Fix Conflict for NonBlocking Interface in Reactor Integration (#6037)

Motivation:
The `reactor.core.scheduler.NonBlocking` interface was introduced to `armeria-core` in #1665 to make `Schedulers.isInNonBlockingThread()` return `true` for an Armeria `EventLoop`. However, a conflict arises when building Java modules because the `NonBlocking` interface clashes with Reactor's own definition.

Modifications:
- Moved `NonBlocking` to `com.linecorp.armeria.common` to resolve the module conflict while retaining its utility for identifying non-blocking threads.
- Updated to call `Schedulers.registerNonBlockingThreadPredicate` if Reactor is available in the classpath.
- Adjusted `CoreBlockHoundIntegration` to update the non-blocking thread predicate.

Result:
- Resolved the Java module conflict with the `NonBlocking` interface.

core/build.gradle

@@ -139,6 +139,9 @@ dependencies {
     // JUnit Pioneer
     testImplementation libs.junit.pioneer
 
+    // Reactor for registering EventLoop as a non blocking thread.
+    optionalImplementation libs.reactor.core
+
     // Reactive Streams
     api libs.reactivestreams
     testImplementation libs.reactivestreams.tck
@@ -217,8 +220,6 @@ if (tasks.findByName('trimShadedJar')) {
     tasks.trimShadedJar.configure {
         // Keep all classes under com.linecorp.armeria, except the internal ones.
         keep "class !com.linecorp.armeria.internal.shaded.**,com.linecorp.armeria.** { *; }"
-        // Keep the 'NonBlocking' tag interface.
-        keep "class reactor.core.scheduler.NonBlocking { *; }"
         // Do not optimize the dependencies that access some fields via sun.misc.Unsafe or reflection only.
         keep "class com.linecorp.armeria.internal.shaded.caffeine.** { *; }"
         keep "class com.linecorp.armeria.internal.shaded.jctools.** { *; }"

core/src/main/java/com/linecorp/armeria/client/ClientFactory.java

@@ -36,6 +36,7 @@
 
 import com.linecorp.armeria.client.endpoint.EndpointGroup;
 import com.linecorp.armeria.common.FlagsProvider;
+import com.linecorp.armeria.common.NonBlocking;
 import com.linecorp.armeria.common.Scheme;
 import com.linecorp.armeria.common.SerializationFormat;
 import com.linecorp.armeria.common.SessionProtocol;
@@ -49,7 +50,6 @@
 import io.micrometer.core.instrument.MeterRegistry;
 import io.netty.channel.EventLoop;
 import io.netty.channel.EventLoopGroup;
-import reactor.core.scheduler.NonBlocking;
 
 /**
  * Creates and manages clients.

core/src/main/java/com/linecorp/armeria/client/HttpChannelPool.java

@@ -47,6 +47,7 @@
 import com.linecorp.armeria.client.proxy.Socks4ProxyConfig;
 import com.linecorp.armeria.client.proxy.Socks5ProxyConfig;
 import com.linecorp.armeria.common.ClosedSessionException;
+import com.linecorp.armeria.common.NonBlocking;
 import com.linecorp.armeria.common.SerializationFormat;
 import com.linecorp.armeria.common.SessionProtocol;
 import com.linecorp.armeria.common.annotation.Nullable;
@@ -76,7 +77,6 @@
 import io.netty.util.AttributeKey;
 import io.netty.util.concurrent.Future;
 import io.netty.util.concurrent.Promise;
-import reactor.core.scheduler.NonBlocking;
 
 final class HttpChannelPool implements AsyncCloseable {
 

core/src/main/java/com/linecorp/armeria/client/HttpClientFactory.java

@@ -42,6 +42,7 @@
 import com.linecorp.armeria.client.proxy.ProxyConfigSelector;
 import com.linecorp.armeria.client.redirect.RedirectConfig;
 import com.linecorp.armeria.common.Http1HeaderNaming;
+import com.linecorp.armeria.common.NonBlocking;
 import com.linecorp.armeria.common.RequestContext;
 import com.linecorp.armeria.common.Scheme;
 import com.linecorp.armeria.common.SerializationFormat;
@@ -71,7 +72,6 @@
 import io.netty.handler.ssl.SslContextBuilder;
 import io.netty.resolver.AddressResolverGroup;
 import io.netty.util.concurrent.FutureListener;
-import reactor.core.scheduler.NonBlocking;
 
 /**
  * A {@link ClientFactory} that creates an HTTP client.

core/src/main/java/com/linecorp/armeria/common/CommonPools.java

@@ -41,6 +41,14 @@ public final class CommonPools {
         MoreMeterBinders
                 .eventLoopMetrics(WORKER_GROUP, new MeterIdPrefix("armeria.netty.common"))
                 .bindTo(Flags.meterRegistry());
+
+        try {
+            Class.forName("reactor.core.scheduler.Schedulers",
+                          true, CommonPools.class.getClassLoader());
+            ReactorNonBlockingUtil.registerEventLoopAsNonBlocking();
+        } catch (ClassNotFoundException e) {
+            // Do nothing.
+        }
     }
 
