Fix leaked futures when executor rejects Runnable

.changes/next-release/bugfix-AWSSDKforJavav2-abc1eac.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "AWS SDK for Java v2",
+    "contributor": "",
+    "description": "This update fixes  an issue where CompletableFutures are leaked/never completed when the submission to the FUTURE_COMPLETE_EXECUTOR is rejected.\n\nBy default, the SDK uses `2 * number of cores` (with a maximum of 64), and uses bounded queue of size 1000. In cases where the throughput to the client exceeds the executor's ability to keep up, it would reject executions. Before this change this would lead to leaked futures."
+}

core/sdk-core/src/main/java/software/amazon/awssdk/core/internal/http/pipeline/stages/MakeAsyncHttpRequestStage.java

@@ -156,15 +156,26 @@ private CompletableFuture<Response<OutputT>> executeHttpRequest(SdkHttpFullReque
             }
         });
 
-        // Offload the completion of the future returned from this stage onto
-        // the future completion executor
-        responseHandlerFuture.whenCompleteAsync((r, t) -> {
-            if (t == null) {
-                responseFuture.complete(r);
-            } else {
-                responseFuture.completeExceptionally(t);
+        // Attempt to offload the completion of the future returned from this
+        // stage onto the future completion executor
+        CompletableFuture<Response<OutputT>> asyncComplete =
+            responseHandlerFuture.whenCompleteAsync((r, t) -> completeResponseFuture(responseFuture, r, t),
+                                                    futureCompletionExecutor);
+
+        // It's possible the async execution above fails. If so, log a warning,
+        // and just complete it synchronously.
+        asyncComplete.whenComplete((ignored, asyncCompleteError) -> {
+            if (asyncCompleteError != null) {
+                log.debug(() -> String.format("Could not complete the service call future on the provided "
+                                              + "FUTURE_COMPLETION_EXECUTOR. The future will be completed synchronously by thread"
+                                              + " %s. This may be an indication that the executor is being overwhelmed by too"
+                                              + " many requests, and it may negatively affect performance. Consider changing "
+                                              + "the configuration of the executor to accommodate the load through the client.",
+                                  Thread.currentThread().getName()),
+                          asyncCompleteError);
+                responseHandlerFuture.whenComplete((r, t) -> completeResponseFuture(responseFuture, r, t));
             }
-        }, futureCompletionExecutor);
+        });
 
         return responseFuture;
     }
@@ -219,6 +230,14 @@ private TimeoutTracker setupAttemptTimer(CompletableFuture<Response<OutputT>> ex
                                                 timeoutMillis);
     }
 
+    private void completeResponseFuture(CompletableFuture<Response<OutputT>> responseFuture, Response<OutputT> r, Throwable t) {
+        if (t == null) {
+            responseFuture.complete(r);
+        } else {
+            responseFuture.completeExceptionally(t);
+        }
+    }
+
     /**
      * When an operation has a streaming input, the customer must supply an {@link AsyncRequestBody} to
      * provide the request content in a non-blocking manner. This adapts that interface to the

core/sdk-core/src/test/java/software/amazon/awssdk/core/internal/http/pipeline/stages/MakeAsyncHttpRequestStageTest.java

@@ -19,6 +19,7 @@
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyLong;
 import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.times;
@@ -31,6 +32,8 @@
 
 import java.time.Duration;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
@@ -47,6 +50,7 @@
 import software.amazon.awssdk.core.interceptor.ExecutionAttributes;
 import software.amazon.awssdk.core.internal.http.HttpClientDependencies;
 import software.amazon.awssdk.core.internal.http.RequestExecutionContext;
+import software.amazon.awssdk.core.internal.http.TransformingAsyncResponseHandler;
 import software.amazon.awssdk.core.internal.http.timers.ClientExecutionAndRequestTimerTestUtils;
 import software.amazon.awssdk.core.internal.util.AsyncResponseHandlerTestUtils;
 import software.amazon.awssdk.http.SdkHttpFullRequest;
@@ -152,6 +156,33 @@ public void testExecute_contextContainsMetricCollector_addsChildToExecuteRequest
         }
     }
 
+    @Test
+    public void execute_futureCompletionExecutorRejectsWhenCompleteAsync_futureCompletedSynchronously() {
+        ExecutorService mockExecutor = mock(ExecutorService.class);
+        doThrow(new RejectedExecutionException("Busy")).when(mockExecutor).execute(any(Runnable.class));
+
+        SdkClientConfiguration config =
+            SdkClientConfiguration.builder()
+                .option(SdkAdvancedAsyncClientOption.FUTURE_COMPLETION_EXECUTOR, mockExecutor)
+                .option(ASYNC_HTTP_CLIENT, sdkAsyncHttpClient)
+                .build();
+        HttpClientDependencies dependencies = HttpClientDependencies.builder().clientConfiguration(config).build();
+
+        TransformingAsyncResponseHandler mockHandler = mock(TransformingAsyncResponseHandler.class);
+        when(mockHandler.prepare()).thenReturn(CompletableFuture.completedFuture(null));
+
+        stage = new MakeAsyncHttpRequestStage<>(mockHandler, dependencies);
+
+        CompletableFuture<SdkHttpFullRequest> requestFuture = CompletableFuture.completedFuture(
+            ValidSdkObjects.sdkHttpFullRequest().build());
+
+        CompletableFuture executeFuture = stage.execute(requestFuture, requestContext());
+
+        long testThreadId = Thread.currentThread().getId();
+        executeFuture.whenComplete((r, t) -> assertThat(Thread.currentThread().getId()).isEqualTo(testThreadId)).join();
+        verify(mockExecutor).execute(any(Runnable.class));
+    }
+
     private HttpClientDependencies clientDependencies(Duration timeout) {
         SdkClientConfiguration configuration = SdkClientConfiguration.builder()
                                                                      .option(SdkAdvancedAsyncClientOption.FUTURE_COMPLETION_EXECUTOR, Runnable::run)