fix(plc4j/drivers/s7): stop S7HMuxImpl from leaking direct buffers (fixes #2248)

plc4j/drivers/s7/src/main/java/org/apache/plc4x/java/s7/readwrite/protocol/S7HMuxImpl.java

@@ -19,6 +19,7 @@
 package org.apache.plc4x.java.s7.readwrite.protocol;
 
 import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelHandler;
 import io.netty.channel.ChannelHandler.Sharable;
@@ -130,9 +131,18 @@ public class S7HMuxImpl extends MessageToMessageCodec<ByteBuf, ByteBuf> implemen
     @Override
     protected void encode(ChannelHandlerContext ctx, ByteBuf outBB, List<Object> list) {
         if ((embedCtx == null) && (ctx.channel() instanceof EmbeddedChannel)) embedCtx = ctx;
-        if ((tcpChannel != null) && (embedCtx == ctx)) {
-            tcpChannel.writeAndFlush(outBB.copy());
+
+        if (ctx == embedCtx) {
+            if (tcpChannel != null) {
+                tcpChannel.writeAndFlush(outBB.copy());
+            }
+            // Netty requires at least one element in the out list, but nothing consumes
+            // the EmbeddedChannel's outbound queue. Use the EMPTY_BUFFER singleton instead
+            // of a copied ByteBuf to avoid leaking direct memory (see issue #2248).
+            list.add(Unpooled.EMPTY_BUFFER);
+            return;
         }
+
         list.add(outBB.copy());
     }
 

plc4j/drivers/s7/src/test/java/org/apache/plc4x/java/s7/readwrite/protocol/S7HMuxLeakTest.java

@@ -0,0 +1,98 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF 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 org.apache.plc4x.java.s7.readwrite.protocol;
+
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.PooledByteBufAllocator;
+import io.netty.channel.embedded.EmbeddedChannel;
+import io.netty.util.ResourceLeakDetector;
+import org.apache.plc4x.java.s7.readwrite.configuration.S7Configuration;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Regression test for issue #2248.
+ *
+ * <p>S7HMuxImpl is installed in the EmbeddedChannel pipeline of S7HPlcConnection. Its encode()
+ * forwards each outbound message to the active TCP channel and propagates a sentinel downstream.
+ * Before the fix, encode() pushed a freshly-copied direct ByteBuf into the EmbeddedChannel's
+ * outbound queue: nothing in plc4j ever drained that queue, so every sent message leaked one
+ * direct buffer until DirectMemory was exhausted (OutOfMemoryError reported on long-running
+ * applications).
+ */
+public class S7HMuxLeakTest {
+
+    private static final int ITERATIONS = 2000;
+    private static final int PAYLOAD_BYTES = 256;
+
+    @Test
+    void embeddedOutboundQueueShouldNotAccumulateRealByteBufs() throws Exception {
+        ResourceLeakDetector.Level previous = ResourceLeakDetector.getLevel();
+        ResourceLeakDetector.setLevel(ResourceLeakDetector.Level.PARANOID);
+        try {
+            S7HMuxImpl mux = new S7HMuxImpl();
+            EmbeddedChannel embedded = new EmbeddedChannel(mux);
+            EmbeddedChannel tcp = new EmbeddedChannel();
+
+            mux.setEmbededhannel(embedded, new S7Configuration());
+            mux.setPrimaryChannel(tcp);
+
+            int forwardedToTcp = 0;
+            for (int i = 0; i < ITERATIONS; i++) {
+                ByteBuf out = PooledByteBufAllocator.DEFAULT.directBuffer(PAYLOAD_BYTES);
+                out.writeZero(PAYLOAD_BYTES);
+                embedded.writeOutbound(out);
+
+                Object msg;
+                while ((msg = tcp.readOutbound()) != null) {
+                    if (msg instanceof ByteBuf) {
+                        ((ByteBuf) msg).release();
+                        forwardedToTcp++;
+                    }
+                }
+            }
+
+            assertEquals(ITERATIONS, forwardedToTcp,
+                "every message written to the embedded channel must be forwarded once to the TCP channel");
+
+            int leakedBuffers = 0;
+            long leakedBytes = 0;
+            Object pending;
+            while ((pending = embedded.readOutbound()) != null) {
+                if (pending instanceof ByteBuf) {
+                    ByteBuf bb = (ByteBuf) pending;
+                    if (bb.capacity() > 0) {
+                        leakedBuffers++;
+                        leakedBytes += bb.capacity();
+                    }
+                    bb.release();
+                }
+            }
+            assertEquals(0, leakedBuffers,
+                "EmbeddedChannel outbound queue must not accumulate non-empty ByteBufs (leaked "
+                    + leakedBytes + " bytes across " + leakedBuffers + " buffers)");
+
+            embedded.finishAndReleaseAll();
+            tcp.finishAndReleaseAll();
+        } finally {
+            ResourceLeakDetector.setLevel(previous);
+        }
+    }
+}