Disallow circular WAIF references

WAIFs containing circular references can cause a space leak in the
database file if the cyclic references are not broken before the WAIFs
are garbage collected. Since LambdaMOO does not allow WAIFs to contain
circular references, we'll disallow them for now as well.

DIFFERENCES.md

@@ -161,11 +161,3 @@ differences compared with LambdaMOO:
 
   * EtaMOO defines the variable `WAIF` within MOO verbs to have the same value
     as `typeof(new_waif())`.
-
-  * Currently EtaMOO allows WAIFs to contain circular references. While this
-    is by itself not a problem, it can cause a space leak in the database file
-    if the cyclic references are not broken before the WAIFs are garbage
-    collected. Note that LambdaMOO does not allow WAIFs to contain circular
-    references, so exporting a database with such references and loading it
-    into LambdaMOO may result in undefined behavior. (This is basically a bug
-    and subject to change.)

TODO

@@ -33,7 +33,6 @@
 ** Enhance version number with git commit info
 ** Unicode strings in database file
 ** User reconnecting to different listening points
-** Circular WAIF references
 ** Deepen value_bytes(waif)
 
 * Before v1.0

src/MOO/WAIF.hs

@@ -26,7 +26,7 @@ import Data.IntMap (IntMap)
 import Data.Monoid ((<>))
 import Data.Typeable (Typeable)
 import Database.VCache (PVar, VRef, VCacheable(put, get), VTx, VSpace,
-                        unsafePVarAddr, deref, vref, vref',
+                        unsafePVarAddr, deref, deref', vref, vref',
                         pvar_space, newPVar, readPVar, modifyPVar)
 
 import qualified Data.HashMap.Lazy as HM
@@ -38,6 +38,7 @@ import MOO.Task
 import MOO.Types
 import MOO.Util
 
+import qualified MOO.List as Lst
 import qualified MOO.String as Str
 
 type PropertyMap = VHashMap StrT (VRef Value)
@@ -148,6 +149,9 @@ storeWaifProperty waif name value = do
   prop <- waifProperty name =<< waifClassObject waif
   unless (propertyPermW prop) $ checkPermission (waifPropertyOwner waif prop)
 
+  -- disallow circular references
+  checkCyclic waif value
+
   let var    = waifData waif  :: PVar PropertyMap
       vspace = pvar_space var :: VSpace
 
@@ -156,6 +160,16 @@ storeWaifProperty waif name value = do
 
   return value
 
+checkCyclic :: WAIF -> Value -> MOO ()
+checkCyclic waif = checkCyclic'
+  where checkCyclic' value = case value of
+          Lst x -> Lst.forM_ x checkCyclic'
+          Waf x | x == waif -> raise E_RECMOVE
+                | otherwise ->
+                    mapM_ (checkCyclic' . deref') . HM.elems . unVHashMap =<<
+                    liftVTx (readPVar $ waifData x)
+          _ -> return ()
+
 callWaifVerb :: Value -> WAIF -> StrT -> [Value] -> MOO Value
 callWaifVerb this waif name = callVerb this (waifClass waif) (prefix <> name)