fix crash enabling PFL after an EffectState is deleted

1 files changed, 15 insertions, 0 deletions

diff --git a/src/engine/effects/engineeffect.cpp b/src/engine/effects/engineeffect.cpp
index 5475a0b13b..7df1ef01ba 100644
--- a/src/engine/effects/engineeffect.cpp
+++ b/src/engine/effects/engineeffect.cpp
@@ -71,6 +71,21 @@ void EngineEffect::loadStatesForInputChannel(const ChannelHandle* inputChannel,
 
 // Called from the main thread for garbage collection after an input channel is disabled
 void EngineEffect::deleteStatesForInputChannel(const ChannelHandle* inputChannel) {
+    // If an output channel is not presently being processed, for example when
+    // PFL is not active, then EngineEffect::process cannot be relied upon to
+    // set the EffectEnableState from Disabling to Disabled. This must be done
+    // before the next time EngineEffect::process is called for that output
+    // channel, otherwise the EffectEnableState will still be in the intermediate
+    // Disabling state and EffectProcessorImpl::processChannel will try to run
+    // with an EffectState that has already been deleted and cause a crash.
+    // Refer to https://bugs.launchpad.net/mixxx/+bug/1741213
+    // NOTE: ChannelHandleMap is like a map in that it associates an object with
+    // a ChannelHandle key, but it actually backed by a QVarLengthArray, not a
+    // QMap. So it is okay that m_effectEnableStateForChannelMatrix may be
+    // accessed concurrently in the audio engine thread in EngineEffect::process.
+    for (EffectEnableState& effectState : m_effectEnableStateForChannelMatrix[*inputChannel]) {
+        effectState = EffectEnableState::Disabled;
+    }
     m_pProcessor->deleteStatesForInputChannel(inputChannel);
 }