ghc-experimental: add mkWeakJSVal

This commit adds a mkWeakJSVal function that can be used to set up a
Weak pointer with a JSVal key to observe the key's lifetime and
optionally attach a finalizer.

(cherry picked from commit 55af20e6)

libraries/ghc-experimental/src/GHC/Wasm/Prim.hs

@@ -4,6 +4,7 @@ module GHC.Wasm.Prim (
   -- User-facing JSVal type and freeJSVal
   JSVal,
   freeJSVal,
+  mkWeakJSVal,
 
   -- The JSString type and conversion from/to Haskell String
   JSString (..),

libraries/ghc-internal/src/GHC/Internal/Wasm/Prim.hs

@@ -4,6 +4,7 @@ module GHC.Internal.Wasm.Prim (
   -- User-facing JSVal type and freeJSVal
   JSVal (..),
   freeJSVal,
+  mkWeakJSVal,
 
   -- The JSString type and conversion from/to Haskell String
   JSString (..),

libraries/ghc-internal/src/GHC/Internal/Wasm/Prim/Types.hs

@@ -8,6 +8,7 @@ module GHC.Internal.Wasm.Prim.Types (
   JSVal# (..),
   JSVal (..),
   freeJSVal,
+  mkWeakJSVal,
   JSString (..),
   fromJSString,
   toJSString,
@@ -26,6 +27,7 @@ import GHC.Internal.IO.Encoding
 import GHC.Internal.Num
 import GHC.Internal.Show
 import GHC.Internal.Stable
+import GHC.Internal.Weak
 
 {-
 
@@ -94,6 +96,12 @@ freeJSVal v@(JSVal _ w sp) = do
   IO $ \s0 -> case finalizeWeak# w s0 of
     (# s1, _, _ #) -> (# s1, () #)
 
+mkWeakJSVal :: JSVal -> Maybe (IO ()) -> IO (Weak JSVal)
+mkWeakJSVal v@(JSVal k _ _) (Just (IO fin)) = IO $ \s0 ->
+  case mkWeak# k v fin s0 of
+    (# s1, w #) -> (# s1, Weak w #)
+mkWeakJSVal (JSVal _ w _) Nothing = pure $ Weak w
+
 foreign import javascript unsafe "if (!__ghc_wasm_jsffi_finalization_registry.unregister($1)) { throw new WebAssembly.RuntimeError('js_callback_unregister'); }"
   js_callback_unregister :: JSVal -> IO ()
 

testsuite/tests/jsffi/jsffigc.hs

@@ -4,7 +4,7 @@ module Test where
 
 import Data.Maybe
 import GHC.Exts
-import GHC.Internal.Wasm.Prim
+import GHC.Wasm.Prim
 import GHC.Weak
 import System.Mem
 
@@ -22,13 +22,6 @@ foreign import javascript "wrapper"
 foreign export javascript "testDynExportFree sync"
   testDynExportFree :: Int -> Int -> Int -> IO ()
 
--- JSVal uses Weak# under the hood for garbage collection support,
--- this exposes the internal Weak# to observe the liveliness of
--- JSVal#. Do not use this in your own codebase since this is purely
--- an implementation detail of JSVal and subject to change!
-jsvalWeak :: JSVal -> Weak JSVal
-jsvalWeak (JSVal _ w _) = Weak w
-
 probeWeak :: Weak v -> IO ()
 probeWeak wk = print =<< isJust <$> deRefWeak wk
 
@@ -42,7 +35,7 @@ testDynExportFree x y z = do
   -- wk_js observe the liveliness of the JavaScript callback on the
   -- Haskell heap. Make sure it's eagerly evaluated and isn't a thunk
   -- that retains cb.
-  let !wk_js = jsvalWeak cb
+  !wk_js <- mkWeakJSVal cb Nothing
   print $ js_to_hs cb x y
   -- Eagerly drop references to both the JavaScript callback and the
   -- Haskell function closure.
@@ -60,7 +53,7 @@ testDynExportGC x y z = do
   let fn a b = a * b + z
   wk_fn <- mkWeak fn () Nothing
   cb <- js_from_hs fn
-  let !wk_js = jsvalWeak cb
+  !wk_js <- mkWeakJSVal cb Nothing
   print $ js_to_hs cb x y
   -- Why performGC twice? The first run gathers some C finalizers
   -- which will be invoked in the second run to free the JSVal