Implement an error handler that collects the stacktrace

src/main.rs

@@ -47,7 +47,7 @@ fn main() -> Result<()> {
     //let globals = lua.globals()?;
     globals.set("X", tbl)?;
 
-    lua.execute(r#"
+    let res = lua.execute(r#"
         require "util"
 
         --[[function dump_table(tbl)
@@ -99,10 +99,33 @@ fn main() -> Result<()> {
         local v2 = Vec2.new(500, 500)
         print("v2 is (" .. v2.x .. ", " .. v2.y .. ")")
 
-        local v_add = v1:add(v2)
+        local v_add = v1 + v2
         v_add.x = 90
         print("v_add is (" .. v_add.x .. ", " .. v_add.y .. ")")
-    "#).unwrap();
+
+        function f1(v1, v2)
+            --print("f1 tb: " .. debug.traceback(1))
+            local v_add = v1:add(v2)
+        end
+
+        function f2(v1, v2)
+            --print("f2 tb: " .. debug.traceback(1))
+            f1(v1, v2)
+        end
+
+        function f3(v1, v2)
+            --print("f3 tb: " .. debug.traceback(1))
+            f2(v1, v2)
+        end
+
+        f3(v1, v2)
+    "#);
+
+    // if unwrapped, the new lines in the message would be escaped making
+    // the traceback in the error difficult to read.
+    if let Err(e) = res {
+        panic!("{}", e);
+    }
 
     unsafe {
         assert_eq!(lua::lua_gettop(lua.state_ptr()), 0); // ensure that nothing is left on the stack
@@ -155,19 +178,24 @@ impl<'a> PushToLuaStack<'a> for LuaRef {
 
 #[derive(Debug, thiserror::Error)]
 pub enum Error {
+    #[error("Syntax error: {0}")]
+    Syntax(String),
     /// An error returned from lua
     #[error("Lua runtime error: {0}")]
     Runtime(String),
+    #[error("Error when running a __gc metamethod")]
+    GcFailure(String),
     /// Ran into a not enough memory error when trying to grow the lua stack.
-    #[error("Ran out of memory when attempting to use `lua_checkstack`")]
-    Oom,
+    #[error("Failed to allocate memory")]
+    MemoryAlloc,
     #[error("Ran into a nill value on the stack")]
     Nil,
     #[error("Unexpected type, expected {0} but got {1}")]
     UnexpectedType(String, String),
-    #[error("Bad argument provided to `{}` at position {arg_index}{}, cause: {error}",
-        .func.clone().unwrap_or("Unknown".to_string()),
-        .arg_name.clone().map(|a| format!(" (name: {})", a)).unwrap_or("".to_string()))]
+    #[error("bad argument #{arg_index}{} to `{}` ({error})",
+        .arg_name.clone().map(|a| format!(" (name: {})", a)).unwrap_or("".to_string()),
+        .func.clone().unwrap_or("Unknown".to_string())
+    )]
     BadArgument {
         func: Option<String>,
         arg_index: i32,

src/state.rs

@@ -1,5 +1,5 @@
 use core::ffi;
-use std::{alloc::{self, Layout}, any::TypeId, collections::HashMap, ffi::{CString, CStr}, mem, ptr::{self, NonNull}, str::Utf8Error};
+use std::{alloc::{self, Layout}, any::TypeId, collections::{HashMap, VecDeque}, ffi::{CStr, CString}, mem, ptr::{self, NonNull}, str::Utf8Error};
 
 use mlua_sys as lua;
 
@@ -150,22 +150,64 @@ impl State {
         }
     }
 
+    unsafe fn get_error_str(&self) -> &str {
+        let error_c = CStr::from_ptr(lua::lua_tostring(self.state_ptr(), -1));
+        let error_str = error_c.to_str()
+            .unwrap_or_else(|_| {
+                let b = error_c.to_bytes();
+                std::str::from_utf8_unchecked(b)
+            });
+        lua::lua_pop(self.state_ptr(), 1);
+        error_str
+    }
+
+    /// A function that handles errors from calling scripts. It modifies the message to include
+    /// the stack traceback.
+    fn state_error_handler(&self, msg: String) -> Result<String> {
+        self.traceback(Some(&msg))
+    }
+
     pub fn execute(&self, text: &str) -> Result<()> {
         let text = format!("{}\0", text);
 
         unsafe {
+            self.ensure_stack(3)?;
+            let _g = StackGuard::new(self);
+
             let lua = self.lua.as_ptr();
-            let text_c = text.as_ptr() as *const ffi::c_char;
-            if lua::luaL_dostring(lua, text_c) != 0 {
-                let error_c = CStr::from_ptr(lua::lua_tostring(lua, -1));
-                let error_str = error_c.to_str()
-                    .unwrap_or_else(|_| {
-                        let b = error_c.to_bytes();
-                        std::str::from_utf8_unchecked(b)
-                    });
-                    //.expect("Error bytes are invalid!");
-                //std::str::from
-                return Err(Error::runtime(error_str));
+            let text_c = text.as_ptr().cast();
+
+            // Subtract one from the length to exclude the null terminator
+            match lua::luaL_loadbuffer(self.state_ptr(), text_c, text.len() - 1, "test.lua\0".as_ptr().cast()) {
+                lua::LUA_ERRSYNTAX => {
+                    return Err(Error::Syntax(self.get_error_str().to_string()));
+                },
+                lua::LUA_ERRMEM => {
+                    return Err(Error::MemoryAlloc);
+                }
+                _ => {},
+            }
+
+            let handler = self.create_function(|lua, msg: String| {
+                lua.state_error_handler(msg)
+            })?;
+            handler.push_to_lua_stack(self)?;
+            lua::lua_insert(lua, -2);
+
+            match lua::lua_pcall(lua, 0, lua::LUA_MULTRET, -2) {
+                lua::LUA_ERRRUN => {
+                    let er = self.get_error_str();
+                    return Err(Error::runtime(er));
+                },
+                lua::LUA_ERRMEM => {
+                    return Err(Error::MemoryAlloc);
+                },
+                lua::LUA_ERRERR => {
+                    let er = self.get_error_str();
+                    // if this panics, its a bug
+                    panic!("Failure when trying to execute error handler function! ({})", er);
+                },
+                _ => {}
             }
         }
 
