String> f3 = new CompletableFuture<>();
+        CompletableFuture<String> result = CompletableFutures.firstOf(List.of(f1, f2, f3));
+        f1.complete("success");
+        assertTrue(result.isDone());
+        assertFalse(result.isCompletedExceptionally());
+        assertEquals("success", result.join());
+    }
+
+    @Test
+    public void firstof_completes_if_any_futures_completes() {
+        CompletableFuture<String> f1 = new CompletableFuture<>();
+        CompletableFuture<String> f2 = new CompletableFuture<>();
+        CompletableFuture<String> f3 = new CompletableFuture<>();
+        CompletableFuture<String> result = CompletableFutures.firstOf(List.of(f1, f2, f3));
+        f1.completeExceptionally(new Throwable("t1"));
+        f2.completeExceptionally(new Throwable("t2"));
+        f3.complete("success");
+        assertTrue(result.isDone());
+        assertFalse(result.isCompletedExceptionally());
+        assertEquals("success", result.join());
+    }
+
+    @Test
+    public void firstof_completes_exceptionally_when_all_futures_have_complete_exceptionally() {
+        CompletableFuture<String> f1 = new CompletableFuture<>();
+        CompletableFuture<String> f2 = new CompletableFuture<>();
+        CompletableFuture<String> f3 = new CompletableFuture<>();
+        CompletableFuture<String> result = CompletableFutures.firstOf(List.of(f1, f2, f3));
+        f1.completeExceptionally(new Throwable("t1"));
+        f2.completeExceptionally(new Throwable("t2"));
+        f3.completeExceptionally(new Throwable("t3"));
+        assertTrue(result.isDone());
+        assertTrue(result.isCompletedExceptionally());
+        try {
+            result.join();
+            fail("Exception expected");
+        } catch (CompletionException e) {
+            Throwable cause = e.getCause();
+            assertEquals("t1", cause.getMessage());
+            assertEquals(2, cause.getSuppressed().length);
+        }
+    }
+
+}
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