make caching thread-safe

pocketfft_hdronly.h

@@ -48,6 +48,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <memory>
 #include <vector>
 #include <complex>
+#include <mutex>
 #ifdef POCKETFFT_OPENMP
 #include <omp.h>
 #endif
@@ -2205,18 +2206,31 @@ template<typename T> shared_ptr<T> get_plan(size_t length)
   constexpr size_t nmax=10;
   static vector<shared_ptr<T>> cache(nmax);
   static size_t lastpos=0;
-  // searching phase
-  size_t pos = lastpos;
-  do {
-    if (!cache[pos]) break;
-    if (cache[pos]->length()==length)
-      return cache[pos];
-    pos = (pos==0) ? nmax-1 : pos-1;
-    } while (pos != lastpos);
-  // adding phase
+  static mutex mut;
+  {
+  lock_guard<mutex> lock(mut);
+  for (size_t i=0; i<nmax; ++i)
+    {
+    if (cache[i]==nullptr)
+      break;
+    if (cache[i]->length()==length)
+      return cache[i];
+    }
+  }
+  auto plan = make_shared<T>(length);
+  {
+  lock_guard<mutex> lock(mut);
+  for (size_t i=0; i<nmax; ++i)
+    {
+    if (cache[i]==nullptr)
+      break;
+    if (cache[i]->length()==length)
+      return cache[i];
+    }
+  cache[lastpos] = plan;
   if (++lastpos>=nmax) lastpos=0;
-  cache[lastpos] = make_shared<T>(length);
-  return cache[lastpos];
+  }
+  return plan;
   }
 
 class arr_info