DocCache: Allow disabling it (again) (#8198)

* Ensure DocCache will always have at least one slot
Fix #8181

frontend/cache.lua

@@ -43,7 +43,7 @@ function Cache:init()
         self.cache = lru.new(self.slots, nil, self.enable_eviction_cb)
     else
         -- Compute the amount of slots in the LRU based on the max size & the average item size
-        self.slots = math.floor(self.size / self.avg_itemsize)
+        self.slots = math.ceil(self.size / self.avg_itemsize)
         self.cache = lru.new(self.slots, self.size, self.enable_eviction_cb)
     end
 

frontend/document/doccache.lua

@@ -11,14 +11,38 @@ local function calcCacheMemSize()
     local min = DGLOBAL_CACHE_SIZE_MINIMUM
     local max = DGLOBAL_CACHE_SIZE_MAXIMUM
     local calc = Cache:_calcFreeMem() * (DGLOBAL_CACHE_FREE_PROPORTION or 0)
-    local size = math.min(max, math.max(min, calc))
+    return math.min(max, math.max(min, calc))
+end
+
+local function computeCacheSize()
+    local size = calcCacheMemSize()
+    local mb_size = size / 1024 / 1024
+
+    -- If we end up with a not entirely ridiculous cache size, use that...
+    if mb_size >= 8 then
+        logger.dbg(string.format("Allocating a %dMB budget for the global document cache", mb_size))
+        return size
+    else
+        return nil
+    end
+end
+
+local function computeCacheSlots()
+    local size = calcCacheMemSize()
+    local mb_size = size / 1024 / 1024
 
-    logger.dbg(string.format("Allocating a %dMB budget for the global document cache", size / 1024 / 1024))
-    return size
+    --- ...otherwise, effectively disable the cache by making it single slot...
+    if mb_size < 8 then
+        logger.dbg(string.format("Setting up a minimal single slot global document cache"))
+        return 1
+    else
+        return nil
+    end
 end
 
 local DocCache = Cache:new{
-    size = calcCacheMemSize(),
+    slots = computeCacheSlots(),
+    size = computeCacheSize(),
     -- Average item size is a screen's worth of bitmap, mixed with a few much smaller tables (pgdim, pglinks, etc.), hence the / 3
     avg_itemsize = math.floor(CanvasContext:getWidth() * CanvasContext:getHeight() * (CanvasContext.is_color_rendering_enabled and 4 or 1) / 3),
     -- Rely on CacheItem's eviction callback to free resources *immediately* on eviction.