libutil: Add LRUCache::getOrNullptr

For heavier objects it doesn't make sense to return
a std::optional with the copy of the data, when it
can be used by const reference.

src/libutil/include/nix/util/lru-cache.hh

@@ -33,6 +33,18 @@ private:
     Data data;
     LRU lru;
 
+    /**
+     * Move this item to the back of the LRU list.
+     */
+    void promote(LRU::iterator it)
+    {
+        /* Think of std::list iterators as stable pointers to the list node,
+         * which never get invalidated. Thus, we can reuse the same lru list
+         * element and just splice it to the back of the list without the need
+         * to update its value in the key -> list iterator map. */
+        lru.splice(/*pos=*/lru.end(), /*other=*/lru, it);
+    }
+
 public:
 
     LRUCache(size_t capacity)
@@ -83,7 +95,9 @@ public:
     /**
      * Look up an item in the cache. If it exists, it becomes the most
      * recently used item.
-     * */
+     *
+     * @returns corresponding cache entry, std::nullopt if it's not in the cache
+     */
     template<typename K>
     std::optional<Value> get(const K & key)
     {
@@ -91,20 +105,30 @@ public:
         if (i == data.end())
             return {};
 
-        /**
-         * Move this item to the back of the LRU list.
-         *
-         * Think of std::list iterators as stable pointers to the list node,
-         * which never get invalidated. Thus, we can reuse the same lru list
-         * element and just splice it to the back of the list without the need
-         * to update its value in the key -> list iterator map.
-         */
         auto & [it, value] = i->second;
-        lru.splice(/*pos=*/lru.end(), /*other=*/lru, it.it);
-
+        promote(it.it);
         return value;
     }
 
+    /**
+     * Look up an item in the cache. If it exists, it becomes the most
+     * recently used item.
+     *
+     * @returns mutable pointer to the corresponding cache entry, nullptr if
+     * it's not in the cache
+     */
+    template<typename K>
+    Value * getOrNullptr(const K & key)
+    {
+        auto i = data.find(key);
+        if (i == data.end())
+            return nullptr;
+
+        auto & [it, value] = i->second;
+        promote(it.it);
+        return &value;
+    }
+
     size_t size() const noexcept
     {
         return data.size();