v0.8.8

If no free registry id found, we call protect_lua! macro while keeping mutex guard to the unref list.
Protected calls can trigger garbage collection and if RegistryKey is placed in userdata being collected, this can lead to deadlock.
The solution is drop mutex guard as soon as possible.
Also this commit includes optimization in creating reference in Lua registry.

CHANGELOG.md

@@ -1,3 +1,28 @@
+## v0.8.8
+
+- Fix potential deadlock when trying to reuse dropped registry keys.
+- Optimize userdata methods call when __index and fields_getters are nil
+
+## v0.8.7
+
+- Minimum Luau updated to 0.555 (`LUAI_MAXCSTACK` limit increased to 100000)
+- `_VERSION` in Luau now includes version number
+- Fixed lifetime of `DebugNames` in `Debug::names()` and `DebugSource` in `Debug::source()`
+- Fixed subtraction overflow when calculating index for `MultiValue::get()`
+
+## v0.8.6
+
+- Fixed bug when recycled Registry slot can be set to Nil
+
+## v0.8.5
+
+- Fixed potential unsoundness when using `Layout::from_size_align_unchecked` and Rust 1.65+
+- Performance optimizations around string and table creation in standalone mode
+- Added fast track path to Table `get`/`set`/`len` methods without metatable
+- Added new methods `push`/`pop`/`raw_push`/`raw_pop` to Table
+- Fix getting caller information from `Lua::load`
+- Better checks and tests when trying to modify a Luau readonly table
+
 ## v0.8.4
 
 - Minimal Luau updated to 0.548

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mlua"
-version = "0.8.4" # remember to update html_root_url and mlua_derive
+version = "0.8.8" # remember to update html_root_url and mlua_derive
 authors = ["Aleksandr Orlenko <zxteam@pm.me>", "kyren <catherine@chucklefish.org>"]
 edition = "2021"
 repository = "https://github.com/khvzak/mlua"
@@ -58,7 +58,7 @@ cc = { version = "1.0" }
 pkg-config = { version = "0.3.17" }
 lua-src = { version = ">= 544.0.0, < 550.0.0", optional = true }
 luajit-src = { version = ">= 210.4.0, < 220.0.0", optional = true }
-luau0-src = { version = "0.4.0", optional = true }
+luau0-src = { version = "0.5.0", optional = true }
 
 [dev-dependencies]
 rustyline = "10.0"

README.md

@@ -7,7 +7,7 @@
 [crates.io]: https://crates.io/crates/mlua
 [API Documentation]: https://docs.rs/mlua/badge.svg
 [docs.rs]: https://docs.rs/mlua
-[Coverage Status]: https://codecov.io/gh/khvzak/mlua/branch/master/graph/badge.svg?token=99339FS1CG
+[Coverage Status]: https://codecov.io/gh/khvzak/mlua/branch/v0.8/graph/badge.svg?token=99339FS1CG
 [codecov.io]: https://codecov.io/gh/khvzak/mlua
 [MSRV]: https://img.shields.io/badge/rust-1.56+-brightgreen.svg?&logo=rust
 

src/ffi/luau/lua.rs

@@ -7,12 +7,15 @@ use std::ptr;
 // Option for multiple returns in 'lua_pcall' and 'lua_call'
 pub const LUA_MULTRET: c_int = -1;
 
+// Max number of Lua stack slots
+const LUAI_MAXCSTACK: c_int = 100000;
+
 //
 // Pseudo-indices
 //
-pub const LUA_REGISTRYINDEX: c_int = -10000;
-pub const LUA_ENVIRONINDEX: c_int = -10001;
-pub const LUA_GLOBALSINDEX: c_int = -10002;
+pub const LUA_REGISTRYINDEX: c_int = -LUAI_MAXCSTACK - 2000;
+pub const LUA_ENVIRONINDEX: c_int = -LUAI_MAXCSTACK - 2001;
+pub const LUA_GLOBALSINDEX: c_int = -LUAI_MAXCSTACK - 2002;
 
 pub const fn lua_upvalueindex(i: c_int) -> c_int {
     LUA_GLOBALSINDEX - i

src/hook.rs

@@ -63,7 +63,7 @@ impl<'lua> Debug<'lua> {
     }
 
     /// Corresponds to the `n` what mask.
-    pub fn names(&self) -> DebugNames<'lua> {
+    pub fn names(&self) -> DebugNames {
         unsafe {
             #[cfg(not(feature = "luau"))]
             mlua_assert!(
@@ -87,7 +87,7 @@ impl<'lua> Debug<'lua> {
     }
 
     /// Corresponds to the `S` what mask.
-    pub fn source(&self) -> DebugSource<'lua> {
+    pub fn source(&self) -> DebugSource {
         unsafe {
             #[cfg(not(feature = "luau"))]
             mlua_assert!(

src/lib.rs

@@ -72,7 +72,7 @@
 //! [`serde::Deserialize`]: https://docs.serde.rs/serde/de/trait.Deserialize.html
 
 // mlua types in rustdoc of other crates get linked to here.
-#![doc(html_root_url = "https://docs.rs/mlua/0.8.4")]
+#![doc(html_root_url = "https://docs.rs/mlua/0.8.8")]
 // Deny warnings inside doc tests / examples. When this isn't present, rustdoc doesn't show *any*
 // warnings at all.
 #![doc(test(attr(deny(warnings))))]

src/lua.rs

@@ -88,6 +88,8 @@ pub(crate) struct ExtraData {
 
     registered_userdata: FxHashMap<TypeId, c_int>,
     registered_userdata_mt: FxHashMap<*const c_void, Option<TypeId>>,
+
+    // When Lua instance dropped, setting `None` would prevent collecting `RegistryKey`s
     registry_unref_list: Arc<Mutex<Option<Vec<c_int>>>>,
 
     #[cfg(not(feature = "send"))]
@@ -402,6 +404,11 @@ impl Lua {
                 return ptr::null_mut();
             }
 
