Added mutex to FFT plan cache function.

changelogs/diplib_next.md

@@ -55,6 +55,10 @@ date: 2020-00-00
   This would throw an exception if the image was protected, but was always a bad thing to do.
   See [issue #170](https://github.com/DIPlib/diplib/issues/170).
 
+- The Fourier Transform, when using the default PocketFFT, used a plan cache that was not thread safe. When calling
+  any function using a Fourier Transform from multiple threads, a race condition could occur. We've added a mutex
+  to the function that maintains the cache to avoid this.
+
 ### Updated dependencies
 
 ### Build changes

src/transform/dft.cpp

@@ -20,6 +20,7 @@
 #include <cmath>
 #include <complex>
 #include <limits>
+#include <mutex>
 #include <vector>
 
 #include "diplib.h"
@@ -91,6 +92,8 @@ using RPlan = pocketfft::pocketfft_r< T >;
 
 namespace {
 
+std::mutex planCacheMutex;
+
 // This is a container that holds plans, and maintains a use count. Plans in use cannot be
 // deleted.
 //
@@ -99,6 +102,8 @@ namespace {
 // PlanType is CPlan< T > or RPlan< T >, T is dip::sfloat or dip::dfloat. There are 4 caches.
 template< typename PlanType >
 PlanType* PlanCache( dip::uint length, bool free = false ) {
+   std::lock_guard< std::mutex > guard( planCacheMutex );
+
    struct Data {
       dip::uint use_count;
       dip::uint last_access;