Avoid further processing when GraphQL request is cancelled

As of gh-1149, CANCEL signals are propagated from the transport request
up to reactive `DataFetcher`s. This efficiently cancels processing and
avoids spending resources when execution results won't be sent to the
client.
Prior to this commit, this would have no effect on non-reactive
`DataFetcher`s because they would still be executed. While we cannot
consistently cancel ongoing blocking operations, we can avoid further
processing and other `DataFetcher`s from being called by returning an
error result instead of the original result.

This commit updates the `ContextDataFetcherDecorator` to detect if the
request has been cancelled and return early a data fetcher result with a
`AbortExecutionException` error instead of the original result.

Closes gh-1153

spring-graphql/src/main/java/org/springframework/graphql/execution/ContextDataFetcherDecorator.java

@@ -21,6 +21,7 @@ import java.util.List;
 import graphql.ExecutionInput;
 import graphql.GraphQLContext;
 import graphql.TrivialDataFetcher;
+import graphql.execution.AbortExecutionException;
 import graphql.execution.DataFetcherResult;
 import graphql.schema.DataFetcher;
 import graphql.schema.DataFetchingEnvironment;
@@ -107,6 +108,11 @@ final class ContextDataFetcherDecorator implements DataFetcher<Object> {
 		if (value == null) {
 			return null;
 		}
+		if (ContextPropagationHelper.isCancelled(graphQlContext)) {
+			return DataFetcherResult.newResult()
+					.error(new AbortExecutionException("GraphQL request has been cancelled by the client."))
+					.build();
+		}
 
 		if (this.subscription) {
 			Flux<?> subscriptionResult = ReactiveAdapterRegistryHelper.toSubscriptionFlux(value)

spring-graphql/src/main/java/org/springframework/graphql/execution/ContextPropagationHelper.java

@@ -131,6 +131,21 @@ public abstract class ContextPropagationHelper {
 		return requestCancelled;
 	}
 
+	/**
+	 * Return {@code true} if the current request has been cancelled, {@code false} otherwise.
+	 * This checks whether a {@link #createCancelPublisher(GraphQLContext) cancellation publisher is present}
+	 * in the given context and the cancel signal has fired already.
+	 * @param context the current GraphQL context
+	 * @since 1.4.0
+	 */
+	public static boolean isCancelled(GraphQLContext context) {
+		Mono<Void> cancelSignal = context.get(CANCEL_PUBLISHER_KEY);
+		if (cancelSignal != null) {
+			return cancelSignal.toFuture().isDone();
+		}
+		return false;
+	}
+
 	/**
 	 * Bind the source {@link Mono} to the publisher from the given {@link GraphQLContext}.
 	 * The returned {@code Mono} will be cancelled when this publisher completes.

spring-graphql/src/test/java/org/springframework/graphql/execution/ContextDataFetcherDecoratorTests.java

@@ -29,6 +29,7 @@ import graphql.GraphQL;
 import graphql.GraphQLError;
 import graphql.GraphqlErrorBuilder;
 import graphql.TrivialDataFetcher;
+import graphql.execution.AbortExecutionException;
 import graphql.execution.DataFetcherResult;
 import graphql.schema.DataFetcher;
 import graphql.schema.DataFetcherFactories;
@@ -55,7 +56,9 @@ import static org.awaitility.Awaitility.await;
 
 /**
  * Tests for {@link ContextDataFetcherDecorator}.
+ *
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  */
 @SuppressWarnings("ReactiveStreamsUnusedPublisher")
 public class ContextDataFetcherDecoratorTests {
@@ -288,7 +291,7 @@ public class ContextDataFetcherDecoratorTests {
 	}
 
 	@Test
-	void cancelMonoDataFetcherWhenRequestCancelled() throws Exception {
+	void cancelMonoDataFetcherWhenRequestCancelled() {
 		AtomicBoolean dataFetcherCancelled = new AtomicBoolean();
 		GraphQL graphQl = GraphQlSetup.schemaContent(SCHEMA_CONTENT)
 				.queryFetcher("greeting", (env) ->
@@ -307,7 +310,7 @@ public class ContextDataFetcherDecoratorTests {
 	}
 
 	@Test
-	void cancelFluxDataFetcherWhenRequestCancelled() throws Exception {
+	void cancelFluxDataFetcherWhenRequestCancelled() {
 		AtomicBoolean dataFetcherCancelled = new AtomicBoolean();
 		GraphQL graphQl = GraphQlSetup.schemaContent(SCHEMA_CONTENT)
 				.queryFetcher("greeting", (env) ->
@@ -325,6 +328,22 @@ public class ContextDataFetcherDecoratorTests {
 		await().atMost(Duration.ofSeconds(2)).until(dataFetcherCancelled::get);
 	}
 
+	@Test
+	void returnAbortExecutionForBlockingDataFetcherWhenRequestCancelled() throws Exception {
+		GraphQL graphQl = GraphQlSetup.schemaContent(SCHEMA_CONTENT)
+				.queryFetcher("greeting", (env) -> "Hello")
+				.toGraphQl();
+
+		ExecutionInput input = ExecutionInput.newExecutionInput().query("{ greeting }").build();
+		Sinks.Empty<Void> requestCancelled = ContextPropagationHelper.createCancelPublisher(input.getGraphQLContext());
+		requestCancelled.tryEmitEmpty();
+		ExecutionResult result = graphQl.executeAsync(input).get();
+
+		assertThat(result.getErrors()).hasSize(1);
+		assertThat(result.getErrors().get(0)).isInstanceOf(AbortExecutionException.class)
+				.extracting("message").asString().isEqualTo("GraphQL request has been cancelled by the client.");
+	}
+
 	@Test
 	void cancelFluxDataFetcherSubscriptionWhenRequestCancelled() throws Exception {
 		AtomicBoolean dataFetcherCancelled = new AtomicBoolean();