+            // Do not allocate more than isize::MAX
+            if nsize > isize::MAX as usize {
+                return ptr::null_mut();
+            }
+
             // Are we fit to the memory limits?
             let mut mem_diff = nsize as isize;
             if !ptr.is_null() {
@@ -411,12 +418,14 @@ impl Lua {
             if mem_info.memory_limit > 0 && new_used_memory > mem_info.memory_limit {
                 return ptr::null_mut();
             }
-
-            let new_layout = Layout::from_size_align_unchecked(nsize, ffi::SYS_MIN_ALIGN);
             mem_info.used_memory += mem_diff;
 
             if ptr.is_null() {
                 // Allocate new memory
+                let new_layout = match Layout::from_size_align(nsize, ffi::SYS_MIN_ALIGN) {
+                    Ok(layout) => layout,
+                    Err(_) => return ptr::null_mut(),
+                };
                 let new_ptr = alloc::alloc(new_layout) as *mut c_void;
                 if new_ptr.is_null() {
                     alloc::handle_alloc_error(new_layout);
@@ -428,7 +437,7 @@ impl Lua {
             let old_layout = Layout::from_size_align_unchecked(osize, ffi::SYS_MIN_ALIGN);
             let new_ptr = alloc::realloc(ptr as *mut u8, old_layout, nsize) as *mut c_void;
             if new_ptr.is_null() {
-                alloc::handle_alloc_error(new_layout);
+                alloc::handle_alloc_error(old_layout);
             }
             new_ptr
         }
@@ -882,10 +891,8 @@ impl Lua {
                         ffi::luaL_sandboxthread(state);
                     } else {
                         // Restore original `LUA_GLOBALSINDEX`
-                        self.ref_thread_exec(|ref_thread| {
-                            ffi::lua_xpush(ref_thread, state, ffi::LUA_GLOBALSINDEX);
-                            ffi::lua_replace(state, ffi::LUA_GLOBALSINDEX);
-                        });
+                        ffi::lua_xpush(self.ref_thread(), state, ffi::LUA_GLOBALSINDEX);
+                        ffi::lua_replace(state, ffi::LUA_GLOBALSINDEX);
                         ffi::luaL_sandbox(state, 0);
                     }
                 })?;
@@ -1366,9 +1373,8 @@ impl Lua {
     where
         S: AsChunk<'lua> + ?Sized,
     {
-        let name = chunk
-            .name()
-            .unwrap_or_else(|| Location::caller().to_string());
+        let caller = Location::caller();
+        let name = chunk.name().unwrap_or_else(|| caller.to_string());
 
         Chunk {
             lua: self,
@@ -1428,11 +1434,15 @@ impl Lua {
         S: AsRef<[u8]> + ?Sized,
     {
         unsafe {
+            if self.unlikely_memory_error() {
+                push_string(self.ref_thread(), s.as_ref(), false)?;
+                return Ok(String(self.pop_ref_thread()));
+            }
+
             let _sg = StackGuard::new(self.state);
             check_stack(self.state, 3)?;
 
-            let protect = !self.unlikely_memory_error();
-            push_string(self.state, s, protect)?;
+            push_string(self.state, s.as_ref(), true)?;
             Ok(String(self.pop_ref()))
         }
     }
@@ -1448,11 +1458,15 @@ impl Lua {
     /// Lua may use these hints to preallocate memory for the new table.
     pub fn create_table_with_capacity(&self, narr: c_int, nrec: c_int) -> Result<Table> {
         unsafe {
+            if self.unlikely_memory_error() {
+                push_table(self.ref_thread(), narr, nrec, false)?;
+                return Ok(Table(self.pop_ref_thread()));
+            }
+
             let _sg = StackGuard::new(self.state);
             check_stack(self.state, 3)?;
 
-            let protect = !self.unlikely_memory_error();
-            push_table(self.state, narr, nrec, protect)?;
+            push_table(self.state, narr, nrec, true)?;
             Ok(Table(self.pop_ref()))
         }
     }
@@ -2016,7 +2030,7 @@ impl Lua {
             check_stack(self.state, 3)?;
 
             let protect = !self.unlikely_memory_error();
-            push_string(self.state, name, protect)?;
+            push_string(self.state, name.as_ref(), protect)?;
             ffi::lua_rawget(self.state, ffi::LUA_REGISTRYINDEX);
 
             self.pop_value()
@@ -2048,34 +2062,38 @@ impl Lua {
     /// [`RegistryKey`]: crate::RegistryKey
     pub fn create_registry_value<'lua, T: ToLua<'lua>>(&'lua self, t: T) -> Result<RegistryKey> {
         let t = t.to_lua(self)?;
+        if t == Value::Nil {
+            // Special case to skip calling `luaL_ref` and use `LUA_REFNIL` instead
+            let unref_list = unsafe { (*self.extra.get()).registry_unref_list.clone() };
+            return Ok(RegistryKey::new(ffi::LUA_REFNIL, unref_list));
+        }
+
         unsafe {
             let _sg = StackGuard::new(self.state);
             check_stack(self.state, 4)?;
 
-            let unref_list = (*self.extra.get()).registry_unref_list.clone();
             self.push_value(t)?;
 
-            // Try to reuse previously allocated RegistryKey
-            let unref_list2 = unref_list.clone();
-            let mut unref_list2 = mlua_expect!(unref_list2.lock(), "unref list poisoned");
-            if let Some(registry_id) = unref_list2.as_mut().and_then(|x| x.pop()) {
+            // Try to reuse previously allocated slot
+            let unref_list = (*self.extra.get()).registry_unref_list.clone();
+            let free_registry_id = mlua_expect!(unref_list.lock(), "unref list poisoned")
+                .as_mut()
+                .and_then(|x| x.pop());
+            if let Some(registry_id) = free_registry_id {
                 // It must be safe to replace the value without triggering memory error
                 ffi::lua_rawseti(self.state, ffi::LUA_REGISTRYINDEX, registry_id as Integer);
-                return Ok(RegistryKey {
-                    registry_id,
-                    unref_list,
-                });
+                return Ok(RegistryKey::new(registry_id, unref_list));
             }
 
