fix: crash when changing a plugin node's input port configuration

src/engine/graphbuilder.cpp

@@ -605,8 +605,14 @@ void GraphBuilder::createRenderingOpsForNode (Processor* const node,
             const auto* const c = graph.getConnection (i);
             if (c->destNode == node->nodeId && c->destPort == port)
             {
-                sourceNodes.add (c->sourceNode);
-                sourcePorts.add (c->sourcePort);
+                // A connection can go stale between a node changing its ports
+                // and removeIllegalConnections() running; feeding one to
+                // BindParameterOp below dereferences a null parameter.
+                if (graph.isConnectionLegal (c))
+                {
+                    sourceNodes.add (c->sourceNode);
+                    sourcePorts.add (c->sourcePort);
+                }
             }
         }
 

test/BusesLayoutTests.cpp

@@ -0,0 +1,111 @@
+// SPDX-FileCopyrightText: Copyright (C) Kushview, LLC.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include <boost/test/unit_test.hpp>
+
+#include <element/context.hpp>
+
+#include "engine/graphnode.hpp"
+#include "nodes/audioprocessor.hpp"
+#include "fixture/TestNode.h"
+
+using namespace juce;
+using namespace element;
+
+namespace {
+
+/** A minimal AudioProcessor with configurable buses and parameters, used to
+    reproduce bus-layout changes on plugin nodes without a real plugin. */
+class BusLayoutTestProcessor : public AudioProcessor {
+public:
+    BusLayoutTestProcessor()
+    {
+        setPlayConfigDetails (8, 2, 44100.0, 512);
+    }
+
+    const String getName() const override { return "BusLayoutTestProcessor"; }
+    void prepareToPlay (double, int) override {}
+    void releaseResources() override {}
+    void processBlock (AudioBuffer<float>&, MidiBuffer&) override {}
+    void processBlock (AudioBuffer<double>&, MidiBuffer&) override {}
+    bool isBusesLayoutSupported (const BusesLayout&) const override { return true; }
+    bool hasEditor() const override { return false; }
+    AudioProcessorEditor* createEditor() override { return nullptr; }
+    double getTailLengthSeconds() const override { return 0.0; }
+    bool acceptsMidi() const override { return false; }
+    bool producesMidi() const override { return false; }
+    bool silenceInProducesSilenceOut() const override { return true; }
+    void getStateInformation (juce::MemoryBlock&) override {}
+    void setStateInformation (const void*, int) override {}
+    int getNumPrograms() override { return 1; }
+    int getCurrentProgram() override { return 0; }
+    void setCurrentProgram (int) override {}
+    const String getProgramName (int) override { return "default"; }
+    void changeProgramName (int, const String&) override {}
+
+    using AudioProcessor::setBusesLayoutWithoutEnabling;
+};
+
+} // namespace
+
+BOOST_AUTO_TEST_SUITE (BusesLayoutTests)
+
+/** Reproduces the crash when an input port configuration change shrinks the
+    plugin's channel count while connections to the old ports still exist.
+    EngineService::changeBusesLayout rebuilds the rendering sequence before
+    removing now-illegal connections, so stale arcs feed the GraphBuilder. */
+BOOST_AUTO_TEST_CASE (ShrinkInputsBuildsSequenceSafely)
+{
+    element::Context context;
+    GraphNode graph (context);
+    graph.setNumPorts (PortType::Audio, 2, true, false);
+    graph.setNumPorts (PortType::Audio, 2, false, false);
+    graph.setNumPorts (PortType::Midi, 1, true, false);
+    graph.setNumPorts (PortType::Midi, 1, false, false);
+
+    auto* proc = new BusLayoutTestProcessor();
+    proc->refreshParameterList();
+    for (int i = 0; i < 8; ++i)
+        proc->addParameter (new AudioParameterFloat ({ "p" + String (i) }, "Param " + String (i), 0.f, 1.f, 0.f));
+
+    ProcessorPtr node = graph.addNode (new AudioProcessorNode (proc));
+    BOOST_REQUIRE (node != nullptr);
+
+    graph.prepareToRender (44100.0, 512);
+
+    // Port order for AudioProcessorNode: [audio ins, audio outs, controls, midi]
+    // 8 ins + 2 outs + 8 params = 18 ports: idx 0..7 in; 8,9 out; 10..17 control
+    BOOST_REQUIRE (node->getNumPorts (PortType::Audio, true) == 8);
+    BOOST_REQUIRE (node->getNumPorts (PortType::Audio, false) == 2);
+    BOOST_REQUIRE (node->getNumPorts (PortType::Control, true) == 8);
+
+    // Connect a source node's audio out to the plugin's LAST audio input
+    // (port 7). Shrinking 8 ins -> 2 renumbers port 7 into the Control range,
+    // so this arc becomes stale in the worst possible way.
+    auto* srcNode = new TestNode (2, 1, 1, 1);
+    srcNode->refreshPorts();
+    ProcessorPtr src = graph.addNode (srcNode);
+
+    const auto srcPort = src->getPortForChannel (PortType::Audio, 0, false);
+    const auto dstPort = node->getPortForChannel (PortType::Audio, 7, true);
+    BOOST_REQUIRE (graph.addConnection (src->nodeId, srcPort, node->nodeId, dstPort));
+
+    // Shrink the plugin to 1-in/1-out, exactly as the I/O configuration window does.
+    AudioProcessor::BusesLayout layout;
+    layout.inputBuses.add (AudioChannelSet::mono());
+    layout.outputBuses.add (AudioChannelSet::mono());
+    proc->setBusesLayoutWithoutEnabling (layout);
+
+    // Mirror EngineService::changeBusesLayout exactly: after port renumbering,
+    // release + re-prepare the graph, which rebuilds the rendering sequence
+    // synchronously BEFORE removeIllegalConnections() runs. A stale arc into
+    // audio_in_8 now points at a Control port; GraphBuilder builds
+    // BindParameterOp with a null source parameter and crashes in its ctor.
+    node->refreshPorts();
+    graph.releaseResources();
+    graph.prepareToRender (44100.0, 512);
+
+    BOOST_CHECK (true); // reaching here means no crash
+}
+
+BOOST_AUTO_TEST_SUITE_END()

test/CMakeLists.txt

@@ -62,6 +62,7 @@ add_test(NAME "AtomicLockTests" COMMAND test_element --run_test=AtomicLockTests)
 add_test(NAME "AudioRoutingTests" COMMAND test_element --run_test=AudioRoutingTests)
 add_test(NAME "AuthTests" COMMAND test_element --run_test=AuthTests)
 add_test(NAME "BytesTest" COMMAND test_element --run_test=BytesTest)
+add_test(NAME "BusesLayoutTests" COMMAND test_element --run_test=BusesLayoutTests)
 add_test(NAME "JuceIntegrationTests" COMMAND test_element --run_test=JuceIntegrationTests)
 add_test(NAME "DataPathTests" COMMAND test_element --run_test=DataPathTests)
 add_test(NAME "DeviceMonitorTests" COMMAND test_element --run_test=DeviceMonitorTests)