Fix slot array corrupted by wrenInterpret() (#730)

src/vm/wren_vm.c

@@ -1447,7 +1447,11 @@ WrenInterpretResult wrenInterpret(WrenVM* vm, const char* module,
   ObjFiber* fiber = wrenNewFiber(vm, closure);
   wrenPopRoot(vm); // closure.
 
-  return runInterpreter(vm, fiber);
+  WrenInterpretResult result = runInterpreter(vm, fiber);
+
+  vm->fiber = NULL;
+  vm->apiStack = NULL;
+  return result;
 }
 
 ObjClosure* wrenCompileSource(WrenVM* vm, const char* module, const char* source,

test/api/new_vm.c

@@ -13,9 +13,43 @@ static void nullConfig(WrenVM* vm)
   wrenFreeVM(otherVM);
 }
 
+static void multipleInterpretCalls(WrenVM* vm)
+{
+  WrenVM* otherVM = wrenNewVM(NULL);
+  WrenInterpretResult result;
+
+  bool correct = true;
+
+  // Handles should be valid across calls into Wren code.
+  WrenHandle* absMethod = wrenMakeCallHandle(otherVM, "abs");
+
+  for (int i = 0; i < 5; i++) {
+    // Calling `wrenEnsureSlots()` before `wrenInterpret()` should not introduce
+    // problems later.
+    wrenEnsureSlots(otherVM, 2);
+
+    result = wrenInterpret(otherVM, "main", "1 + 2");
+    correct = correct && (result == WREN_RESULT_SUCCESS);
+
+    wrenEnsureSlots(otherVM, 2);
+    wrenSetSlotDouble(otherVM, 0, -i);
+    result = wrenCall(otherVM, absMethod);
+    correct = correct && (result == WREN_RESULT_SUCCESS);
+
+    double absValue = wrenGetSlotDouble(otherVM, 0);
+    correct = correct && (absValue == (double)i);
+  }
+
+  wrenSetSlotBool(vm, 0, correct);
+
+  wrenReleaseHandle(otherVM, absMethod);
+  wrenFreeVM(otherVM);
+}
+
 WrenForeignMethodFn newVMBindMethod(const char* signature)
 {
   if (strcmp(signature, "static VM.nullConfig()") == 0) return nullConfig;
+  if (strcmp(signature, "static VM.multipleInterpretCalls()") == 0) return multipleInterpretCalls;
 
   return NULL;
 }

test/api/new_vm.wren

@@ -1,6 +1,8 @@
 class VM {
   foreign static nullConfig()
+  foreign static multipleInterpretCalls()
 }
 // TODO: Other configuration settings.
 
 System.print(VM.nullConfig()) // expect: true
+System.print(VM.multipleInterpretCalls()) // expect: true