BearerTokenServerAuthenticationEntryPoint waits for subscriber

Fixes: gh-5742

oauth2/oauth2-resource-server/src/main/java/org/springframework/security/oauth2/server/resource/web/server/BearerTokenServerAuthenticationEntryPoint.java

@@ -58,14 +58,16 @@ public final class BearerTokenServerAuthenticationEntryPoint implements
 
 	@Override
 	public Mono<Void> commence(ServerWebExchange exchange, AuthenticationException authException) {
-		HttpStatus status = getStatus(authException);
-
-		Map<String, String> parameters = createParameters(authException);
-		String wwwAuthenticate = computeWWWAuthenticateHeaderValue(parameters);
-		ServerHttpResponse response = exchange.getResponse();
-		response.getHeaders().set(HttpHeaders.WWW_AUTHENTICATE, wwwAuthenticate);
-		response.setStatusCode(status);
-		return response.setComplete();
+		return Mono.defer(() -> {
+			HttpStatus status = getStatus(authException);
+
+			Map<String, String> parameters = createParameters(authException);
+			String wwwAuthenticate = computeWWWAuthenticateHeaderValue(parameters);
+			ServerHttpResponse response = exchange.getResponse();
+			response.getHeaders().set(HttpHeaders.WWW_AUTHENTICATE, wwwAuthenticate);
+			response.setStatusCode(status);
+			return response.setComplete();
+		});
 	}
 
 	private Map<String, String> createParameters(AuthenticationException authException) {

oauth2/oauth2-resource-server/src/test/java/org/springframework/security/oauth2/server/resource/web/server/BearerTokenServerAuthenticationEntryPointTests.java

@@ -91,6 +91,14 @@ public class BearerTokenServerAuthenticationEntryPointTests {
 		assertThat(getResponse().getStatusCode()).isEqualTo(HttpStatus.BAD_REQUEST);
 	}
 
+	@Test
+	public void commenceWhenNoSubscriberThenNothingHappens() {
+		this.entryPoint.commence(this.exchange, new BadCredentialsException(""));
+
+		assertThat(getResponse().getHeaders()).isEmpty();
+		assertThat(getResponse().getStatusCode()).isNull();
+	}
+
 	private MockServerHttpResponse getResponse() {
 		return this.exchange.getResponse();
 	}