Lua: preserve common state of custom Lua readers

The common state is transferred to Lua when calling a custom Lua reader,
and is now also transferred back after the reader has finished. This
ensures that info messages, warnings, and mediabag entries are available
to the main program and all subsequent processing steps.

pandoc-lua-engine/src/Text/Pandoc/Lua/Custom.hs

@@ -148,12 +148,19 @@ readerField = "Pandoc Reader function"
 writerField :: Name
 writerField = "Pandoc Writer function"
 
--- | Runs a Lua action in a continueable environment.
-inLua :: MonadIO m => GCManagedState -> LuaE PandocError a -> m a
-inLua st = liftIO . withGCManagedState @PandocError st
+-- | Runs a Lua action in a continuable environment and transfers the common
+-- state after the Lua action has finished.
+inLua :: (PandocMonad m, MonadIO m)
+  => GCManagedState -> LuaE PandocError a -> m a
+inLua st luaAction = do
+  let inLua' = liftIO . withGCManagedState @PandocError st
+  result <- inLua' luaAction
+  cstate <- inLua' (unPandocLua PandocMonad.getCommonState)
+  PandocMonad.putCommonState cstate
+  return result
 
 -- | Returns the ByteStringReader function
-byteStringReader :: MonadIO m => GCManagedState -> Reader m
+byteStringReader :: (PandocMonad m, MonadIO m) => GCManagedState -> Reader m
 byteStringReader st = ByteStringReader $ \ropts input -> inLua st $ do
   getfield registryindex readerField
   push input
@@ -163,12 +170,12 @@ byteStringReader st = ByteStringReader $ \ropts input -> inLua st $ do
     _ -> throwErrorAsException
 
 -- | Returns the TextReader function
-textReader :: MonadIO m => GCManagedState -> Reader m
+textReader :: (PandocMonad m, MonadIO m) => GCManagedState -> Reader m
 textReader st = TextReader $ \ropts srcs -> inLua st $ do
   let input = toSources srcs
   getfield registryindex readerField
   push input
   push ropts
   pcallTrace 2 1 >>= \case
-    OK -> forcePeek $ peekPandoc top
+    OK -> forcePeek (peekPandoc top)
     _ -> throwErrorAsException