Lua: set `lpeg`, `re` as globals; allow shared lib access via require

The `lpeg` and `re` modules are loaded into globals of the respective
name, but they are not necessarily registered as loaded packages. This
ensures that

- the built-in library versions are preferred when setting the globals,
- a shared library is used if pandoc has been compiled without `lpeg`,
  and
- the `require` mechanism can be used to load the shared library if
  available, falling back to the internal version if possible and
  necessary.

3 files changed, 52 insertions, 33 deletions

diff --git a/doc/lua-filters.md b/doc/lua-filters.md
index 38790ca5d..9fc90a13f 100644
--- a/doc/lua-filters.md
+++ b/doc/lua-filters.md
@@ -253,17 +253,26 @@ variables.
 
 `lpeg`
 :   This variable holds the `lpeg` module, a package based on
-    Parsing Expression Grammars (PEG). It provides excellent
+    Parsing Expression Grammars (PEG).  It provides excellent
     parsing utilities and is documented on the official [LPeg
-    homepage]. Pandoc will try to load the module through the
-    normal package mechanism, and fall back to a built-in version
-    if necessary.
+    homepage].  Pandoc uses a built-int version of the library,
+    unless it has been configured by the package maintainer to
+    rely on a system-wide installation.
+
+    Note that the result of `require 'lpeg'` is not necessarily
+    equal to this value; the `require` mechanism prefers the
+    system's lpeg library over the built-in version.
 
 `re`
-:   Contains the LPeg.re module, which is built on top of LPeg and
-    offers an implementation of a [regex engine]. Pandoc will try
-    to load the module through the normal package mechanism, and
-    fall back to a built-in version if necessary.
+:   Contains the LPeg.re module, which is built on top of LPeg
+    and offers an implementation of a [regex engine].  Pandoc
+    uses a built-in version of the library, unless it has been
+    configured by the package maintainer to rely on a system-wide
+    installation.
+
+    Note that the result of `require 're` is not necessarily
+    equal to this value; the `require` mechanism prefers the
+    system's lpeg library over the built-in version.
 
 [LPeg homepage]: http://www.inf.puc-rio.br/~roberto/lpeg/
 [regex engine]: http://www.inf.puc-rio.br/~roberto/lpeg/re.html
diff --git a/src/Text/Pandoc/Lua/Init.hs b/src/Text/Pandoc/Lua/Init.hs
index bf87f0a41..72a06f556 100644
--- a/src/Text/Pandoc/Lua/Init.hs
+++ b/src/Text/Pandoc/Lua/Init.hs
@@ -47,7 +47,8 @@ initLuaState = do
   liftPandocLua Lua.openlibs
   installPandocPackageSearcher
   initPandocModule
-  requireGlobalModules
+  installLpegSearcher
+  setGlobalModules
   loadInitScript "init.lua"
  where
   initPandocModule :: PandocLua ()
@@ -76,33 +77,32 @@ initLuaState = do
         PandocLuaError msg -> msg
         _                  -> T.pack $ show err
 
-  requireGlobalModules :: PandocLua ()
-  requireGlobalModules = liftPandocLua $
+  setGlobalModules :: PandocLua ()
+  setGlobalModules = liftPandocLua $
     forM_ [ ("lpeg", LPeg.luaopen_lpeg_ptr)
           , ("re", LPeg.luaopen_re_ptr)
           ] $
       \(pkgname, luaopen) -> do
-        -- Try loading via the normal package loading mechanism, and
-        -- fall back to manual module loading if the normal mechanism
-        -- fails. This means the system installation of the package,
-        -- should it be available, is preferred.
-        Lua.getglobal "require"
-        Lua.pushName pkgname
-        Lua.pcall 1 1 Nothing >>= \case
+        Lua.pushcfunction luaopen
+        Lua.pcall 0 1 Nothing >>= \case
           OK -> pure ()          -- all good, loading succeeded
-          _  -> do               -- default mechanism failed, load included lib
+          _  -> do               -- built-in library failed, load system lib
             Lua.pop 1  -- ignore error message
-            Lua.pushcfunction luaopen
-            Lua.call 0 1  -- Throws an exception if loading failed again!
-            -- Success. Add module to table @_LOADED@ in the registry
-            _ <- Lua.getfield Lua.registryindex Lua.loaded
-            Lua.pushvalue (Lua.nth 2)  -- push module to top
-            Lua.setfield (Lua.nth 2) pkgname
-            Lua.pop 1  -- pop _LOADED
+            -- 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!
 
         -- Module on top of stack. Register as global
         Lua.setglobal pkgname
 
+  installLpegSearcher :: PandocLua ()
+  installLpegSearcher = liftPandocLua $ do
+    Lua.getglobal' "package.searchers"
+    Lua.pushHaskellFunction $ Lua.state >>= liftIO . LPeg.lpeg_searcher
+    Lua.rawseti (Lua.nth 2) . (+1) . fromIntegral =<< Lua.rawlen (Lua.nth 2)
+    Lua.pop 1  -- remove 'package.searchers' from stack
+
 -- | AST elements are marshaled via normal constructor functions in the
 -- @pandoc@ module. However, accessing Lua globals from Haskell is
 -- expensive (due to error handling). Accessing the Lua registry is much
diff --git a/test/Tests/Lua.hs b/test/Tests/Lua.hs
index 6ee07f8fa..5e81cec6a 100644
--- a/test/Tests/Lua.hs
+++ b/test/Tests/Lua.hs
@@ -17,7 +17,7 @@ module Tests.Lua ( runLuaTest, tests ) where
 import Control.Monad (when)
 import HsLua as Lua hiding (Operation (Div), error)
 import System.FilePath ((</>))
-import Test.Tasty (TestTree, localOption)
+import Test.Tasty (TestTree, testGroup, localOption)
 import Test.Tasty.HUnit ((@=?), Assertion, HasCallStack, assertEqual, testCase)
 import Test.Tasty.QuickCheck (QuickCheckTests (..), ioProperty, testProperty)
 import Text.Pandoc.Arbitrary ()
@@ -211,13 +211,23 @@ tests = map (localOption (QuickCheckTests 20))
       ty <- Lua.ltype Lua.top
       Lua.liftIO $ assertEqual "module should be a table" Lua.TypeTable ty
 
-  , testCase "module 'lpeg' is loaded into a global" . runLuaTest $ do
-      s <- Lua.dostring "assert(type(lpeg)=='table');assert(lpeg==require'lpeg')"
-      Lua.liftIO $ Lua.OK @=? s
+  , testGroup "global modules"
+    [ testCase "module 'lpeg' is loaded into a global" . runLuaTest $ do
+        s <- Lua.dostring "assert(type(lpeg)=='table')"
+        Lua.liftIO $ Lua.OK @=? s
 
-  , testCase "module 're' is available" . runLuaTest $ do
-      s <- Lua.dostring "require 're'"
-      Lua.liftIO $ Lua.OK @=? s
+    , testCase "module 're' is loaded into a global" . runLuaTest $ do
+        s <- Lua.dostring "assert(type(re)=='table')"
+        Lua.liftIO $ Lua.OK @=? s
+
+    , testCase "module 'lpeg' is available via `require`" . runLuaTest $ do
+        s <- Lua.dostring "require 'lpeg'"
+        Lua.liftIO $ Lua.OK @=? s
+
+    , testCase "module 're' is available via `require`" . runLuaTest $ do
+        s <- Lua.dostring "require 're'"
+        Lua.liftIO $ Lua.OK @=? s
+    ]
 
   , testCase "informative error messages" . runLuaTest $ do
       Lua.pushboolean True