             // Allocate a new RegistryKey
-            let registry_id = protect_lua!(self.state, 1, 0, |state| {
-                ffi::luaL_ref(state, ffi::LUA_REGISTRYINDEX)
-            })?;
-
-            Ok(RegistryKey {
-                registry_id,
-                unref_list,
-            })
+            let registry_id = if self.unlikely_memory_error() {
+                ffi::luaL_ref(self.state, ffi::LUA_REGISTRYINDEX)
+            } else {
+                protect_lua!(self.state, 1, 0, |state| {
+                    ffi::luaL_ref(state, ffi::LUA_REGISTRYINDEX)
+                })?
+            };
+            Ok(RegistryKey::new(registry_id, unref_list))
         }
     }
 
@@ -2090,16 +2108,16 @@ impl Lua {
             return Err(Error::MismatchedRegistryKey);
         }
 
-        let value = unsafe {
-            let _sg = StackGuard::new(self.state);
-            check_stack(self.state, 1)?;
+        let value = match key.is_nil() {
+            true => Value::Nil,
+            false => unsafe {
+                let _sg = StackGuard::new(self.state);
+                check_stack(self.state, 1)?;
 
-            ffi::lua_rawgeti(
-                self.state,
-                ffi::LUA_REGISTRYINDEX,
-                key.registry_id as Integer,
-            );
-            self.pop_value()
+                let id = key.registry_id as Integer;
+                ffi::lua_rawgeti(self.state, ffi::LUA_REGISTRYINDEX, id);
+                self.pop_value()
+            },
         };
         T::from_lua(value, self)
     }
@@ -2138,20 +2156,31 @@ impl Lua {
         }
 
         let t = t.to_lua(self)?;
+        if t == Value::Nil && key.is_nil() {
+            // Nothing to replace
+            return Ok(());
+        } else if t != Value::Nil && key.registry_id == ffi::LUA_REFNIL {
+            // We cannot update `LUA_REFNIL` slot
+            let err = "cannot replace nil value with non-nil".to_string();
+            return Err(Error::RuntimeError(err));
+        }
+
         unsafe {
             let _sg = StackGuard::new(self.state);
             check_stack(self.state, 2)?;
 
-            self.push_value(t)?;
+            let id = key.registry_id as Integer;
+            if t == Value::Nil {
+                self.push_value(Value::Integer(id))?;
+                key.set_nil(true);
+            } else {
+                self.push_value(t)?;
+                key.set_nil(false);
+            }
             // It must be safe to replace the value without triggering memory error
-            ffi::lua_rawseti(
-                self.state,
-                ffi::LUA_REGISTRYINDEX,
-                key.registry_id as Integer,
-            );
-
-            Ok(())
+            ffi::lua_rawseti(self.state, ffi::LUA_REGISTRYINDEX, id);
         }
+        Ok(())
     }
 
     /// Returns true if the given `RegistryKey` was created by a `Lua` which shares the underlying
@@ -2439,6 +2468,13 @@ impl Lua {
         LuaRef { lua: self, index }
     }
 
+    // Same as `pop_ref` but assumes the value is already on the reference thread
+    pub(crate) unsafe fn pop_ref_thread(&self) -> LuaRef {
+        let extra = &mut *self.extra.get();
+        let index = ref_stack_pop(extra);
+        LuaRef { lua: self, index }
+    }
+
     pub(crate) fn clone_ref<'lua>(&'lua self, lref: &LuaRef<'lua>) -> LuaRef<'lua> {
         unsafe {
             let extra = &mut *self.extra.get();
@@ -2457,16 +2493,6 @@ impl Lua {
         }
     }
 
-    /// Executes the function provided on the ref thread
-    #[inline]
-    pub(crate) unsafe fn ref_thread_exec<F, R>(&self, f: F) -> R
-    where
-        F: FnOnce(*mut ffi::lua_State) -> R,
-    {
-        let ref_thread = (*self.extra.get()).ref_thread;
-        f(ref_thread)
-    }
-
     unsafe fn push_userdata_metatable<T: 'static + UserData>(&self) -> Result<()> {
         let extra = &mut *self.extra.get();
 
@@ -2969,6 +2995,13 @@ impl Lua {
     }
 }
 
+impl LuaInner {
+    #[inline(always)]
+    pub(crate) fn ref_thread(&self) -> *mut ffi::lua_State {
+        unsafe { (*self.extra.get()).ref_thread }
+    }
+}
+
 struct StateGuard<'a>(&'a mut LuaInner, *mut ffi::lua_State);
 
 impl<'a> StateGuard<'a> {

src/luau.rs

@@ -22,6 +22,12 @@ impl Lua {
         globals.raw_set("require", self.create_function(lua_require)?)?;
         globals.raw_set("vector", self.create_c_function(lua_vector)?)?;
 
+        // Set `_VERSION` global to include version number
+        // The environment variable `LUAU_VERSION` set by the build script
+        if let Some(version) = option_env!("LUAU_VERSION") {
+            globals.raw_set("_VERSION", format!("Luau {version}"))?;
+        }
+
         Ok(())
     }
 }

src/serde/de.rs

@@ -71,7 +71,7 @@ impl Options {
     ///
     /// [`deny_recursive_tables`]: #structfield.deny_recursive_tables
     #[must_use]
-    pub fn deny_recursive_tables(mut self, enabled: bool) -> Self {
+    pub const fn deny_recursive_tables(mut self, enabled: bool) -> Self {
         self.deny_recursive_tables = enabled;
         self
     }

src/serde/mod.rs

@@ -10,7 +10,7 @@ use crate::ffi;
 use crate::lua::Lua;
 use crate::table::Table;
 use crate::types::LightUserData;
-use crate::util::{assert_stack, check_stack, StackGuard};
+use crate::util::check_stack;
 use crate::value::Value;
 
 /// Trait for serializing/deserializing Lua values using Serde.
@@ -205,12 +205,8 @@ impl<'lua> LuaSerdeExt<'lua> for Lua {
 
     fn array_metatable(&'lua self) -> Table<'lua> {
         unsafe {
-            let _sg = StackGuard::new(self.state);
-            assert_stack(self.state, 1);
-
-            push_array_metatable(self.state);
-
-            Table(self.pop_ref())
+            push_array_metatable(self.ref_thread());
+            Table(self.pop_ref_thread())
         }
     }
 

src/string.rs

@@ -13,7 +13,6 @@ use {
 use crate::error::{Error, Result};
 use crate::ffi;
 use crate::types::LuaRef;
-use crate::util::{assert_stack, StackGuard};
 
