Reduce memory leaks when switching credocuments

CRE cache, hyphdict and fonts can be initialized only once
when first credocument is opened. Previously, they were
recreated for each document, and as previous instances were probably
not free'd, this caused memory leaks.

frontend/document/credocument.lua

@@ -9,14 +9,16 @@ local ffi = require("ffi")
 local lfs = require("libs/libkoreader-lfs")
 local logger = require("logger")
 
+-- engine can be initialized only once, on first document opened
+local engine_initialized = false
+
 local CreDocument = Document:new{
     -- this is defined in kpvcrlib/crengine/crengine/include/lvdocview.h
     SCROLL_VIEW_MODE = 0,
     PAGE_VIEW_MODE = 1,
 
     _document = false,
     _loaded = false,
-    engine_initilized = false,
 
     line_space_percent = 100,
     default_font = G_reader_settings:readSetting("cre_font") or "Noto Serif",
@@ -47,7 +49,7 @@ function CreDocument:cacheInit()
 end
 
 function CreDocument:engineInit()
-    if not self.engine_initilized then
+    if not engine_initialized then
         require "libs/libkoreader-cre"
         -- initialize cache
         self:cacheInit()
@@ -66,7 +68,7 @@ function CreDocument:engineInit()
             end
         end
 
-        self.engine_initilized = true
+        engine_initialized = true
     end
 end
 

frontend/document/documentregistry.lua

@@ -28,6 +28,11 @@ function DocumentRegistry:getProvider(file)
 end
 
 function DocumentRegistry:openDocument(file)
+    -- force a GC, so that any previous document used memory can be reused
+    -- immediately by this new document without having to wait for the
+    -- next regular gc. The second call may help reclaming more memory.
+    collectgarbage()
+    collectgarbage()
     if not self.registry[file] then
         local provider = self:getProvider(file)
         if provider ~= nil then