Fix thread recovery when pushing a bad arg

We should not erase thread stack if a bad argument is pushed before resuming the thread.

Author: khvzak

src/thread.rs

@@ -156,7 +156,6 @@ impl Thread {
         let thread_state = self.state();
         unsafe {
             let _sg = StackGuard::new(state);
-            let _thread_sg = StackGuard::with_top(thread_state, 0);
 
             let nargs = args.push_into_stack_multi(&lua)?;
             if nargs > 0 {
@@ -165,6 +164,7 @@ impl Thread {
                 pushed_nargs += nargs;
             }
 
+            let _thread_sg = StackGuard::with_top(thread_state, 0);
             let (_, nresults) = self.resume_inner(&lua, pushed_nargs)?;
             check_stack(state, nresults + 1)?;
             ffi::lua_xmove(thread_state, state, nresults);
@@ -192,12 +192,12 @@ impl Thread {
         let thread_state = self.state();
         unsafe {
             let _sg = StackGuard::new(state);
-            let _thread_sg = StackGuard::with_top(thread_state, 0);
 
             check_stack(state, 1)?;
             error.push_into_stack(&lua)?;
             ffi::lua_xmove(state, thread_state, 1);
 
+            let _thread_sg = StackGuard::with_top(thread_state, 0);
             let (_, nresults) = self.resume_inner(&lua, ffi::LUA_RESUMEERROR)?;
             check_stack(state, nresults + 1)?;
             ffi::lua_xmove(thread_state, state, nresults);

tests/thread.rs

@@ -1,6 +1,6 @@
 use std::panic::catch_unwind;
 
-use mlua::{Error, Function, Lua, Result, Thread, ThreadStatus};
+use mlua::{Error, Function, IntoLua, Lua, Result, Thread, ThreadStatus, Value};
 
 #[test]
 fn test_thread() -> Result<()> {
@@ -252,3 +252,24 @@ fn test_thread_resume_error() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_thread_resume_bad_arg() -> Result<()> {
+    let lua = Lua::new();
+
+    struct BadArg;
+
+    impl IntoLua for BadArg {
+        fn into_lua(self, _lua: &Lua) -> Result<Value> {
+            Err(Error::runtime("bad arg"))
+        }
+    }
+
+    let f = lua.create_thread(lua.create_function(|_, ()| Ok("okay"))?)?;
+    let res = f.resume::<()>((123, BadArg));
+    assert!(matches!(res, Err(Error::RuntimeError(msg)) if msg == "bad arg"));
+    let res = f.resume::<String>(()).unwrap();
+    assert_eq!(res, "okay");
+
+    Ok(())
+}