 /// Handle to an internal Lua string.
 ///
@@ -40,6 +39,7 @@ impl<'lua> String<'lua> {
     /// # Ok(())
     /// # }
     /// ```
+    #[inline]
     pub fn to_str(&self) -> Result<&str> {
         str::from_utf8(self.as_bytes()).map_err(|e| Error::FromLuaConversionError {
             from: "string",
@@ -66,6 +66,7 @@ impl<'lua> String<'lua> {
     /// # Ok(())
     /// # }
     /// ```
+    #[inline]
     pub fn to_string_lossy(&self) -> Cow<'_, str> {
         StdString::from_utf8_lossy(self.as_bytes())
     }
@@ -87,6 +88,7 @@ impl<'lua> String<'lua> {
     /// # Ok(())
     /// # }
     /// ```
+    #[inline]
     pub fn as_bytes(&self) -> &[u8] {
         let nulled = self.as_bytes_with_nul();
         &nulled[..nulled.len() - 1]
@@ -94,21 +96,17 @@ impl<'lua> String<'lua> {
 
     /// Get the bytes that make up this string, including the trailing nul byte.
     pub fn as_bytes_with_nul(&self) -> &[u8] {
-        let lua = self.0.lua;
+        let ref_thread = self.0.lua.ref_thread();
         unsafe {
-            let _sg = StackGuard::new(lua.state);
-            assert_stack(lua.state, 1);
-
-            lua.push_ref(&self.0);
             mlua_debug_assert!(
-                ffi::lua_type(lua.state, -1) == ffi::LUA_TSTRING,
+                ffi::lua_type(ref_thread, self.0.index) == ffi::LUA_TSTRING,
                 "string ref is not string type"
             );
 
             let mut size = 0;
             // This will not trigger a 'm' error, because the reference is guaranteed to be of
             // string type
-            let data = ffi::lua_tolstring(lua.state, -1, &mut size);
+            let data = ffi::lua_tolstring(ref_thread, self.0.index, &mut size);
 
             slice::from_raw_parts(data as *const u8, size + 1)
         }
@@ -121,8 +119,8 @@ impl<'lua> String<'lua> {
     /// Typically this function is used only for hashing and debug information.
     #[inline]
     pub fn to_pointer(&self) -> *const c_void {
-        let lua = self.0.lua;
-        unsafe { lua.ref_thread_exec(|refthr| ffi::lua_topointer(refthr, self.0.index)) }
+        let ref_thread = self.0.lua.ref_thread();
+        unsafe { ffi::lua_topointer(ref_thread, self.0.index) }
     }
 }
 

src/table.rs

@@ -58,6 +58,11 @@ impl<'lua> Table<'lua> {
     ///
     /// [`raw_set`]: #method.raw_set
     pub fn set<K: ToLua<'lua>, V: ToLua<'lua>>(&self, key: K, value: V) -> Result<()> {
+        // Fast track
+        if !self.has_metatable() {
+            return self.raw_set(key, value);
+        }
+
         let lua = self.0.lua;
         let key = key.to_lua(lua)?;
         let value = value.to_lua(lua)?;
@@ -98,6 +103,11 @@ impl<'lua> Table<'lua> {
     ///
     /// [`raw_get`]: #method.raw_get
     pub fn get<K: ToLua<'lua>, V: FromLua<'lua>>(&self, key: K) -> Result<V> {
+        // Fast track
+        if !self.has_metatable() {
+            return self.raw_get(key);
+        }
+
         let lua = self.0.lua;
         let key = key.to_lua(lua)?;
 
@@ -116,18 +126,54 @@ impl<'lua> Table<'lua> {
 
     /// Checks whether the table contains a non-nil value for `key`.
     pub fn contains_key<K: ToLua<'lua>>(&self, key: K) -> Result<bool> {
-        let lua = self.0.lua;
-        let key = key.to_lua(lua)?;
+        Ok(self.get::<_, Value>(key)? != Value::Nil)
+    }
 
+    /// Appends a value to the back of the table.
+    pub fn push<V: ToLua<'lua>>(&self, value: V) -> Result<()> {
+        // Fast track
+        if !self.has_metatable() {
+            return self.raw_push(value);
+        }
+
+        let lua = self.0.lua;
+        let value = value.to_lua(lua)?;
         unsafe {
             let _sg = StackGuard::new(lua.state);
             check_stack(lua.state, 4)?;
 
             lua.push_ref(&self.0);
-            lua.push_value(key)?;
-            protect_lua!(lua.state, 2, 1, fn(state) ffi::lua_gettable(state, -2))?;
-            Ok(ffi::lua_isnil(lua.state, -1) == 0)
+            lua.push_value(value)?;
+            protect_lua!(lua.state, 2, 0, fn(state) {
+                let len = ffi::luaL_len(state, -2) as Integer;
+                ffi::lua_seti(state, -2, len + 1);
+            })?
         }
+        Ok(())
+    }
+
+    /// Removes the last element from the table and returns it.
+    pub fn pop<V: FromLua<'lua>>(&self) -> Result<V> {
+        // Fast track
+        if !self.has_metatable() {
+            return self.raw_pop();
+        }
+
+        let lua = self.0.lua;
+        let value = unsafe {
+            let _sg = StackGuard::new(lua.state);
+            check_stack(lua.state, 4)?;
+
+            lua.push_ref(&self.0);
+            protect_lua!(lua.state, 1, 1, fn(state) {
+                let len = ffi::luaL_len(state, -1) as Integer;
+                ffi::lua_geti(state, -1, len);
+                ffi::lua_pushnil(state);
+                ffi::lua_seti(state, -3, len);
+            })?;
+            lua.pop_value()
+        };
+        V::from_lua(value, lua)
     }
 
     /// Compares two tables for equality.
@@ -188,6 +234,9 @@ impl<'lua> Table<'lua> {
 
