Fix PCM buffer thread safety with proper mutex

src/libprojectM/Audio/PCM.cpp

@@ -1,5 +1,7 @@
 #include "Audio/PCM.hpp"
 
+#include <mutex>
+
 namespace libprojectM {
 namespace Audio {
 
@@ -17,6 +19,7 @@ void PCM::AddToBuffer(
         return;
     }
 
+    std::lock_guard<std::mutex> lock(m_pcmMutex);
     for (size_t i = 0; i < sampleCount; i++)
     {
         size_t const bufferOffset = (m_start + i) % AudioBufferSamples;
@@ -48,9 +51,12 @@ void PCM::Add(int16_t const* const samples, uint32_t channels, size_t const coun
 
 void PCM::UpdateFrameAudioData(double secondsSinceLastFrame, uint32_t frame)
 {
-    // 1. Copy audio data from input buffer
-    CopyNewWaveformData(m_inputBufferL, m_waveformL);
-    CopyNewWaveformData(m_inputBufferR, m_waveformR);
+    // 1. Copy audio data from input buffer (lock to prevent tearing with audio thread writes)
+    {
+        std::lock_guard<std::mutex> lock(m_pcmMutex);
+        CopyNewWaveformData(m_inputBufferL, m_waveformL);
+        CopyNewWaveformData(m_inputBufferR, m_waveformR);
+    }
 
     // 2. Update spectrum analyzer data for both channels
     UpdateSpectrum(m_waveformL, m_spectrumL);
@@ -110,7 +116,7 @@ void PCM::UpdateSpectrum(const WaveformBuffer& waveformData, SpectrumBuffer& spe
 
 void PCM::CopyNewWaveformData(const WaveformBuffer& source, WaveformBuffer& destination)
 {
-    auto const bufferStartIndex = m_start.load();
+    auto const bufferStartIndex = m_start;
 
     for (size_t i = 0; i < AudioBufferSamples; i++)
     {

src/libprojectM/Audio/PCM.hpp

@@ -15,9 +15,9 @@
 
 #include <projectM-4/projectM_cxx_export.h>
 
-#include <atomic>
 #include <cstdint>
 #include <cstdlib>
+#include <mutex>
 
 
 namespace libprojectM {
@@ -89,10 +89,12 @@ class PROJECTM_CXX_EXPORT PCM
      */
     void CopyNewWaveformData(const WaveformBuffer& source, WaveformBuffer& destination);
 
+    std::mutex m_pcmMutex; //!< Protects the circular input buffer from concurrent access.
+
     // External input buffer
     WaveformBuffer m_inputBufferL{0.f}; //!< Circular buffer for left-channel PCM data.
     WaveformBuffer m_inputBufferR{0.f}; //!< Circular buffer for right-channel PCM data.
-    std::atomic<size_t> m_start{0};     //!< Circular buffer start index.
+    size_t m_start{0};                  //!< Circular buffer start index.
 
     // Frame waveform data
     WaveformBuffer m_waveformL{0.f}; //!< Left-channel waveform data, aligned. Only the first WaveformSamples number of samples are valid.