@@ -176,7 +218,7 @@ impl State {
         let lua = self.lua.as_ptr();
 
         unsafe {
-            let base = "_G".as_ptr() as *const i8;
+            let base = "_G\0".as_ptr().cast();
             lua::luaL_requiref(lua, base, lua::luaopen_base, 1);
             assert_eq!(lua::lua_tonumber(lua, -1), 0.0); // ensure that loading base worked
             lua::lua_pop(lua, 1);
@@ -210,7 +252,7 @@ impl State {
     /// The result will be Ok if the stack has space for `n` values.
     pub unsafe fn ensure_stack(&self, n: i32) -> Result<()> {
         if lua::lua_checkstack(self.state_ptr(), n) == 0 {
-            Err(Error::Oom)
+            Err(Error::MemoryAlloc)
         } else {
             Ok(())
         }
@@ -328,14 +370,8 @@ impl State {
             let ptr = lua::lua_newuserdata(s, mem::size_of::<T>()).cast::<T>();
             ptr::write(ptr, data);
 
-            let name_cstr = format!("{}\0", name);
-            let name_cstr = name_cstr.as_str();
-            let name_cstr = name_cstr.as_ptr() as *const i8;
-
-            // attempt to get the metatable
-
-            let udmts = &mut self.get_extra_space().userdata_metatables;//.get(&TypeId::of::<T>())
-
+            // get the current metatable, or create a new one and push it to the stack.
+            let udmts = &mut self.get_extra_space().userdata_metatables;
             if !udmts.contains_key(&TypeId::of::<T>()) {
                 // create the userdata's metatable and store it in extra space
                 let mt = self.create_userdata_metatable::<T>()?;
@@ -428,4 +464,61 @@ impl State {
 
         Ok(mt)
     }
+
+    pub(crate) unsafe fn debug_info(&self) -> Box<lua::lua_Debug> {
+        let s = self.state_ptr();
+
+        // lua_Debug doesn't implement default
+        let ar = alloc::alloc(Layout::new::<lua::lua_Debug>())
+            .cast::<lua::lua_Debug>();
+        lua::lua_getstack(s, 1, ar);
+        let what = "nSl".as_ptr() as *const i8;
+        lua::lua_getinfo(s, what, ar);
+
+        Box::from_raw(ar)
+    }
+
+    pub fn current_line(&self) -> usize {
+        unsafe {
+            let ar = self.debug_info();
+            ar.currentline as usize
+        }
+    }
+
+    pub fn traceback(&self, msg: Option<&str>) -> Result<String> {
+        unsafe {
+            let _g = StackGuard::new(self);
+            let s = self.state_ptr();
+
+            let globals = self.globals()?;
+            let debug = globals.get::<_, Table>("debug")?;
+            let tb_fn = debug.get::<_, Function>("traceback")?;
+            tb_fn.push_to_lua_stack(self)?;
+
+            if let Some(msg) = msg {
+                lua::lua_pushstring(s, msg.as_ptr().cast());
+            } else {
+                lua::lua_pushnil(s);
+            }
+
+            // skip the call to debug.traceback, and the
+            // error handler (which hopefully called this)
+            lua::lua_pushnumber(s, 2.0);
+
+            match lua::lua_pcall(s, 2, 1, 0) {
+                lua::LUA_ERRRUN => {
+                    let er = self.get_error_str();
+                    return Err(Error::runtime(er));
+                },
+                lua::LUA_ERRMEM => {
+                    return Err(Error::MemoryAlloc)
+                },
+                _ => {}
+            }
+
+            let v = Value::from_lua_stack(self)?;
+            let s = v.into_string()?;
+            Ok(s)
+        }
+    }
 }

src/value.rs

@@ -63,6 +63,15 @@ impl<'a> Value<'a> {
             }
         }
     }
+
+    pub fn into_string(self) -> crate::Result<String> {
+        match self {
+            Value::String(s) => Ok(s),
+            _ => {
+                Err(crate::Error::UnexpectedType("String".to_string(), self.type_name().to_string()))
+            }
+        }
+    }
 }
 
 impl<'a> PushToLuaStack<'a> for Value<'a> {
@@ -241,6 +250,19 @@ impl<'a> FromLuaVec<'a> for ValueVec<'a> {
     }
 }
 
+impl<'a, T: FromLua<'a>> FromLuaVec<'a> for T {
+    fn from_lua_value_vec(state: &'a State, mut values: ValueVec<'a>) -> crate::Result<Self> {
+        if let Some(val) = values.pop_front() {
+            T::from_lua(state, val)
+        } else {
+            Err(crate::Error::IncorrectArgCount {
+                arg_expected: 1,
+                arg_count: values.len() as i32,
+            })
+        }
+    }
+}
+
 macro_rules! impl_from_lua_vec_tuple {
     ( $count: expr, $first: tt, $( $name: tt ),+ ) => (
         #[allow(non_snake_case)]