     /// Sets a key-value pair without invoking metamethods.
     pub fn raw_set<K: ToLua<'lua>, V: ToLua<'lua>>(&self, key: K, value: V) -> Result<()> {
+        #[cfg(feature = "luau")]
+        self.check_readonly_write()?;
+
         let lua = self.0.lua;
         let key = key.to_lua(lua)?;
         let value = value.to_lua(lua)?;
@@ -199,6 +248,7 @@ impl<'lua> Table<'lua> {
             lua.push_ref(&self.0);
             lua.push_value(key)?;
             lua.push_value(value)?;
+
             if lua.unlikely_memory_error() {
                 ffi::lua_rawset(lua.state, -3);
                 ffi::lua_pop(lua.state, 1);
@@ -254,6 +304,56 @@ impl<'lua> Table<'lua> {
         }
     }
 
+    /// Appends a value to the back of the table without invoking metamethods.
+    pub fn raw_push<V: ToLua<'lua>>(&self, value: V) -> Result<()> {
+        #[cfg(feature = "luau")]
+        self.check_readonly_write()?;
+
+        let lua = self.0.lua;
+        let value = value.to_lua(lua)?;
+
+        unsafe {
+            let _sg = StackGuard::new(lua.state);
+            check_stack(lua.state, 4)?;
+
+            lua.push_ref(&self.0);
+            lua.push_value(value)?;
+
+            unsafe fn callback(state: *mut ffi::lua_State) {
+                let len = ffi::lua_rawlen(state, -2) as Integer;
+                ffi::lua_rawseti(state, -2, len + 1);
+            }
+
+            if lua.unlikely_memory_error() {
+                callback(lua.state);
+            } else {
+                protect_lua!(lua.state, 2, 0, fn(state) callback(state))?;
+            }
+        }
+        Ok(())
+    }
+
+    /// Removes the last element from the table and returns it, without invoking metamethods.
+    pub fn raw_pop<V: FromLua<'lua>>(&self) -> Result<V> {
+        #[cfg(feature = "luau")]
+        self.check_readonly_write()?;
+
+        let lua = self.0.lua;
+        let value = unsafe {
+            let _sg = StackGuard::new(lua.state);
+            check_stack(lua.state, 3)?;
+
+            lua.push_ref(&self.0);
+            let len = ffi::lua_rawlen(lua.state, -1) as Integer;
+            ffi::lua_rawgeti(lua.state, -1, len);
+            // Set slot to nil (it must be safe to do)
+            ffi::lua_pushnil(lua.state);
+            ffi::lua_rawseti(lua.state, -3, len);
+            lua.pop_value()
+        };
+        V::from_lua(value, lua)
+    }
+
     /// Removes a key from the table.
     ///
     /// If `key` is an integer, mlua shifts down the elements from `table[key+1]`,
@@ -295,6 +395,11 @@ impl<'lua> Table<'lua> {
     ///
     /// [`raw_len`]: #method.raw_len
     pub fn len(&self) -> Result<Integer> {
+        // Fast track
+        if !self.has_metatable() {
+            return Ok(self.raw_len());
+        }
+
         let lua = self.0.lua;
         unsafe {
             let _sg = StackGuard::new(lua.state);
@@ -307,14 +412,8 @@ impl<'lua> Table<'lua> {
 
     /// Returns the result of the Lua `#` operator, without invoking the `__len` metamethod.
     pub fn raw_len(&self) -> Integer {
-        let lua = self.0.lua;
-        unsafe {
-            let _sg = StackGuard::new(lua.state);
-            assert_stack(lua.state, 1);
-
-            lua.push_ref(&self.0);
-            ffi::lua_rawlen(lua.state, -1) as Integer
-        }
+        let ref_thread = self.0.lua.ref_thread();
+        unsafe { ffi::lua_rawlen(ref_thread, self.0.index) as Integer }
     }
 
     /// Returns a reference to the metatable of this table, or `None` if no metatable is set.
@@ -340,6 +439,12 @@ impl<'lua> Table<'lua> {
     /// If `metatable` is `None`, the metatable is removed (if no metatable is set, this does
     /// nothing).
     pub fn set_metatable(&self, metatable: Option<Table<'lua>>) {
+        // Workaround to throw readonly error without returning Result
+        #[cfg(feature = "luau")]
+        if self.is_readonly() {
+            panic!("attempt to modify a readonly table");
+        }
+
         let lua = self.0.lua;
         unsafe {
             let _sg = StackGuard::new(lua.state);
@@ -355,21 +460,33 @@ impl<'lua> Table<'lua> {
         }
     }
 
+    /// Returns true if the table has metatable attached.
+    #[doc(hidden)]
+    #[inline]
+    pub fn has_metatable(&self) -> bool {
+        let ref_thread = self.0.lua.ref_thread();
+        unsafe {
+            if ffi::lua_getmetatable(ref_thread, self.0.index) != 0 {
+                ffi::lua_pop(ref_thread, 1);
+                return true;
+            }
+        }
+        false
+    }
+
     /// Sets `readonly` attribute on the table.
     ///
     /// Requires `feature = "luau"`
     #[cfg(any(feature = "luau", doc))]
     #[cfg_attr(docsrs, doc(cfg(feature = "luau")))]
     pub fn set_readonly(&self, enabled: bool) {
-        let lua = self.0.lua;
+        let ref_thread = self.0.lua.ref_thread();
         unsafe {
-            lua.ref_thread_exec(|refthr| {
-                ffi::lua_setreadonly(refthr, self.0.index, enabled as _);
-                if !enabled {
-                    // Reset "safeenv" flag
-                    ffi::lua_setsafeenv(refthr, self.0.index, 0);
-                }
-            });
+            ffi::lua_setreadonly(ref_thread, self.0.index, enabled as _);
+            if !enabled {
+                // Reset "safeenv" flag
+                ffi::lua_setsafeenv(ref_thread, self.0.index, 0);
+            }
         }
     }
 
