fix cache implementation

the cache would behave badly when the same item was insert()ed twice:
it would add the size twice to memory consumption, but would never substract
it twice when purging the (actually single) object from cache. So the cache
would seem to fill up while in fact it wasn't.

frontend/cache.lua

@@ -66,7 +66,32 @@ function Cache:new(o)
     return o
 end
 
+-- internal: remove reference in cache_order list
+function Cache:_unref(key)
+    for i = #self.cache_order, 1, -1 do
+        if self.cache_order[i] == key then
+            table.remove(self.cache_order, i)
+        end
+    end
+end
+
+-- internal: free cache item
+function Cache:_free(key)
+    if not self.cache[key] then return end
+    self.current_memsize = self.current_memsize - self.cache[key].size
+    self.cache[key]:onFree()
+    self.cache[key] = nil
+end
+
+-- drop an item named via key from the cache
+function Cache:drop(key)
+    self:_unref(key)
+    self:_free(key)
+end
+
 function Cache:insert(key, object)
+    -- make sure that one key only exists once: delete existing
+    self:drop(key)
     -- guarantee that we have enough memory in cache
     if(object.size > self.max_memsize) then
         DEBUG("too much memory claimed for", key)
@@ -76,9 +101,7 @@ function Cache:insert(key, object)
     -- (they are at the end of the cache_order array)
     while self.current_memsize + object.size > self.max_memsize do
         local removed_key = table.remove(self.cache_order)
-        self.current_memsize = self.current_memsize - self.cache[removed_key].size
-        self.cache[removed_key]:onFree()
-        self.cache[removed_key] = nil
+        self:_free(removed_key)
     end
     -- insert new object in front of the LRU order
     table.insert(self.cache_order, 1, key)
@@ -94,11 +117,7 @@ function Cache:check(key, ItemClass)
     if self.cache[key] then
         if self.cache_order[1] ~= key then
             -- put key in front of the LRU list
-            for k, v in ipairs(self.cache_order) do
-                if v == key then
-                    table.remove(self.cache_order, k)
-                end
-            end
+            self:_unref(key)
             table.insert(self.cache_order, 1, key)
         end
         return self.cache[key]