wasm: properly pin the raiseJSException closure

We used to use keepAlive# to pin the raiseJSException closure when
blocking on a JSFFI import thunk, since it can potentially be used by
RTS. But raiseJSException may be used in other places as well (e.g.
the promise.throwTo logic), and it's better to simply unconditionally
pin it in the JSFFI initialization logic.

(cherry picked from commit da34f0aa)

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

@@ -5,6 +5,7 @@
 {-# LANGUAGE UnliftedFFITypes #-}
 
 module GHC.Internal.Wasm.Prim.Imports (
+  raiseJSException,
   stg_blockPromise,
   stg_messagePromiseUnit,
   stg_messagePromiseJSVal,
@@ -94,13 +95,7 @@ stg_blockPromise err_msg p msg_p = unsafeDupablePerformIO $ IO $ \s0 ->
             --    and prevents dmdanal from being naughty
             (# s4, _ #) -> case unIO (freeJSVal p) s4 of
               (# s5, _ #) ->
-                -- raiseJSException_closure is used by the RTS in case
-                -- the Promise is rejected, and it is likely a CAF. So
-                -- we need to keep it alive when we block waiting for
-                -- the Promise to resolve or reject, and also mark it
-                -- as OPAQUE just to be sure.
-                keepAlive# raiseJSException s5 $
-                  readMVar# mv#
+                readMVar# mv# s5
 
 foreign import prim "stg_jsffi_check"
   stg_jsffi_check :: Any -> State# RealWorld -> (# State# RealWorld #)

rts/wasm/JSFFI.c

@@ -7,6 +7,7 @@
 
 extern HsBool rts_JSFFI_flag;
 extern HsStablePtr rts_threadDelay_impl;
+extern StgClosure ghczminternal_GHCziInternalziWasmziPrimziImports_raiseJSException_closure;
 extern StgClosure ghczminternal_GHCziInternalziWasmziPrimziConcziInternal_threadDelay_closure;
 
 int __main_void(void);
@@ -20,6 +21,7 @@ int __main_argc_argv(int argc, char *argv[]) {
   hs_init_ghc(&argc, &argv, __conf);
   // See Note [threadDelay on wasm] for details.
   rts_JSFFI_flag = HS_BOOL_TRUE;
+  getStablePtr((StgPtr)&ghczminternal_GHCziInternalziWasmziPrimziImports_raiseJSException_closure);
   rts_threadDelay_impl = getStablePtr((StgPtr)&ghczminternal_GHCziInternalziWasmziPrimziConcziInternal_threadDelay_closure);
   return 0;
 }
@@ -145,7 +147,6 @@ INLINE_HEADER void pushClosure   (StgTSO *tso, StgWord c) {
 }
 
 extern const StgInfoTable stg_scheduler_loop_info;
-extern StgClosure ghczminternal_GHCziInternalziWasmziPrimziImports_raiseJSException_closure;
 
 // schedule a future round of RTS scheduler loop via setImmediate(),
 // to avoid jamming the JavaScript main thread