@@ -379,8 +496,8 @@ impl<'lua> Table<'lua> {
     #[cfg(any(feature = "luau", doc))]
     #[cfg_attr(docsrs, doc(cfg(feature = "luau")))]
     pub fn is_readonly(&self) -> bool {
-        let lua = self.0.lua;
-        unsafe { lua.ref_thread_exec(|refthr| ffi::lua_getreadonly(refthr, self.0.index) != 0) }
+        let ref_thread = self.0.lua.ref_thread();
+        unsafe { ffi::lua_getreadonly(ref_thread, self.0.index) != 0 }
     }
 
     /// Converts the table to a generic C pointer.
@@ -391,8 +508,8 @@ impl<'lua> Table<'lua> {
     /// Typically this function is used only for hashing and debug information.
     #[inline]
     pub fn to_pointer(&self) -> *const c_void {
-        let lua = self.0.lua;
-        unsafe { lua.ref_thread_exec(|refthr| ffi::lua_topointer(refthr, self.0.index)) }
+        let ref_thread = self.0.lua.ref_thread();
+        unsafe { ffi::lua_topointer(ref_thread, self.0.index) }
     }
 
     /// Consume this table and return an iterator over the pairs of the table.
@@ -532,6 +649,16 @@ impl<'lua> Table<'lua> {
             ffi::lua_rawequal(lua.state, -1, -2) != 0
         }
     }
+
+    #[cfg(feature = "luau")]
+    #[inline(always)]
+    pub(crate) fn check_readonly_write(&self) -> Result<()> {
+        if self.is_readonly() {
+            let err = "attempt to modify a readonly table".to_string();
+            return Err(Error::RuntimeError(err));
+        }
+        Ok(())
+    }
 }
 
 impl<'lua> PartialEq for Table<'lua> {

src/thread.rs

@@ -118,8 +118,7 @@ impl<'lua> Thread<'lua> {
             let _sg = StackGuard::new(lua.state);
             check_stack(lua.state, cmp::max(nargs + 1, 3))?;
 
-            let thread_state =
-                lua.ref_thread_exec(|ref_thread| ffi::lua_tothread(ref_thread, self.0.index));
+            let thread_state = ffi::lua_tothread(lua.ref_thread(), self.0.index);
 
             let status = ffi::lua_status(thread_state);
             if status != ffi::LUA_YIELD && ffi::lua_gettop(thread_state) == 0 {
@@ -160,8 +159,7 @@ impl<'lua> Thread<'lua> {
     pub fn status(&self) -> ThreadStatus {
         let lua = self.0.lua;
         unsafe {
-            let thread_state =
-                lua.ref_thread_exec(|ref_thread| ffi::lua_tothread(ref_thread, self.0.index));
+            let thread_state = ffi::lua_tothread(lua.ref_thread(), self.0.index);
 
             let status = ffi::lua_status(thread_state);
             if status != ffi::LUA_OK && status != ffi::LUA_YIELD {
@@ -326,7 +324,7 @@ impl<'lua> Thread<'lua> {
     pub fn sandbox(&self) -> Result<()> {
         let lua = self.0.lua;
         unsafe {
-            let thread = lua.ref_thread_exec(|t| ffi::lua_tothread(t, self.0.index));
+            let thread = ffi::lua_tothread(lua.ref_thread(), self.0.index);
             check_stack(thread, 1)?;
             check_stack(lua.state, 3)?;
             // Inherit `LUA_GLOBALSINDEX` from the caller
@@ -366,8 +364,7 @@ impl<'lua, R> Drop for AsyncThread<'lua, R> {
                 if !lua.recycle_thread(&mut self.thread) {
                     #[cfg(feature = "lua54")]
                     if self.thread.status() == ThreadStatus::Error {
-                        let thread_state =
-                            lua.ref_thread_exec(|t| ffi::lua_tothread(t, self.thread.0.index));
+                        let thread_state = ffi::lua_tothread(lua.ref_thread(), self.thread.0.index);
                         ffi::lua_resetthread(thread_state);
                     }
                 }

src/types.rs

@@ -1,6 +1,7 @@
 use std::cell::UnsafeCell;
 use std::hash::{Hash, Hasher};
 use std::os::raw::{c_int, c_void};
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
 use std::{fmt, mem, ptr};
 
@@ -104,6 +105,7 @@ pub(crate) struct DestructedUserdata;
 /// [`AnyUserData::get_user_value`]: crate::AnyUserData::get_user_value
 pub struct RegistryKey {
     pub(crate) registry_id: c_int,
+    pub(crate) is_nil: AtomicBool,
     pub(crate) unref_list: Arc<Mutex<Option<Vec<c_int>>>>,
 }
 
@@ -129,15 +131,27 @@ impl Eq for RegistryKey {}
 
 impl Drop for RegistryKey {
     fn drop(&mut self) {
-        let mut unref_list = mlua_expect!(self.unref_list.lock(), "unref list poisoned");
-        if let Some(list) = unref_list.as_mut() {
-            list.push(self.registry_id);
+        // We don't need to collect nil slot
+        if self.registry_id > ffi::LUA_REFNIL {
+            let mut unref_list = mlua_expect!(self.unref_list.lock(), "unref list poisoned");
+            if let Some(list) = unref_list.as_mut() {
+                list.push(self.registry_id);
+            }
         }
     }
 }
 
 impl RegistryKey {
-    // Destroys the RegistryKey without adding to the drop list
+    // Creates a new instance of `RegistryKey`
+    pub(crate) const fn new(id: c_int, unref_list: Arc<Mutex<Option<Vec<c_int>>>>) -> Self {
+        RegistryKey {
+            registry_id: id,
+            is_nil: AtomicBool::new(id == ffi::LUA_REFNIL),
+            unref_list,
+        }
+    }
+
+    // Destroys the `RegistryKey` without adding to the unref list
     pub(crate) fn take(self) -> c_int {
         let registry_id = self.registry_id;
         unsafe {
@@ -146,6 +160,21 @@ impl RegistryKey {
         }
         registry_id
     }
+
+    // Returns true if this `RegistryKey` holds a nil value
+    #[inline(always)]
+    pub(crate) fn is_nil(&self) -> bool {
+        self.is_nil.load(Ordering::Relaxed)
+    }
+
+    // Marks value of this `RegistryKey` as `Nil`
+    #[inline(always)]
+    pub(crate) fn set_nil(&self, enabled: bool) {
+        // We cannot replace previous value with nil in as this will break
+        // Lua mechanism to find free keys.
+        // Instead, we set a special flag to mark value as nil.
+        self.is_nil.store(enabled, Ordering::Relaxed);
+    }
 }
 
 pub(crate) struct LuaRef<'lua> {

src/util.rs

@@ -242,15 +242,11 @@ pub unsafe fn pop_error(state: *mut ffi::lua_State, err_code: c_int) -> Error {
     }
 }
 
