DSP: Ensured that FFTW is initialised and destroyed in a thread-safe way

modules/juce_dsp/frequency/juce_FFT.cpp

@@ -641,6 +641,8 @@ struct FFTWImpl  : public FFT::Instance
     FFTWImpl (size_t orderToUse, DynamicLibrary&& libraryToUse, const Symbols& symbols)
         : fftwLibrary (std::move (libraryToUse)), fftw (symbols), order (static_cast<size_t> (orderToUse))
     {
+        ScopedLock lock (getFFTWPlanLock());
+
         auto n = (1u << order);
         HeapBlock<Complex<float>> in (n), out (n);
 
@@ -653,6 +655,8 @@ struct FFTWImpl  : public FFT::Instance
 
     ~FFTWImpl() override
     {
+        ScopedLock lock (getFFTWPlanLock());
+
         fftw.destroy_fftw (c2cForward);
         fftw.destroy_fftw (c2cInverse);
         fftw.destroy_fftw (r2c);
@@ -697,6 +701,15 @@ struct FFTWImpl  : public FFT::Instance
         FloatVectorOperations::multiply ((float*) inputOutputData, 1.0f / static_cast<float> (n), (int) n);
     }
 
+    //==============================================================================
+    // fftw's plan_* and destroy_* methods are NOT thread safe. So we need to share
+    // a lock between all instances of FFTWImpl
+    static CriticalSection& getFFTWPlanLock() noexcept
+    {
+        static CriticalSection cs;
+        return cs;
+    }
+
     //==============================================================================
     DynamicLibrary fftwLibrary;
     Symbols fftw;