Fix PCM buffer thread safety with proper mutex

The circular audio buffer was protected only by a std::atomic on the
write index, which is insufficient: a multi-sample AddToBuffer() call
can be interrupted mid-write by UpdateFrameAudioData() reading the
buffer, producing torn waveform data.

Replace the atomic index with a std::mutex that protects both the
buffer writes in AddToBuffer() and the bulk copy in
UpdateFrameAudioData(). The lock is held only for the buffer access,
not for the downstream spectrum analysis.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: struktured

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.