-// Uses 3 stack spaces, does not call checkstack.
-#[inline]
-pub unsafe fn push_string<S: AsRef<[u8]> + ?Sized>(
-    state: *mut ffi::lua_State,
-    s: &S,
-    protect: bool,
-) -> Result<()> {
-    let s = s.as_ref();
-    if protect {
+// Uses 3 (or 1 if unprotected) stack spaces, does not call checkstack.
+#[inline(always)]
+pub unsafe fn push_string(state: *mut ffi::lua_State, s: &[u8], protect: bool) -> Result<()> {
+    // Always use protected mode if the string is too long
+    if protect || s.len() > (1 << 30) {
         protect_lua!(state, 0, 1, |state| {
             ffi::lua_pushlstring(state, s.as_ptr() as *const c_char, s.len());
         })
@@ -432,11 +428,17 @@ unsafe fn init_userdata_metatable_index(state: *mut ffi::lua_State) -> Result<()
     }
     ffi::lua_pop(state, 1);
 
-    // Create and cache `__index` helper
+    // Create and cache `__index` generator
     let code = cstr!(
         r#"
             local error, isfunction = ...
             return function (__index, field_getters, methods)
+                -- Fastpath to return methods table for index access
+                if __index == nil and field_getters == nil then
+                    return methods
+                end
+
+                -- Alternatively return a function for index access
                 return function (self, key)
                     if field_getters ~= nil then
                         local field_getter = field_getters[key]
@@ -486,7 +488,7 @@ pub unsafe fn init_userdata_metatable_newindex(state: *mut ffi::lua_State) -> Re
     }
     ffi::lua_pop(state, 1);
 
-    // Create and cache `__newindex` helper
+    // Create and cache `__newindex` generator
     let code = cstr!(
         r#"
             local error, isfunction = ...
@@ -548,7 +550,7 @@ pub unsafe fn init_userdata_metatable<T>(
         // Push `__index` generator function
         init_userdata_metatable_index(state)?;
 
-        push_string(state, "__index", true)?;
+        push_string(state, b"__index", true)?;
         let index_type = ffi::lua_rawget(state, -3);
         match index_type {
             ffi::LUA_TNIL | ffi::LUA_TTABLE | ffi::LUA_TFUNCTION => {
@@ -573,7 +575,7 @@ pub unsafe fn init_userdata_metatable<T>(
         // Push `__newindex` generator function
         init_userdata_metatable_newindex(state)?;
 
-        push_string(state, "__newindex", true)?;
+        push_string(state, b"__newindex", true)?;
         let newindex_type = ffi::lua_rawget(state, -3);
         match newindex_type {
             ffi::LUA_TNIL | ffi::LUA_TTABLE | ffi::LUA_TFUNCTION => {
@@ -893,7 +895,7 @@ pub unsafe fn init_error_registry(state: *mut ffi::lua_State) -> Result<()> {
                 }
             }?;
 
-            push_string(state, &*err_buf, true)?;
+            push_string(state, (*err_buf).as_bytes(), true)?;
             (*err_buf).clear();
 
             Ok(1)

src/value.rs

@@ -111,11 +111,11 @@ impl<'lua> Value<'lua> {
                 Value::LightUserData(ud) => ud.0,
                 Value::Table(t) => t.to_pointer(),
                 Value::String(s) => s.to_pointer(),
-                Value::Function(Function(v))
-                | Value::Thread(Thread(v))
-                | Value::UserData(AnyUserData(v)) => v
-                    .lua
-                    .ref_thread_exec(|refthr| ffi::lua_topointer(refthr, v.index)),
+                Value::Function(Function(r))
+                | Value::Thread(Thread(r))
+                | Value::UserData(AnyUserData(r)) => {
+                    ffi::lua_topointer(r.lua.ref_thread(), r.index)
+                }
                 _ => ptr::null(),
             }
         }
@@ -197,7 +197,6 @@ pub struct MultiValue<'lua>(Vec<Value<'lua>>);
 
 impl<'lua> MultiValue<'lua> {
     /// Creates an empty `MultiValue` containing no values.
-    #[inline]
     pub const fn new() -> MultiValue<'lua> {
         MultiValue(Vec::new())
     }
@@ -276,7 +275,10 @@ impl<'lua> MultiValue<'lua> {
 
     #[inline]
     pub fn get(&self, index: usize) -> Option<&Value<'lua>> {
-        self.0.get(self.0.len() - index - 1)
+        if index < self.0.len() {
+            return self.0.get(self.0.len() - index - 1);
+        }
+        None
     }
 
     #[inline]

tests/luau.rs

@@ -1,7 +1,9 @@
 #![cfg(feature = "luau")]
 
 use std::env;
+use std::fmt::Debug;
 use std::fs;
+use std::panic::{catch_unwind, AssertUnwindSafe};
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::Arc;
 
@@ -75,18 +77,33 @@ fn test_vectors() -> Result<()> {
 fn test_readonly_table() -> Result<()> {
     let lua = Lua::new();
 
-    let t = lua.create_table()?;
+    let t = lua.create_sequence_from([1])?;
     assert!(!t.is_readonly());
     t.set_readonly(true);
     assert!(t.is_readonly());
 
