HttpHandlerConnector uses non-blocking thread

Closes gh-23936

spring-test/src/main/java/org/springframework/test/web/reactive/server/HttpHandlerConnector.java

@@ -25,6 +25,7 @@ import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.core.publisher.MonoProcessor;
+import reactor.core.scheduler.Schedulers;
 
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.http.HttpCookie;
@@ -75,6 +76,13 @@ public class HttpHandlerConnector implements ClientHttpConnector {
 	public Mono<ClientHttpResponse> connect(HttpMethod httpMethod, URI uri,
 			Function<? super ClientHttpRequest, Mono<Void>> requestCallback) {
 
+		return Mono.defer(() -> doConnect(httpMethod, uri, requestCallback))
+				.subscribeOn(Schedulers.parallel());
+	}
+
+	private Mono<ClientHttpResponse> doConnect(
+			HttpMethod httpMethod, URI uri, Function<? super ClientHttpRequest, Mono<Void>> requestCallback) {
+
 		MonoProcessor<ClientHttpResponse> result = MonoProcessor.create();
 
 		MockClientHttpRequest mockClientRequest = new MockClientHttpRequest(httpMethod, uri);

spring-test/src/test/java/org/springframework/test/web/reactive/server/HttpHandlerConnectorTests.java

@@ -24,6 +24,7 @@ import java.util.function.Function;
 
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
 
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
@@ -51,7 +52,7 @@ public class HttpHandlerConnectorTests {
 
 
 	@Test
-	public void adaptRequest() throws Exception {
+	public void adaptRequest() {
 
 		TestHttpHandler handler = new TestHttpHandler(response -> {
 			response.setStatusCode(HttpStatus.OK);
@@ -79,7 +80,7 @@ public class HttpHandlerConnectorTests {
 	}
 
 	@Test
-	public void adaptResponse() throws Exception {
+	public void adaptResponse() {
 
 		ResponseCookie cookie = ResponseCookie.from("custom-cookie", "c0").build();
 
@@ -104,6 +105,22 @@ public class HttpHandlerConnectorTests {
 		assertThat(DataBufferTestUtils.dumpString(buffer, UTF_8)).isEqualTo("Custom body");
 	}
 
+	@Test // gh-23936
+	public void handlerOnNonBlockingThread() {
+
+		TestHttpHandler handler = new TestHttpHandler(response -> {
+
+			assertThat(Schedulers.isInNonBlockingThread()).isTrue();
+
+			response.setStatusCode(HttpStatus.OK);
+			return response.setComplete();
+		});
+
+		new HttpHandlerConnector(handler)
+				.connect(HttpMethod.POST, URI.create("/path"), request -> request.writeWith(Mono.empty()))
+				.block(Duration.ofSeconds(5));
+	}
+
 	private DataBuffer toDataBuffer(String body) {
 		return new DefaultDataBufferFactory().wrap(body.getBytes(UTF_8));
 	}