     /**

core/src/main/java/com/linecorp/armeria/common/CoreBlockHoundIntegration.java

@@ -30,6 +30,8 @@
 public final class CoreBlockHoundIntegration implements BlockHoundIntegration {
     @Override
     public void applyTo(Builder builder) {
+        builder.nonBlockingThreadPredicate(predicate -> predicate.or(NonBlocking.class::isInstance));
+
         // short locks
         builder.allowBlockingCallsInside("com.linecorp.armeria.client.HttpClientFactory",
                                          "pool");

core/src/main/java/reactor/core/scheduler/NonBlocking.java → core/src/main/java/com/linecorp/armeria/common/NonBlocking.java

@@ -13,9 +13,18 @@
  * License for the specific language governing permissions and limitations
  * under the License.
  */
-package reactor.core.scheduler;
+package com.linecorp.armeria.common;
 
 /**
- * A dummy interface that makes Project Reactor recognize Armeria's event loop threads as non-blocking.
+ * An interface that indicates a non-blocking thread. You can use this interface to check if the current
+ * thread is a non-blocking thread. For example:
+ * <pre>{@code
+ * if (Thread.currentThread() instanceof NonBlocking) {
+ *     // Avoid blocking operations.
+ *     closeable.closeAsync();
+ * } else {
+ *     closeable.close();
+ * }
+ * }</pre>
  */
 public interface NonBlocking {}

core/src/main/java/reactor/core/scheduler/package-info.java → core/src/main/java/com/linecorp/armeria/common/ReactorNonBlockingUtil.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019 LINE Corporation
+ * Copyright 2024 LINE Corporation
  *
  * LINE Corporation licenses this file to you under the Apache License,
  * version 2.0 (the "License"); you may not use this file except in compliance
@@ -13,11 +13,15 @@
  * License for the specific language governing permissions and limitations
  * under the License.
  */
+package com.linecorp.armeria.common;
 
-/**
- * Provides a dummy interface that makes Project Reactor recognize Armeria's event loop threads as non-blocking.
- */
-@NonNullByDefault
-package reactor.core.scheduler;
+import reactor.core.scheduler.Schedulers;
+
+final class ReactorNonBlockingUtil {
+
+    static void registerEventLoopAsNonBlocking() {
+        Schedulers.registerNonBlockingThreadPredicate(NonBlocking.class::isInstance);
+    }
 
-import com.linecorp.armeria.common.annotation.NonNullByDefault;
+    private ReactorNonBlockingUtil() {}
+}

core/src/main/java/com/linecorp/armeria/common/util/EventLoopCheckingFuture.java

@@ -31,10 +31,9 @@
 import com.google.common.collect.MapMaker;
 
 import com.linecorp.armeria.common.Flags;
+import com.linecorp.armeria.common.NonBlocking;
 import com.linecorp.armeria.common.annotation.Nullable;
 
-import reactor.core.scheduler.NonBlocking;
-
 /**
  * A {@link CompletableFuture} that warns the user if they call a method that blocks the event loop.
  */

core/src/main/java/com/linecorp/armeria/internal/common/util/EventLoopThread.java

@@ -15,15 +15,15 @@
  */
 package com.linecorp.armeria.internal.common.util;
 
+import com.linecorp.armeria.common.NonBlocking;
 import com.linecorp.armeria.common.annotation.Nullable;
 
 import io.netty.util.concurrent.FastThreadLocal;
 import io.netty.util.concurrent.FastThreadLocalThread;
-import reactor.core.scheduler.NonBlocking;
 
 /**
  * An event loop thread with support for {@link TemporaryThreadLocals}, Netty {@link FastThreadLocal} and
- * Project Reactor {@link NonBlocking}.
+ * {@link NonBlocking} interface.
  */
 public final class EventLoopThread extends FastThreadLocalThread implements NonBlocking {
 

reactor3/src/test/java/com/linecorp/armeria/common/reactor3/EventLoopNonBlockingTest.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2024 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package com.linecorp.armeria.common.reactor3;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+import com.linecorp.armeria.common.CommonPools;
+
+import io.netty.util.concurrent.Future;
+import reactor.core.scheduler.Schedulers;
+
+final class EventLoopNonBlockingTest {
+
+    /**
+     * Verifies that the current thread is registered a non-blocking thread via {@code ReactorNonBlockingUtil}.
+     */
+    @Test
+    void checkEventLoopNonBlocking() throws Exception {
+        final Future<Boolean> submit = CommonPools.workerGroup().submit(Schedulers::isInNonBlockingThread);
+        assertThat(submit.get()).isTrue();
+    }
+}