-    match t.set("key", "value") {
-        Err(Error::RuntimeError(err)) if err.contains("attempt to modify a readonly table") => {}
-        r => panic!(
-            "expected RuntimeError(...) with a specific message, got {:?}",
-            r
-        ),
-    };
+    #[track_caller]
+    fn check_readonly_error<T: Debug>(res: Result<T>) {
+        match res {
+            Err(Error::RuntimeError(e)) if e.contains("attempt to modify a readonly table") => {}
+            r => panic!("expected RuntimeError(...) with a specific message, got {r:?}"),
+        }
+    }
+
+    check_readonly_error(t.set("key", "value"));
+    check_readonly_error(t.raw_set("key", "value"));
+    check_readonly_error(t.raw_insert(1, "value"));
+    check_readonly_error(t.raw_remove(1));
+    check_readonly_error(t.push("value"));
+    check_readonly_error(t.pop::<Value>());
+    check_readonly_error(t.raw_push("value"));
+    check_readonly_error(t.raw_pop::<Value>());
+
+    // Special case
+    match catch_unwind(AssertUnwindSafe(|| t.set_metatable(None))) {
+        Ok(_) => panic!("expected panic, got nothing"),
+        Err(_) => {}
+    }
 
     Ok(())
 }
@@ -220,7 +237,7 @@ fn test_interrupts() -> Result<()> {
 }
 
 #[test]
-fn test_coverate() -> Result<()> {
+fn test_coverage() -> Result<()> {
     let lua = Lua::new();
 
     lua.set_compiler(Compiler::default().set_coverage_level(1));

tests/table.rs

@@ -109,6 +109,49 @@ fn test_table() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_table_push_pop() -> Result<()> {
+    let lua = Lua::new();
+
+    // Test raw access
+    let table1 = lua.create_sequence_from(vec![123])?;
+    table1.raw_push(321)?;
+    assert_eq!(
+        table1
+            .clone()
+            .raw_sequence_values::<i64>()
+            .collect::<Result<Vec<_>>>()?,
+        vec![123, 321]
+    );
+    assert_eq!(table1.raw_pop::<i64>()?, 321);
+    assert_eq!(table1.raw_pop::<i64>()?, 123);
+    assert_eq!(table1.raw_pop::<Value>()?, Value::Nil); // An extra pop should do nothing
+    assert_eq!(table1.raw_len(), 0);
+
+    // Test access through metamethods
+    let table2 = lua
+        .load(
+            r#"
+        local proxy_table = {234}
+        table2 = setmetatable({}, {
+            __len = function() return #proxy_table end,
+            __index = proxy_table,
+            __newindex = proxy_table,
+        })
+        return table2
+    "#,
+        )
+        .eval::<Table>()?;
+    table2.push(345)?;
+    assert_eq!(table2.len()?, 2);
+    assert_eq!(table2.pop::<i64>()?, 345);
+    assert_eq!(table2.pop::<i64>()?, 234);
+    assert_eq!(table2.pop::<Value>()?, Value::Nil);
+    assert_eq!(table2.len()?, 0);
+
+    Ok(())
+}
+
 #[test]
 fn test_table_sequence_from() -> Result<()> {
     let lua = Lua::new();

tests/tests.rs

@@ -276,7 +276,10 @@ fn test_error() -> Result<()> {
             end, 3)
 
             local function handler(err)
-                if string.match(_VERSION, ' 5%.1$') or string.match(_VERSION, ' 5%.2$') or _VERSION == "Luau" then
+                if string.match(_VERSION, " 5%.1$")
+                    or string.match(_VERSION, " 5%.2$")
+                    or string.match(_VERSION, "Luau")
+                then
                     -- Special case for Lua 5.1/5.2 and Luau
                     local caps = string.match(err, ': (%d+)$')
                     if caps then
@@ -793,6 +796,17 @@ fn test_replace_registry_value() -> Result<()> {
     let key = lua.create_registry_value::<i32>(42)?;
     lua.replace_registry_value(&key, "new value")?;
     assert_eq!(lua.registry_value::<String>(&key)?, "new value");
+    lua.replace_registry_value(&key, Value::Nil)?;
+    assert_eq!(lua.registry_value::<Value>(&key)?, Value::Nil);
+    lua.replace_registry_value(&key, 123)?;
+    assert_eq!(lua.registry_value::<i32>(&key)?, 123);
+
+    // It should be impossible to replace (initial) nil value with non-nil
+    let key2 = lua.create_registry_value(Value::Nil)?;
+    match lua.replace_registry_value(&key2, "abc") {
+        Err(Error::RuntimeError(_)) => {}
+        r => panic!("expected RuntimeError, got {r:?}"),
+    }
 
     Ok(())
 }
@@ -844,6 +858,28 @@ fn test_mismatched_registry_key() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_registry_value_reuse() -> Result<()> {
+    let lua = Lua::new();
+
+    let r1 = lua.create_registry_value("value1")?;
+    let r1_slot = format!("{r1:?}");
+    drop(r1);
+
+    // Previous slot must not be reused by nil value
+    let r2 = lua.create_registry_value(Value::Nil)?;
+    let r2_slot = format!("{r2:?}");
+    assert_ne!(r1_slot, r2_slot);
+    drop(r2);
+
+    // But should be reused by non-nil value
+    let r3 = lua.create_registry_value("value3")?;
+    let r3_slot = format!("{r3:?}");
+    assert_eq!(r1_slot, r3_slot);
+
+    Ok(())
+}
+
 #[test]
 fn test_application_data() -> Result<()> {
     let lua = Lua::new();

tests/value.rs

@@ -1,6 +1,6 @@
 use std::ptr;
 
-use mlua::{Lua, Result, Value};
+use mlua::{Lua, MultiValue, Result, Value};
 
 #[test]
 fn test_value_eq() -> Result<()> {
@@ -63,3 +63,21 @@ fn test_value_eq() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_multi_value() {
+    let mut multi_value = MultiValue::new();
+    assert_eq!(multi_value.len(), 0);
+    assert_eq!(multi_value.get(0), None);
+
+    multi_value.push_front(Value::Number(2.));
+    multi_value.push_front(Value::Number(1.));
+    assert_eq!(multi_value.get(0), Some(&Value::Number(1.)));
+    assert_eq!(multi_value.get(1), Some(&Value::Number(2.)));
+
+    assert_eq!(multi_value.pop_front(), Some(Value::Number(1.)));
+    assert_eq!(multi_value[0], Value::Number(2.));
+
+    multi_value.clear();
+    assert!(multi_value.is_empty());
+}