Prefer existing Context in ReactorContextTestExecutionListener

Issue gh-4718

test/src/main/java/org/springframework/security/test/context/support/ReactorContextTestExecutionListener.java

@@ -77,7 +77,7 @@ public class ReactorContextTestExecutionListener
 					return context;
 				}
 				Context toMerge = ReactiveSecurityContextHolder.withAuthentication(authentication);
-				return context.putAll(toMerge);
+				return toMerge.putAll(context);
 			}
 
 			@Override

test/src/test/java/org/springframework/security/test/context/support/ReactorContextTestExecutionListenerTests.java

@@ -92,6 +92,25 @@ public class ReactorContextTestExecutionListenerTests {
 		assertAuthentication(expectedAuthentication);
 	}
 
+	@Test
+	public void beforeTestMethodWhenExistingAuthenticationThenReactorContextHasOriginalAuthentication() throws Exception {
+		TestingAuthenticationToken expectedAuthentication = new TestingAuthenticationToken("user", "password", "ROLE_USER");
+		TestingAuthenticationToken contextHolder = new TestingAuthenticationToken("contextHolder", "password", "ROLE_USER");
+		TestSecurityContextHolder.setContext(new SecurityContextImpl(contextHolder));
+
+		this.listener.beforeTestMethod(this.testContext);
+
+		Mono<Authentication> authentication = Mono.just("any")
+			.flatMap(s -> ReactiveSecurityContextHolder.getContext()
+				.map(SecurityContext::getAuthentication)
+			)
+			.subscriberContext(ReactiveSecurityContextHolder.withAuthentication(expectedAuthentication));
+
+		StepVerifier.create(authentication)
+			.expectNext(expectedAuthentication)
+			.verifyComplete();
+	}
+
 	@Test
 	public void afterTestMethodWhenSecurityContextEmptyThenNoError() throws Exception {
 		this.listener.beforeTestMethod(this.testContext);