#2438 resolves broadcaster error that happens during tuner source swapout (#2550)

Co-authored-by: denny <dennis.sheirer@gmail.com>

src/main/java/io/github/dsheirer/dsp/filter/channelizer/PolyphaseChannelSource.java

@@ -23,6 +23,7 @@ import io.github.dsheirer.dsp.filter.channelizer.output.OneChannelOutputProcesso
 import io.github.dsheirer.dsp.filter.channelizer.output.TwoChannelOutputProcessor;
 import io.github.dsheirer.dsp.filter.design.FilterDesignException;
 import io.github.dsheirer.sample.Listener;
+import io.github.dsheirer.sample.SerializedListener;
 import io.github.dsheirer.sample.complex.ComplexSamples;
 import io.github.dsheirer.source.SourceEvent;
 import io.github.dsheirer.source.tuner.channel.TunerChannel;
@@ -45,7 +46,8 @@ public class PolyphaseChannelSource extends TunerChannelSource implements Listen
 {
     private Logger mLog = LoggerFactory.getLogger(PolyphaseChannelSource.class);
     private IPolyphaseChannelOutputProcessor mPolyphaseChannelOutputProcessor;
-    private Listener<ComplexSamples> mSamplesListener;
+    //Output processor replacement can overlap the outgoing and incoming dispatcher threads, and consumers aren't thread safe
+    private final SerializedListener<ComplexSamples> mSamplesListener = new SerializedListener<>();
     private double mChannelSampleRate;
     private long mIndexCenterFrequency;
     private List<Integer> mOutputProcessorIndexes = new ArrayList<>();
@@ -148,23 +150,20 @@ public class PolyphaseChannelSource extends TunerChannelSource implements Listen
     @Override
     public void setListener(final Listener<ComplexSamples> listener)
     {
-        mSamplesListener = listener;
+        mSamplesListener.setListener(listener);
         mPolyphaseChannelOutputProcessor.setListener(this);
     }
 
     @Override
     public void receive(ComplexSamples complexSamples)
     {
-        if(mSamplesListener != null)
+        try
         {
-            try
-            {
-                mSamplesListener.receive(complexSamples);
-            }
-            catch(Throwable t)
-            {
-                mLog.error("Error dispatching complex samples to listener [" + mSamplesListener.getClass() + "]", t);
-            }
+            mSamplesListener.receive(complexSamples);
+        }
+        catch(Throwable t)
+        {
+            mLog.error("Error dispatching complex samples to listener", t);
         }
     }
 

src/main/java/io/github/dsheirer/sample/SerializedListener.java

@@ -0,0 +1,71 @@
+/*
+ * *****************************************************************************
+ * Copyright (C) 2014-2026 Dennis Sheirer
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ * ****************************************************************************
+ */
+
+package io.github.dsheirer.sample;
+
+import java.util.concurrent.locks.ReentrantLock;
+
+/**
+ * Delivers to a listener that isn't thread safe from producers that can overlap.  Dispatcher.stop() doesn't wait for
+ * an in-progress buffer, so when a producer is replaced, the outgoing dispatcher thread can still be inside the listener
+ * when the incoming dispatcher thread delivers its first buffer.  This class makes the incoming thread wait for the
+ * outgoing thread to leave the listener, which means that the thread that replaces the producer doesn't have to wait.
+ *
+ * Producers should acquire no other locks before calling receive(), so that this is always the outermost lock.
+ *
+ * @param <T> type of element delivered to the listener
+ */
+public class SerializedListener<T> implements Listener<T>
+{
+    private final ReentrantLock mLock = new ReentrantLock();
+    private volatile Listener<T> mListener;
+
+    /**
+     * Sets or changes the listener.  This does not wait for a delivery that is in progress to the previous listener.
+     * @param listener to receive elements, or null to ignore elements.
+     */
+    public void setListener(Listener<T> listener)
+    {
+        mListener = listener;
+    }
+
+    /**
+     * Delivers the element to the listener once no other thread is delivering to the listener.
+     * @param t element to deliver
+     */
+    @Override
+    public void receive(T t)
+    {
+        mLock.lock();
+
+        try
+        {
+            Listener<T> listener = mListener;
+
+            if(listener != null)
+            {
+                listener.receive(t);
+            }
+        }
+        finally
+        {
+            mLock.unlock();
+        }
+    }
+}

src/main/java/io/github/dsheirer/source/tuner/channel/MultiFrequencyTunerChannelSource.java

@@ -21,6 +21,7 @@ package io.github.dsheirer.source.tuner.channel;
 
 import com.google.common.eventbus.Subscribe;
 import io.github.dsheirer.sample.Listener;
+import io.github.dsheirer.sample.SerializedListener;
 import io.github.dsheirer.sample.complex.ComplexSamples;
 import io.github.dsheirer.source.Source;
 import io.github.dsheirer.source.SourceEvent;
@@ -51,7 +52,8 @@ public class MultiFrequencyTunerChannelSource extends TunerChannelSource
     private List<Long> mLockedFrequencies = new ArrayList<>();
     private int mFrequencyListPointer = 0;
     private ChannelSpecification mChannelSpecification;
-    private Listener<ComplexSamples> mComplexSamplesListener;
+    //Rotating to the next source can overlap the outgoing and incoming dispatcher threads and consumers aren't thread safe
+    private final SerializedListener<ComplexSamples> mComplexSamplesListener = new SerializedListener<>();
     private Listener<Heartbeat> mHeartbeatListener;
     private String mPreferredTuner;
     private AtomicBoolean mChangingChannels = new AtomicBoolean();
@@ -285,11 +287,11 @@ public class MultiFrequencyTunerChannelSource extends TunerChannelSource
     @Override
     public void setListener(Listener<ComplexSamples> complexSamplesListener)
     {
-        mComplexSamplesListener = complexSamplesListener;
+        mComplexSamplesListener.setListener(complexSamplesListener);
 
         if(mTunerChannelSource != null)
         {
-            mTunerChannelSource.setListener(complexSamplesListener);
+            mTunerChannelSource.setListener(mComplexSamplesListener);
         }
     }