propagate errors during async refresh

hollow/src/main/java/com/netflix/hollow/api/consumer/HollowConsumer.java

@@ -244,8 +244,8 @@ public void triggerRefresh() {
      * <p>
      * This is an asynchronous call.
      */
-    public void triggerAsyncRefresh() {
-        triggerAsyncRefreshWithDelay(0);
+    public CompletableFuture<Void> triggerAsyncRefresh() {
+        return triggerAsyncRefreshWithDelay(0);
     }
 
     /**
@@ -256,10 +256,10 @@ public void triggerAsyncRefresh() {
      *
      * @param delayMillis the delay, in millseconds, before triggering the refresh
      */
-    public void triggerAsyncRefreshWithDelay(int delayMillis) {
+    public CompletableFuture<Void> triggerAsyncRefreshWithDelay(int delayMillis) {
         final long targetBeginTime = System.currentTimeMillis() + delayMillis;
 
-        refreshExecutor.execute(() -> {
+        return CompletableFuture.runAsync(() -> {
             try {
                 long delay = targetBeginTime - System.currentTimeMillis();
                 if (delay > 0)
@@ -278,7 +278,7 @@ public void triggerAsyncRefreshWithDelay(int delayMillis) {
                 LOG.log(Level.SEVERE, "Async refresh failed", e);
                 throw e;
             }
-        });
+        }, refreshExecutor);
     }
 
     /**

hollow/src/test/java/com/netflix/hollow/api/consumer/HollowProducerConsumerTests.java

@@ -32,6 +32,7 @@
 import com.netflix.hollow.tools.compact.HollowCompactor.CompactionConfig;
 import java.time.Duration;
 import java.util.BitSet;
+import java.util.concurrent.CompletionException;
 import java.util.concurrent.atomic.AtomicInteger;
 import org.junit.Assert;
 import org.junit.Before;
@@ -487,6 +488,28 @@ public void consumerFilteringSupport() {
         Assert.fail();  // fail if UnsupportedOperationException was not thrown
     }
 
+    @Test
+    public void consumerErrorsDuringRefreshArePropagated() {
+        HollowProducer producer = HollowProducer.withPublisher(blobStore)
+            .withAnnouncer(announcement)
+            .withBlobStager(new HollowInMemoryBlobStager())
+            .build();
+        long v1 = runCycle(producer, 1);
+
+        InMemoryBlobStore otherBlobStore = new InMemoryBlobStore();
+        HollowConsumer consumer = HollowConsumer.withBlobRetriever(otherBlobStore)
+            .withAnnouncementWatcher(announcement)
+            .build();
+
+        try {
+            consumer.triggerAsyncRefresh().toCompletableFuture().join();
+            Assert.fail("Expected exception to be thrown by async refresh.");
+        } catch (Exception e) {
+            Assert.assertTrue(e instanceof CompletionException);
+            Assert.assertTrue(e.getCause() instanceof IllegalArgumentException);
+        }
+    }
+
     private long runCycle(HollowProducer producer, final int cycleNumber) {
         return producer.runCycle(state -> state.add(cycleNumber));
     }