Do not allow already running coroutines to be reset or resumed.

This is a wrong use of the Lua API and is not supported.
See #416 for the reference.

src/error.rs

@@ -101,7 +101,7 @@ pub enum Error {
     /// [`Thread::resume`] was called on an inactive coroutine.
     ///
     /// A coroutine is inactive if its main function has returned or if an error has occurred inside
-    /// the coroutine.
+    /// the coroutine. Already running coroutines are also marked as inactive (unresumable).
     ///
     /// [`Thread::status`] can be used to check if the coroutine can be resumed without causing this
     /// error.

src/thread.rs

@@ -142,6 +142,10 @@ impl<'lua> Thread<'lua> {
         A: IntoLuaMulti<'lua>,
         R: FromLuaMulti<'lua>,
     {
+        if self.status() != ThreadStatus::Resumable {
+            return Err(Error::CoroutineInactive);
+        }
+
         let lua = self.0.lua;
         let state = lua.state();
         let thread_state = self.state();
@@ -165,10 +169,6 @@ impl<'lua> Thread<'lua> {
         let state = lua.state();
         let thread_state = self.state();
 
-        if self.status() != ThreadStatus::Resumable {
-            return Err(Error::CoroutineInactive);
-        }
-
         let nargs = args.push_into_stack_multi(lua)?;
         if nargs > 0 {
             check_stack(thread_state, nargs)?;
@@ -196,6 +196,10 @@ impl<'lua> Thread<'lua> {
     /// Gets the status of the thread.
     pub fn status(&self) -> ThreadStatus {
         let thread_state = self.state();
+        if thread_state == self.0.lua.state() {
+            // The coroutine is currently running
+            return ThreadStatus::Unresumable;
+        }
         unsafe {
             let status = ffi::lua_status(thread_state);
             if status != ffi::LUA_OK && status != ffi::LUA_YIELD {
@@ -243,6 +247,9 @@ impl<'lua> Thread<'lua> {
     pub fn reset(&self, func: crate::function::Function<'lua>) -> Result<()> {
         let lua = self.0.lua;
         let thread_state = self.state();
+        if thread_state == lua.state() {
+            return Err(Error::runtime("cannot reset a running thread"));
+        }
         unsafe {
             #[cfg(all(feature = "lua54", not(feature = "vendored")))]
             let status = ffi::lua_resetthread(thread_state);

tests/thread.rs

@@ -90,6 +90,19 @@ fn test_thread() -> Result<()> {
         _ => panic!("resuming dead coroutine did not return error"),
     }
 
+    // Already running thread must be unresumable
+    let thread = lua.create_thread(lua.create_function(|lua, ()| {
+        assert_eq!(lua.current_thread().status(), ThreadStatus::Unresumable);
+        let result = lua.current_thread().resume::<_, ()>(());
+        assert!(
+            matches!(result, Err(Error::CoroutineInactive)),
+            "unexpected result: {result:?}",
+        );
+        Ok(())
+    })?)?;
+    let result = thread.resume::<_, ()>(());
+    assert!(result.is_ok(), "unexpected result: {result:?}");
+
     Ok(())
 }
 
@@ -146,6 +159,21 @@ fn test_thread_reset() -> Result<()> {
         assert_eq!(thread.status(), ThreadStatus::Resumable);
     }
 
+    // Try reset running thread
+    let thread = lua.create_thread(lua.create_function(|lua, ()| {
+        let this = lua.current_thread();
+        this.reset(lua.create_function(|_, ()| Ok(()))?)?;
+        Ok(())
+    })?)?;
+    let result = thread.resume::<_, ()>(());
+    assert!(
+        matches!(result, Err(Error::CallbackError{ ref cause, ..})
+            if matches!(cause.as_ref(), Error::RuntimeError(ref err)
+                if err == "cannot reset a running thread")
+        ),
+        "unexpected result: {result:?}",
+    );
+
     Ok(())
 }