Lua: fix global module loading (#7701)

Author: tarleb

src/Text/Pandoc/Lua/Init.hs

@@ -14,10 +14,11 @@ module Text.Pandoc.Lua.Init
   ( runLua
   ) where
 
-import Control.Monad (forM_, when)
+import Control.Monad (forM, forM_, when)
 import Control.Monad.Catch (throwM, try)
 import Control.Monad.Trans (MonadIO (..))
 import Data.Data (Data, dataTypeConstrs, dataTypeOf, showConstr)
+import Data.Maybe (catMaybes)
 import HsLua as Lua hiding (status, try)
 import GHC.IO.Encoding (getForeignEncoding, setForeignEncoding, utf8)
 import Text.Pandoc.Class.PandocMonad (PandocMonad, readDataFile)
@@ -78,23 +79,42 @@ initLuaState = do
         _                  -> T.pack $ show err
 
   setGlobalModules :: PandocLua ()
-  setGlobalModules = liftPandocLua $
-    forM_ [ ("lpeg", LPeg.luaopen_lpeg_ptr)
-          , ("re", LPeg.luaopen_re_ptr)
-          ] $
+  setGlobalModules = liftPandocLua $ do
+    let globalModules =
+          [ ("lpeg", LPeg.luaopen_lpeg_ptr)  -- must be loaded first
+          , ("re", LPeg.luaopen_re_ptr)      -- re depends on lpeg
+          ]
+    loadedBuiltInModules <- fmap catMaybes . forM globalModules $
       \(pkgname, luaopen) -> do
         Lua.pushcfunction luaopen
-        Lua.pcall 0 1 Nothing >>= \case
-          OK -> pure ()          -- all good, loading succeeded
+        usedBuiltIn <- Lua.pcall 0 1 Nothing >>= \case
+          OK -> do               -- all good, loading succeeded
+            -- register as loaded module so later modules can rely on this
+            Lua.getfield Lua.registryindex Lua.loaded
+            Lua.pushvalue (Lua.nth 2)
+            Lua.setfield (Lua.nth 2) pkgname
+            Lua.pop 1  -- pop _LOADED
+            return True
           _  -> do               -- built-in library failed, load system lib
             Lua.pop 1  -- ignore error message
             -- Try loading via the normal package loading mechanism.
             Lua.getglobal "require"
             Lua.pushName pkgname
             Lua.call 1 1  -- Throws an exception if loading failed again!
+            return False
 
         -- Module on top of stack. Register as global
         Lua.setglobal pkgname
+        return $ if usedBuiltIn then Just pkgname else Nothing
+
+    -- Remove module entry from _LOADED table in registry if we used a
+    -- built-in library. This ensures that later calls to @require@ will
+    -- prefer the shared library, if any.
+    forM_ loadedBuiltInModules $ \pkgname -> do
+      Lua.getfield Lua.registryindex Lua.loaded
+      Lua.pushnil
+      Lua.setfield (Lua.nth 2) pkgname
+      Lua.pop 1  -- registry
 
   installLpegSearcher :: PandocLua ()
   installLpegSearcher = liftPandocLua $ do