fix #301: QueuedChannel futures don't wait forever when delegates throw

changelog/@unreleased/pr-387.v2.yml

@@ -0,0 +1,5 @@
+type: fix
+fix:
+  description: QueuedChannel futures don't wait forever when delegates throw
+  links:
+  - https://github.com/palantir/dialogue/pull/387

dialogue-core/src/main/java/com/palantir/dialogue/core/NeverThrowLimitedChannel.java

@@ -0,0 +1,50 @@
+/*
+ * (c) Copyright 2020 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed 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 com.palantir.dialogue.core;
+
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.palantir.dialogue.Endpoint;
+import com.palantir.dialogue.Request;
+import com.palantir.dialogue.Response;
+import java.util.Optional;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The contract of {@link LimitedChannel} requires that the {@link LimitedChannel#maybeExecute} method never throws.
+ * This is a defensive backstop so that callers can rely on this invariant.
+ */
+final class NeverThrowLimitedChannel implements LimitedChannel {
+
+    private static final Logger log = LoggerFactory.getLogger(NeverThrowLimitedChannel.class);
+    private final LimitedChannel delegate;
+
+    NeverThrowLimitedChannel(LimitedChannel delegate) {
+        this.delegate = delegate;
+    }
+
+    @Override
+    public Optional<ListenableFuture<Response>> maybeExecute(Endpoint endpoint, Request request) {
+        try {
+            return delegate.maybeExecute(endpoint, request);
+        } catch (RuntimeException | Error e) {
+            log.error("Dialogue channels should never throw. This may be a bug in the channel implementation", e);
+            return Optional.of(Futures.immediateFailedFuture(e));
+        }
+    }
+}

dialogue-core/src/main/java/com/palantir/dialogue/core/QueuedChannel.java

@@ -75,7 +75,7 @@ final class QueuedChannel implements Channel {
     @VisibleForTesting
     @SuppressWarnings("FutureReturnValueIgnored")
     QueuedChannel(LimitedChannel delegate, int maxQueueSize, DispatcherMetrics metrics) {
-        this.delegate = delegate;
+        this.delegate = new NeverThrowLimitedChannel(delegate);
         this.queuedCalls = new LinkedBlockingDeque<>(maxQueueSize);
 
         metrics.callsQueued(queuedCalls::size);

dialogue-core/src/test/java/com/palantir/dialogue/core/QueuedChannelTest.java

@@ -37,6 +37,7 @@
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.Mock;
+import org.mockito.Mockito;
 import org.mockito.junit.jupiter.MockitoExtension;
 import org.mockito.stubbing.OngoingStubbing;
 
@@ -114,6 +115,30 @@ public void testQueuedRequestExecutedOnNextSubmission() {
         verify(delegate, times(3)).maybeExecute(endpoint, request);
     }
 
+    @Test
+    public void testQueuedRequestExecutedOnNextSubmission_throws() throws ExecutionException, InterruptedException {
+        // First request is limited by the channel and queued
+        Request queuedRequest = Mockito.mock(Request.class);
+        when(delegate.maybeExecute(endpoint, queuedRequest)).thenReturn(Optional.empty());
+        ListenableFuture<Response> queuedFuture = queuedChannel.execute(endpoint, queuedRequest);
+        verify(delegate, times(2)).maybeExecute(endpoint, queuedRequest);
+        assertThat(queuedFuture).isNotDone();
+
+        // Second request succeeds and the queued request is attempted, but throws an exception
+        futureResponse.set(mockResponse);
+        when(delegate.maybeExecute(endpoint, request)).thenReturn(maybeResponse);
+        when(delegate.maybeExecute(endpoint, queuedRequest)).thenThrow(new NullPointerException("expected"));
+        ListenableFuture<Response> completed = queuedChannel.execute(endpoint, request);
+        // Both results should be completed. The thrown exception should
+        // be converted into a failed future by NeverThrowLimitedChannel
+        assertThat(completed).isDone();
+        assertThat(queuedFuture).isDone();
+        assertThat(completed.get()).isEqualTo(mockResponse);
+        assertThatThrownBy(queuedFuture::get).hasRootCauseMessage("expected");
+        verify(delegate, times(1)).maybeExecute(endpoint, request);
+        verify(delegate, times(3)).maybeExecute(endpoint, queuedRequest);
+    }
+
     @Test
     @SuppressWarnings("FutureReturnValueIgnored")
     public void testQueuedRequestExecutedWhenRunningRequestCompletes() {