Add option for expected errors to GraphQlTester

Closes: gh-175

spring-graphql-test/src/main/java/org/springframework/graphql/test/tester/DefaultGraphQlTester.java

@@ -203,13 +203,19 @@ class DefaultGraphQlTester implements GraphQlTester {
 			this.assertDecorator.accept(task);
 		}
 
-		void filterErrors(@Nullable Predicate<GraphQLError> predicate) {
+		boolean filterErrors(@Nullable Predicate<GraphQLError> predicate) {
+			boolean filtered = false;
 			if (predicate != null) {
-				this.errors.forEach((error) -> {
-					// Error marked "filtered" if true
-					error.applyErrorFilterPredicate(predicate);
-				});
+				for (TestGraphQlError error : this.errors) {
+					filtered |= error.apply(predicate);
+				}
 			}
+			return filtered;
+		}
+
+		void expectErrors(@Nullable Predicate<GraphQLError> predicate) {
+			boolean filtered = filterErrors(predicate);
+			this.assertDecorator.accept(() -> AssertionErrors.assertTrue("No matching errors.", filtered));
 		}
 
 		void consumeErrors(Consumer<List<GraphQLError>> consumer) {
@@ -265,6 +271,12 @@ class DefaultGraphQlTester implements GraphQlTester {
 			return this;
 		}
 
+		@Override
+		public ErrorSpec expect(Predicate<GraphQLError> predicate) {
+			this.responseContainer.expectErrors(predicate);
+			return this;
+		}
+
 		@Override
 		public TraverseSpec verify() {
 			this.responseContainer.verifyErrors();

spring-graphql-test/src/main/java/org/springframework/graphql/test/tester/GraphQlTester.java

@@ -90,10 +90,8 @@ public interface GraphQlTester {
 	interface Builder<T extends Builder<T>> {
 
 		/**
-		 * Add a global filter for expected errors. All errors that match the
-		 * given predicate are treated as expected and ignored on
-		 * {@link GraphQlTester.ErrorSpec#verify()} or when
-		 * {@link TraverseSpec#path(String) traversing} to a data path.
+		 * Configure a global {@link ErrorSpec#filter(Predicate) filter} that
+		 * applies to all requests.
 		 * @param predicate the error filter to add
 		 * @return the same builder instance
 		 */
@@ -450,14 +448,32 @@ public interface GraphQlTester {
 	interface ErrorSpec {
 
 		/**
-		 * Add a filter for expected errors. All errors that match the predicate are
-		 * treated as expected and ignored on {@link #verify()} or when
+		 * Use this to filter out errors that are expected and can be ignored.
+		 * This can be useful for warnings or other notifications returned along
+		 * with the data.
+		 * <p>The configured filters are applied to all errors. Those that match
+		 * are treated as expected and are ignored on {@link #verify()} or when
 		 * {@link TraverseSpec#path(String) traversing} to a data path.
+		 * <p>In contrast to {@link #expect(Predicate)}, filters do not have to
+		 * match any errors, and don't imply that the errors must be present.
 		 * @param errorPredicate the error filter to add
 		 * @return the same spec to add more filters before {@link #verify()}
 		 */
 		ErrorSpec filter(Predicate<GraphQLError> errorPredicate);
 
+		/**
+		 * Use this to declare errors that are expected.
+		 * <p>Errors that match are treated as expected and are ignored on
+		 * {@link #verify()} or when {@link TraverseSpec#path(String) traversing}
+		 * to a data path.
+		 * <p>In contrast to {@link #filter(Predicate)}, use of this option
+		 * does imply that errors are  present or else an {@link AssertionError}
+		 * is raised.
+		 * @param errorPredicate the predicate for the expected error
+		 * @return the same spec to add more filters or expected errors
+		 */
+		ErrorSpec expect(Predicate<GraphQLError> errorPredicate);
+
 		/**
 		 * Verify there are either no errors or that there no unexpected errors that have
 		 * not been {@link #filter(Predicate) filtered out}.

spring-graphql-test/src/main/java/org/springframework/graphql/test/tester/TestGraphQlError.java

@@ -148,9 +148,12 @@ final class TestGraphQlError implements GraphQLError {
 	/**
 	 * Mark this error as expected if it matches the predicate.
 	 * @param predicate the error predicate
+	 * @return whether the predicate matched
 	 */
-	void applyErrorFilterPredicate(Predicate<GraphQLError> predicate) {
-		this.expected |= predicate.test(this);
+	boolean apply(Predicate<GraphQLError> predicate) {
+		boolean match = predicate.test(this);
+		this.expected |= match;
+		return match;
 	}
 
 	@Override

spring-graphql-test/src/test/java/org/springframework/graphql/test/tester/GraphQlTesterTests.java

@@ -276,6 +276,40 @@ public class GraphQlTesterTests {
 		assertThat(this.inputCaptor.getValue().getQuery()).contains(query);
 	}
 
+	@Test
+	void errorsExpected() throws Exception {
+
+		String query = "{me {name, friends}}";
+		setResponse(
+				GraphqlErrorBuilder.newError().message("some error").build(),
+				GraphqlErrorBuilder.newError().message("some other error").build());
+
+		this.graphQlTester.query(query)
+				.execute()
+				.errors()
+				.expect((error) -> error.getMessage().startsWith("some "))
+				.verify()
+				.path("me")
+				.pathDoesNotExist();
+
+		assertThat(this.inputCaptor.getValue().getQuery()).contains(query);
+	}
+
+	@Test
+	void errorsExpectedButNotFound() throws Exception {
+
+		String query = "{me {name, friends}}";
+		setResponse(
+				GraphqlErrorBuilder.newError().message("some error").build(),
+				GraphqlErrorBuilder.newError().message("some other error").build());
+
+		assertThatThrownBy(() ->
+				this.graphQlTester.query(query)
+						.execute()
+						.errors().expect((error) -> error.getMessage().startsWith("another ")))
+				.hasMessageStartingWith("No matching errors.");
+	}
+
 	@Test
 	void errorsConsumed() throws Exception {