Lua: keep CommonState object in the registry

The state is an internal value and should be treated as such. The
`PANDOC_STATE` global is merely a copy; unsetting the global no longer
breaks the Lua engine.

Author: tarleb

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

@@ -77,10 +77,12 @@ instance PandocMonad PandocLua where
   getModificationTime = IO.getModificationTime
 
   getCommonState = PandocLua $ do
-    Lua.getglobal "PANDOC_STATE"
+    Lua.getfield registryindex "PANDOC_STATE"
     forcePeek $ peekCommonState Lua.top `lastly` pop 1
   putCommonState cst = PandocLua $ do
     pushCommonState cst
+    Lua.pushvalue Lua.top
+    Lua.setfield registryindex "PANDOC_STATE"
     Lua.setglobal "PANDOC_STATE"
 
   logOutput = IO.logOutput

pandoc-lua-engine/test/lua/module/globals.lua

@@ -147,5 +147,11 @@ return {
     test('has property "verbosity"', function ()
       assert.are_equal(type(PANDOC_STATE.verbosity), 'string')
     end),
+    test('can be deleted without breaking PandocLua monad functions', function()
+      local state = PANDOC_STATE
+      PANDOC_STATE = nil
+      assert.is_nil(pandoc.mediabag.lookup('does-not-exist'))
+      PANDOC_STATE = state
+    end),
   },
 }