Tweak a few things in the new GC.

- Make sure it handles an empty gray set.
- Make sure growing the gray stack doesn't itself trigger a GC.
- Make sure it works when stress testing is enabled.
- Ensure the tests kick off a GC.

src/vm/wren_value.c

@@ -867,15 +867,14 @@ void wrenGrayObj(WrenVM* vm, Obj* obj)
   
   // Add it to the gray list so it can be recursively explored for
   // more marks later.
-  if (vm->grayDepth >= vm->maxGray)
+  if (vm->grayCount >= vm->grayCapacity)
   {
-    size_t oldSize = vm->maxGray * sizeof(Obj*);
-    size_t newSize = vm->grayDepth * 2 * sizeof(Obj*);
-    vm->gray = (Obj**)wrenReallocate(vm, vm->gray, oldSize, newSize);
-    vm->maxGray = vm->grayDepth * 2;
+    vm->grayCapacity = vm->grayCount * 2;
+    vm->gray = vm->config.reallocateFn(vm->gray,
+                                       vm->grayCapacity * sizeof(Obj*));
   }
   
-  vm->gray[vm->grayDepth++] = obj;
+  vm->gray[vm->grayCount++] = obj;
 }
 
 void wrenGrayValue(WrenVM* vm, Value value)
@@ -1095,12 +1094,12 @@ static void blackenObject(WrenVM* vm, Obj* obj)
 
 void wrenBlackenObjects(WrenVM* vm)
 {
-  do
+  while (vm->grayCount > 0)
   {
     // Pop an item from the gray stack.
-    Obj* obj = vm->gray[--vm->grayDepth];
+    Obj* obj = vm->gray[--vm->grayCount];
     blackenObject(vm, obj);
-  } while (vm->grayDepth > 0);
+  }
 }
 
 void wrenFreeObj(WrenVM* vm, Obj* obj)

src/vm/wren_vm.c

@@ -46,29 +46,25 @@ void wrenInitConfiguration(WrenConfiguration* config)
   config->heapGrowthPercent = 50;
 }
 
-static void initializeGC(WrenVM* vm)
-{
-  vm->gray = (Obj**)wrenReallocate(vm, NULL, 0, 5 * sizeof(Obj*));
-  vm->grayDepth = 0;
-  vm->maxGray = 5;
-}
-
 WrenVM* wrenNewVM(WrenConfiguration* config)
 {
   WrenVM* vm = (WrenVM*)config->reallocateFn(NULL, sizeof(*vm));
   memset(vm, 0, sizeof(WrenVM));
   memcpy(&vm->config, config, sizeof(WrenConfiguration));
 
+  // TODO: Should we allocate and free this during a GC?
+  vm->grayCount = 0;
+  // TODO: Tune this.
+  vm->grayCapacity = 4;
+  vm->gray = vm->config.reallocateFn(NULL, vm->grayCapacity * sizeof(Obj*));
   vm->nextGC = config->initialHeapSize;
-
+  
   wrenSymbolTableInit(&vm->methodNames);
 
   vm->modules = wrenNewMap(vm);
 
   wrenInitializeCore(vm);
 
-  initializeGC(vm);
-
   // TODO: Lazy load these.
   #if WREN_OPT_META
     wrenLoadMetaModule(vm);
@@ -94,7 +90,7 @@ void wrenFreeVM(WrenVM* vm)
   }
 
   // Free up the GC gray set.
-  wrenReallocate(vm, vm->gray, vm->maxGray * sizeof(Obj*), 0);
+  vm->gray = vm->config.reallocateFn(vm->gray, 0);
 
   // Tell the user if they didn't free any handles. We don't want to just free
   // them here because the host app may still have pointers to them that they

src/vm/wren_vm.h

@@ -72,8 +72,8 @@ struct WrenVM
   // The "gray" set for the garbage collector. This is the stack of unprocessed
   // objects while a garbage collection pass is in process.
   Obj** gray;
-  int grayDepth;
-  int maxGray;
+  int grayCount;
+  int grayCapacity;
 
   // The list of temporary roots. This is for temporary or new objects that are
   // not otherwise reachable but should not be collected.

test/language/deeply_nested_gc.wren

@@ -4,4 +4,5 @@ for (i in 1..400000) {
   head = { "next" : head }
 }
 
-System.print("done") // expect: done
+System.gc()
+System.print("done") // expect: done

test/language/many_reallocations.wren

@@ -10,5 +10,7 @@ for (i in 1..1000) {
   }
   found.add(bar)
 }
+
+System.gc()
 System.print(found.all {|i| i == 1337}) // expect: true
-System.print("DONE!") // expect: DONE!
+System.print("done") // expect: done