Fix processing params clear when XMP sync enabled

Fix caching of XMP when clearing processing parameters so that it knows
if the pp3 can be deleted without affecting the rank and color labels.

rtengine/utils.h

@@ -18,6 +18,7 @@
  */
 #pragma once
 
+#include <functional>
 #include <vector>
 
 #include <type_traits>
@@ -26,6 +27,37 @@
 namespace rtengine
 {
 
+/**
+ * A function object that supplies a value and returns the same value for
+ * subsequent calls.
+ */
+template <typename T>
+struct MemoizingSupplier {
+    using Supplier = std::function<T()>;
+
+    /**
+     * @param supplier The delegate supplier.
+     */
+    explicit MemoizingSupplier(const Supplier &supplier) :
+        supplier(supplier)
+    {
+    }
+
+    T operator()() const
+    {
+        if (!is_cached) {
+            value = supplier();
+            is_cached = true;
+        }
+        return value;
+    }
+
+private:
+    const Supplier supplier;
+    mutable T value;
+    mutable bool is_cached{false};
+};
+
 // Update a point of a Cairo::Surface by accessing the raw data
 void poke255_uc(unsigned char*& dest, unsigned char r, unsigned char g, unsigned char b);
 // Update a point of a Cairo::Surface by accessing the raw data

rtgui/thumbnail.cc

@@ -37,6 +37,7 @@
 #include "rtengine/metadata.h"
 #include "rtengine/profilestore.h"
 #include "rtengine/settings.h"
+#include "rtengine/utils.h"
 #include "guiutils.h"
 #include "batchqueue.h"
 #include "extprog.h"
@@ -1418,10 +1419,9 @@ void Thumbnail::updateProcParamsProperties(bool forceUpdate)
         pparamsValid = true;
     }
 
-    const auto getXmpSidecar = [this]() {
-        static auto xmp = rtengine::Exiv2Metadata::getXmpSidecar(fname);
-        return xmp;
-    };
+    const rtengine::MemoizingSupplier<Exiv2::XmpData> getXmpSidecar([this]() {
+        return rtengine::Exiv2Metadata::getXmpSidecar(fname);
+    });
 
     // save procparams rank and color also when options.thumbnailRankColorMode == Options::ThumbnailPropertyMode::XMP
     // so they'll be kept in sync