From f370edcb5d471fadfd6401d434f735693169111d Mon Sep 17 00:00:00 2001
From: hogliux <fabian.renn@gmail.com>
Date: Tue, 4 Oct 2022 16:31:50 +0200
Subject: [PATCH] VST3: Fix an issue with sidechain configurations in Cubase

---
 .../VST3/juce_VST3_Wrapper.cpp                | 47 +++++++++++++++----
 .../processors/juce_AudioProcessor.h          |  2 +-
 2 files changed, 39 insertions(+), 10 deletions(-)

diff --git a/modules/juce_audio_plugin_client/VST3/juce_VST3_Wrapper.cpp b/modules/juce_audio_plugin_client/VST3/juce_VST3_Wrapper.cpp
index 3d33c26a3e..7d3f7ce83c 100644
--- a/modules/juce_audio_plugin_client/VST3/juce_VST3_Wrapper.cpp
+++ b/modules/juce_audio_plugin_client/VST3/juce_VST3_Wrapper.cpp
@@ -3256,13 +3256,21 @@ public:
         if (numIns > numInputBuses || numOuts > numOutputBuses)
             return false;
 
-        auto requested = pluginInstance->getBusesLayout();
+        // see the following documentation to understand the correct way to react to this callback
+        // https://steinbergmedia.github.io/vst3_doc/vstinterfaces/classSteinberg_1_1Vst_1_1IAudioProcessor.html#ad3bc7bac3fd3b194122669be2a1ecc42
 
-        for (int i = 0; i < numIns; ++i)
-            requested.getChannelSet (true,  i) = getChannelSetForSpeakerArrangement (inputs[i]);
+        const auto requestedLayout = [&]
+        {
+            auto result = pluginInstance->getBusesLayout();
 
-        for (int i = 0; i < numOuts; ++i)
-            requested.getChannelSet (false, i) = getChannelSetForSpeakerArrangement (outputs[i]);
+            for (int i = 0; i < numIns; ++i)
+                result.getChannelSet (true,  i) = getChannelSetForSpeakerArrangement (inputs[i]);
+
+            for (int i = 0; i < numOuts; ++i)
+                result.getChannelSet (false, i) = getChannelSetForSpeakerArrangement (outputs[i]);
+
+            return result;
+        }();
 
        #ifdef JucePlugin_PreferredChannelConfigurations
         short configs[][2] = { JucePlugin_PreferredChannelConfigurations };
@@ -3270,11 +3278,32 @@ public:
             return kResultFalse;
        #endif
 
-        if (! pluginInstance->setBusesLayoutWithoutEnabling (requested))
-            return kResultFalse;
+        if (pluginInstance->checkBusesLayoutSupported (requestedLayout))
+        {
+            if (! pluginInstance->setBusesLayoutWithoutEnabling (requestedLayout))
+                return kResultFalse;
 
-        bufferMapper.updateFromProcessor (*pluginInstance);
-        return kResultTrue;
+            bufferMapper.updateFromProcessor (*pluginInstance);
+            return kResultTrue;
+        }
+
+        // apply layout changes in reverse order as Steinberg says we should prioritize main buses
+        const auto nextBest = [this, numInputBuses, numOutputBuses, &requestedLayout]
+        {
+            auto layout = pluginInstance->getBusesLayout();
+
+            for (auto busIdx = jmax (numInputBuses, numOutputBuses) - 1; busIdx >= 0; --busIdx)
+                for (const auto isInput : { true, false })
+                    if (auto* bus = pluginInstance->getBus (isInput, busIdx))
+                        bus->isLayoutSupported (requestedLayout.getChannelSet (isInput, busIdx), &layout);
+
+            return layout;
+        }();
+
+        if (pluginInstance->setBusesLayoutWithoutEnabling (nextBest))
+            bufferMapper.updateFromProcessor (*pluginInstance);
+
+        return kResultFalse;
     }
 
     tresult PLUGIN_API getBusArrangement (Vst::BusDirection dir, Steinberg::int32 index, Vst::SpeakerArrangement& arr) override
diff --git a/modules/juce_audio_processors/processors/juce_AudioProcessor.h b/modules/juce_audio_processors/processors/juce_AudioProcessor.h
index 88c12b650e..2ea13dc9d6 100644
--- a/modules/juce_audio_processors/processors/juce_AudioProcessor.h
+++ b/modules/juce_audio_processors/processors/juce_AudioProcessor.h
@@ -418,7 +418,7 @@ public:
             @param set           The AudioChannelSet which is to be probed.
             @param currentLayout If non-null, pretend that the current layout of the AudioProcessor is
                                  currentLayout. On exit, currentLayout will be modified to
-                                 to represent the buses layouts of the AudioProcessor as if the layout
+                                 represent the buses layouts of the AudioProcessor as if the layout
                                  of the receiver had been successfully changed. This is useful as changing
                                  the layout of the receiver may change the bus layout of other buses.