Handler failure from GraphQlExceptionHandler method

Closes gh-1090

Author: rstoyanchev

spring-graphql/src/main/java/org/springframework/graphql/data/method/annotation/support/AnnotatedControllerExceptionResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -356,7 +356,16 @@ Method getMethod() {
 			return this.method;
 		}
 
+		@SuppressWarnings("unchecked")
 		Mono<List<GraphQLError>> adapt(@Nullable Object result, Throwable ex) {
+			if (result instanceof Mono<?> errorMono && this.adapter != ReturnValueAdapter.forMono) {
+				return (Mono<List<GraphQLError>>) errorMono.onErrorMap((ex2) -> {
+					if (logger.isWarnEnabled()) {
+						logger.warn("Failure in @GraphQlExceptionHandler " + this.method, ex2);
+					}
+					return ex; // fall back to original exception
+				});
+			}
 			return this.adapter.adapt(result, this.returnType, ex);
 		}
 

spring-graphql/src/test/java/org/springframework/graphql/data/method/annotation/support/AnnotatedControllerExceptionResolverTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -111,6 +111,19 @@ void leaveUnresolvedViaNullReturnValue() {
 		StepVerifier.create(resolver.resolveException(ex, this.environment, controller)).verifyComplete();
 	}
 
+	@Test // gh-1090
+	void failureFromResolver() {
+		ExceptionThrowingController controller = new ExceptionThrowingController();
+
+		Exception ex = new IllegalArgumentException("Bad input");
+		AnnotatedControllerExceptionResolver resolver = exceptionResolver();
+		resolver.registerController(controller.getClass());
+
+		StepVerifier.create(resolver.resolveException(ex, this.environment, controller))
+				.expectErrorSatisfies(actual -> assertThat(actual).isSameAs(ex))
+				.verify();
+	}
+
 	@Test
 	void resolveWithControllerAdvice() {
 		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
@@ -304,4 +317,14 @@ public String handle(IllegalArgumentException ex) {
 
 	}
 
+
+	private static class ExceptionThrowingController {
+
+		@GraphQlExceptionHandler
+		GraphQLError handle(IllegalArgumentException ex) {
+			throw new IllegalStateException("failure in exception handler");
+		}
+
+	}
+
 }