rts: add hs_try_putmvar_with_value to RTS API

This commit adds hs_try_putmvar_with_value to rts. It allows more
flexibility than hs_try_putmvar by taking an additional value argument
as a closure to be put into the MVar. This function is used & tested
by the wasm backend runtime, though it makes sense to expose it as a
public facing RTS API function as well.

(cherry picked from commit f75e823e)

docs/users_guide/exts/ffi.rst

@@ -976,6 +976,8 @@ wake up a Haskell thread from C/C++.
 
   void hs_try_putmvar (int capability, HsStablePtr sp);
 
+  void hs_try_putmvar_with_value (int capability, HsStablePtr sp, StgClosure *value);
+
 The C call ``hs_try_putmvar(cap, mvar)`` is equivalent to the Haskell
 call ``tryPutMVar mvar ()``, except that it is
 
@@ -988,6 +990,15 @@ call ``tryPutMVar mvar ()``, except that it is
   the ``MVar`` is empty; if it is full, ``hs_try_putmvar()`` will have
   no effect.
 
+The C call ``hs_try_putmvar_with_value(cap, mvar, value)`` takes an
+additional ``value`` argument, which is an RTS closure pointer of the
+value to be put into the MVar. It works the same way as
+``hs_try_putmvar`` while offering a bit more flexibility: for a C
+value to be passed to Haskell, you can directly call one of the
+``rts_mk`` functions to wrap the C value and put it into the MVar,
+instead of writing it to a heap location and peeking it from a pointer
+in Haskell.
+
 **Example**. Suppose we have a C/C++ function to call that will return and then
 invoke a callback at some point in the future, passing us some data.
 We want to wait in Haskell for the callback to be called, and retrieve

rts/RtsAPI.c

@@ -942,12 +942,23 @@ void rts_done (void)
    it would be very difficult for the caller to arrange to free the StablePtr
    in all circumstances.
 
+   There's also hs_try_putmvar_with_value(cap, mvar, value) which
+   allows putting a custom value other than () in the MVar, typically
+   a closure created by one of rts_mk*() functions.
+
    For more details, see the section "Waking up Haskell threads from C" in the
    User's Guide.
    -------------------------------------------------------------------------- */
 
 void hs_try_putmvar (/* in */ int capability,
                      /* in */ HsStablePtr mvar)
+{
+    hs_try_putmvar_with_value(capability, mvar, TAG_CLOSURE(1, Unit_closure));
+}
+
+void hs_try_putmvar_with_value (/* in */ int capability,
+                                /* in */ HsStablePtr mvar,
+                                /* in */ StgClosure *value)
 {
     Task *task = getMyTask();
     Capability *cap;
@@ -963,7 +974,7 @@ void hs_try_putmvar (/* in */ int capability,
 
 #if !defined(THREADED_RTS)
 
-    performTryPutMVar(cap, (StgMVar*)deRefStablePtr(mvar), Unit_closure);
+    performTryPutMVar(cap, (StgMVar*)deRefStablePtr(mvar), value);
     freeStablePtr(mvar);
 
 #else
@@ -976,7 +987,7 @@ void hs_try_putmvar (/* in */ int capability,
         task->cap = cap;
         RELEASE_LOCK(&cap->lock);
 
-        performTryPutMVar(cap, (StgMVar*)deRefStablePtr(mvar), Unit_closure);
+        performTryPutMVar(cap, (StgMVar*)deRefStablePtr(mvar), value);
 
         freeStablePtr(mvar);
 

rts/RtsSymbols.c

@@ -597,6 +597,7 @@ extern char **environ;
       SymI_HasProto(hs_hpc_module)                                      \
       SymI_HasProto(hs_thread_done)                                     \
       SymI_HasProto(hs_try_putmvar)                                     \
+      SymI_HasProto(hs_try_putmvar_with_value)                          \
       SymI_HasProto(defaultRtsConfig)                                   \
       SymI_HasProto(initLinker)                                         \
       SymI_HasProto(initLinker_)                                        \

rts/include/HsFFI.h

@@ -21,6 +21,7 @@ extern "C" {
 
 /* get types from GHC's runtime system */
 #include "ghcconfig.h"
+#include "rts/Types.h"
 #include "stg/Types.h"
 
 /* get limits for floating point types */
@@ -138,6 +139,7 @@ extern int hs_spt_keys(StgPtr keys[], int szKeys);
 extern int hs_spt_key_count (void);
 
 extern void hs_try_putmvar (int capability, HsStablePtr sp);
+extern void hs_try_putmvar_with_value (int capability, HsStablePtr sp, StgClosure *value);
 
 /* -------------------------------------------------------------------------- */