Do not rewrap CompletionException in DataFetcher instrumentation

This commit ensures that, when an instrumented DataFetcher returns a
`CompletionException`, we do not re-wrap it with the same exception
type. This aligns with the behavior enforced in the JDK
`CompletableFuture`.

Fixes gh-784

spring-graphql/src/main/java/org/springframework/graphql/observation/GraphQlObservationInstrumentation.java

@@ -147,9 +147,15 @@ public DataFetcher<?> instrumentDataFetcher(DataFetcher<?> dataFetcher,
 						return completion.handle((result, error) -> {
 							observationContext.setValue(result);
 							if (error != null) {
-								dataFetcherObservation.error(error);
-								dataFetcherObservation.stop();
-								throw new CompletionException(error);
+								if (error instanceof CompletionException completionException) {
+									dataFetcherObservation.error(error.getCause());
+									dataFetcherObservation.stop();
+									throw completionException;
+								} else {
+									dataFetcherObservation.error(error);
+									dataFetcherObservation.stop();
+									throw new CompletionException(error);
+								}
 							}
 							dataFetcherObservation.stop();
 							return result;

spring-graphql/src/test/java/org/springframework/graphql/observation/GraphQlObservationInstrumentationTests.java

@@ -37,6 +37,7 @@
 import reactor.core.publisher.Mono;
 
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionException;
 import java.util.stream.Stream;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -168,8 +169,7 @@ void instrumentGraphQlRequestWhenDataFetchingFailure(DataFetcher<?> dataFetcher)
 								.errorType(ErrorType.BAD_REQUEST).build());
 		Mono<ExecutionGraphQlResponse> responseMono = graphQlSetup
 				.exceptionResolver(resolver)
-				.queryFetcher("bookById", env ->
-						CompletableFuture.failedStage(new IllegalStateException("book fetching failure")))
+				.queryFetcher("bookById", dataFetcher)
 				.toGraphQlService()
 				.execute(TestExecutionRequest.forDocument(document));
 		ResponseHelper response = ResponseHelper.forResponse(responseMono);
@@ -190,13 +190,14 @@ void instrumentGraphQlRequestWhenDataFetchingFailure(DataFetcher<?> dataFetcher)
 	}
 
 	static Stream<Arguments> failureDataFetchers() {
-		DataFetcher<Book> bookDataFetcher = environment -> {
-			throw new IllegalStateException("book fetching failure");
-		};
 		return Stream.of(
-				Arguments.of(bookDataFetcher),
+				Arguments.of((DataFetcher<?>) environment -> {
+					throw new IllegalStateException("book fetching failure");
+				}),
+				Arguments.of((DataFetcher<?>) environment ->
+						CompletableFuture.failedStage(new IllegalStateException("book fetching failure"))),
 				Arguments.of((DataFetcher<?>) environment ->
-						CompletableFuture.failedStage(new IllegalStateException("book fetching failure")))
+						CompletableFuture.failedStage(new CompletionException(new IllegalStateException("book fetching failure"))))
 		);
 	}