Use MethodHandle to avoid hard dependency on newer netty version

servicetalk-http-netty/src/main/java/io/servicetalk/http/netty/OptimizedHttp2FrameCodecBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2021 Apple Inc. and the ServiceTalk project authors
+ * Copyright © 2021-2022 Apple Inc. and the ServiceTalk project authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,13 +21,45 @@
 import io.netty.handler.codec.http2.Http2FrameCodecBuilder;
 import io.netty.handler.codec.http2.Http2RemoteFlowController;
 import io.netty.handler.codec.http2.UniformStreamByteDistributor;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import javax.annotation.Nullable;
+
+import static io.servicetalk.utils.internal.PlatformDependent.throwException;
+import static java.lang.invoke.MethodType.methodType;
 
 /**
  * Optimized variant of {@link Http2FrameCodecBuilder} that allows us to use {@link UniformStreamByteDistributor}
  * for {@link Http2RemoteFlowController}.
  */
 final class OptimizedHttp2FrameCodecBuilder extends Http2FrameCodecBuilder {
 
+    private static final Logger LOGGER = LoggerFactory.getLogger(OptimizedHttp2FrameCodecBuilder.class);
+
+    @Nullable
+    private static final MethodHandle FLUSH_PREFACE;
+
+    static {
+        MethodHandle flushPreface;
+        try {
+            // Find a new method that exists only in Netty starting from 4.1.78.Final:
+            flushPreface = MethodHandles.publicLookup()
+                    .findVirtual(Http2FrameCodecBuilder.class, "flushPreface",
+                            methodType(Http2FrameCodecBuilder.class, boolean.class));
+            // Verify the method is working as expected:
+            disableFlushPreface(flushPreface, Http2FrameCodecBuilder.forClient());
+        } catch (Throwable cause) {
+            LOGGER.debug("Http2FrameCodecBuilder#flushPreface(boolean) is available only starting from " +
+                            "Netty 4.1.78.Final. Detected Netty version: {}",
+                    Http2FrameCodecBuilder.class.getPackage().getImplementationVersion(), cause);
+            flushPreface = null;
+        }
+        FLUSH_PREFACE = flushPreface;
+    }
+
     private final boolean server;
 
     /**
@@ -37,10 +69,7 @@ final class OptimizedHttp2FrameCodecBuilder extends Http2FrameCodecBuilder {
      */
     OptimizedHttp2FrameCodecBuilder(final boolean server) {
         this.server = server;
-        // We manage flushes at ST level and don't want netty to flush the preface & settings only. Instead, we write
-        // headers or entire message and flush them all together. Netty changed the default flush behavior starting from
-        // 4.1.78.Final. For context, see https://github.com/netty/netty/pull/12349.
-        flushPreface(false);
+        disableFlushPreface(FLUSH_PREFACE, this);
     }
 
     @Override
@@ -56,4 +85,30 @@ public Http2FrameCodec build() {
         connection(connection);
         return super.build();
     }
+
+    /**
+     * We manage flushes at ST level and don't want netty to flush the preface & settings only. Instead, we write
+     * headers or entire message and flush them all together. Netty changed the default flush behavior starting from
+     * 4.1.78.Final. To avoid a strict dependency on Netty 4.1.78.Final in the classpath, we use {@link MethodHandle} to
+     * check if the new method is available or not.
+     *
+     * @param flushPrefaceMethod {@link MethodHandle} for {@link Http2FrameCodecBuilder#flushPreface(boolean)}
+     * @param builderInstance an instance of {@link Http2FrameCodecBuilder} where the flush behavior should be disabled
+     * @return {@link Http2FrameCodecBuilder} or {@code null} if {@code flushPrefaceMethod == null}
+     * @see <a href="https://github.com/netty/netty/pull/12349">Netty PR#12349</a>
+     */
+    @Nullable
+    private static Http2FrameCodecBuilder disableFlushPreface(@Nullable final MethodHandle flushPrefaceMethod,
+                                                              final Http2FrameCodecBuilder builderInstance) {
+        if (flushPrefaceMethod == null) {
+            return null;
+        }
+        try {
+            // invokeExact requires return type cast to match the type signature
+            return (Http2FrameCodecBuilder) flushPrefaceMethod.invokeExact(builderInstance, false);
+        } catch (Throwable t) {
+            throwException(t);
+            return null;
+        }
+    }
 }