Get rid of fiber for finalizers.

Instead, finalizers just get access to the foreign object's raw bytes.
This is deliberately limiting, since it discourages the user from
interacting with the VM in the middle of a GC.

src/vm/wren_vm.c

@@ -66,8 +66,6 @@ WrenVM* wrenNewVM(WrenConfiguration* config)
 
   wrenInitializeCore(vm);
   
-  vm->finalizerFiber = wrenNewFiber(vm, NULL);
-
   // TODO: Lazy load these.
   #if WREN_OPT_META
     wrenLoadMetaModule(vm);
@@ -139,9 +137,8 @@ void wrenCollectGarbage(WrenVM* vm)
     wrenGrayObj(vm, vm->tempRoots[i]);
   }
 
-  // The rooted fibers.
+  // The rooted fiber.
   wrenGrayObj(vm, (Obj*)vm->fiber);
-  wrenGrayObj(vm, (Obj*)vm->finalizerFiber);
 
   // The value handles.
   for (WrenValue* value = vm->valueHandles;
@@ -628,7 +625,7 @@ static void bindForeignClass(WrenVM* vm, ObjClass* classObj, ObjModule* module)
 
   if (methods.finalize != NULL)
   {
-    method.fn.foreign = methods.finalize;
+    method.fn.foreign = (WrenForeignMethodFn)methods.finalize;
     wrenBindMethod(vm, classObj, symbol, method);
   }
 }
@@ -699,23 +696,8 @@ void wrenFinalizeForeign(WrenVM* vm, ObjForeign* foreign)
 
   ASSERT(method->type == METHOD_FOREIGN, "Finalizer should be foreign.");
 
-  // Pass the foreign object to the finalizer.
-  *vm->finalizerFiber->stackTop++ = OBJ_VAL(foreign);
-  ObjFiber* previousFiber = vm->fiber;
-  vm->fiber = vm->finalizerFiber;
-  
-  // TODO: What if apiStack is not currently NULL? This can happen if a foreign
-  // method causes an allocation which triggers a GC. The simplest solution
-  // might be to make finalizers have a different function type which only
-  // passes in the foreign bytes and not the VM. Then we wouldn't need a fiber
-  // for the finalizer.
-  vm->apiStack = vm->finalizerFiber->stack;
-
-  method->fn.foreign(vm);
-  
-  vm->fiber = previousFiber;
-  vm->apiStack = NULL;
-  wrenResetFiber(vm, vm->finalizerFiber, NULL);
+  WrenFinalizerFn finalizer = (WrenFinalizerFn)method->fn.foreign;
+  finalizer(foreign->data);
 }
 
 // The main bytecode interpreter loop. This is where the magic happens. It is
@@ -1618,9 +1600,6 @@ int wrenGetSlotCount(WrenVM* vm)
 
 void wrenEnsureSlots(WrenVM* vm, int numSlots)
 {
-  ASSERT(vm->fiber != vm->finalizerFiber,
-         "Cannot grow the stack in a finalizer.");
-  
   // If we don't have a fiber accessible, create one for the API to use.
   if (vm->fiber == NULL && vm->apiStack == NULL)
   {

src/vm/wren_vm.h

@@ -105,12 +105,6 @@ struct WrenVM
   // There is a single global symbol table for all method names on all classes.
   // Method calls are dispatched directly by index in this table.
   SymbolTable methodNames;
-  
-  // A special fiber whose stack is used for communicating with foreign
-  // finalizer methods. Finalizers run during GC, so we don't want to do any
-  // allocation in order to execute one, so we set up this fiber ahead of time
-  // to ensure it's available.
-  ObjFiber* finalizerFiber;
 };
 
 // A generic allocation function that handles all explicit memory management.