path: root/src/util.rs

use std::mem;
use std::ptr;
use std::sync::Arc;
use std::ffi::CStr;
use std::any::Any;
use std::os::raw::{c_char, c_int, c_void};
use std::panic::{catch_unwind, resume_unwind, UnwindSafe};

use ffi;
use error::{LuaResult, LuaError};

macro_rules! cstr {
  ($s:expr) => (
    concat!($s, "\0") as *const str as *const [c_char] as *const c_char
  );
}

pub unsafe fn check_stack(state: *mut ffi::lua_State, amount: c_int) -> LuaResult<()> {
    if ffi::lua_checkstack(state, amount) == 0 {
        Err(LuaError::StackOverflow)
    } else {
        Ok(())
    }
}

// Run an operation on a lua_State and automatically clean up the stack before returning.  Takes
// the lua_State, the expected stack size change, and an operation to run.  If the operation
// results in success, then the stack is inspected to make sure the change in stack size matches
// the expected change and otherwise this is a logic error and will panic.  If the operation
// results in an error, the stack is shrunk to the value before the call.  If the operation
// results in an error and the stack is smaller than the value before the call, then this is
// unrecoverable and this will panic.
pub unsafe fn stack_guard<F, R>(state: *mut ffi::lua_State, change: c_int, op: F) -> LuaResult<R>
where
    F: FnOnce() -> LuaResult<R>,
{
    let expected = ffi::lua_gettop(state) + change;
    assert!(
        expected >= 0,
        "lua stack error, too many values would be popped"
    );
    let res = op();
    let top = ffi::lua_gettop(state);
    if res.is_ok() {
        assert_eq!(
            ffi::lua_gettop(state),
            expected,
            "lua stack error, expected stack to be {}, got {}",
            expected,
            top
        );
    } else {
        assert!(
            top >= expected,
            "lua stack error, {} too many values popped",
            top - expected
        );
        if top > expected {
            ffi::lua_settop(state, expected);
        }
    }
    res
}

// Call the given rust function in a protected lua context, similar to pcall.
// The stack given to the protected function is a separate protected stack. This
// catches all calls to lua_error, but ffi functions that can call lua_error are
// still longjmps, and have all the same dangers as longjmps, so extreme care
// must still be taken in code that uses this function.  Does not call
// lua_checkstack, and uses 2 extra stack spaces.
pub unsafe fn error_guard<F, R>(
    state: *mut ffi::lua_State,
    nargs: c_int,
    nresults: c_int,
    func: F,
) -> LuaResult<R>
where
    F: FnOnce(*mut ffi::lua_State) -> LuaResult<R> + UnwindSafe,
{
    unsafe extern "C" fn call_impl<F>(state: *mut ffi::lua_State) -> c_int
    where
        F: FnOnce(*mut ffi::lua_State) -> c_int,
    {
        let func = ffi::lua_touserdata(state, -1) as *mut F;
        let func = mem::replace(&mut *func, mem::uninitialized());
        ffi::lua_pop(state, 1);
        func(state)
    }

    unsafe fn cpcall<F>(
        state: *mut ffi::lua_State,
        nargs: c_int,
        nresults: c_int,
        mut func: F,
    ) -> LuaResult<()>
    where
        F: FnOnce(*mut ffi::lua_State) -> c_int,
    {
        ffi::lua_pushcfunction(state, call_impl::<F>);
        ffi::lua_insert(state, -(nargs + 1));
        ffi::lua_pushlightuserdata(state, &mut func as *mut F as *mut c_void);
        mem::forget(func);
        if pcall_with_traceback(state, nargs + 1, nresults) != ffi::LUA_OK {
            Err(pop_error(state))
        } else {
            Ok(())
        }
    }

    let mut res = None;
    cpcall(state, nargs, nresults, |state| {
        res = Some(callback_error(state, || func(state)));
        ffi::lua_gettop(state)
    })?;
    Ok(res.unwrap())
}

// If the return code indicates an error, pops the error off of the stack and
// returns Err. If the error was actually a rust panic, clears the current lua
// stack and panics.
pub unsafe fn handle_error(state: *mut ffi::lua_State, ret: c_int) -> LuaResult<()> {
    if ret != ffi::LUA_OK && ret != ffi::LUA_YIELD {
        Err(pop_error(state))
    } else {
        Ok(())
    }
}

pub unsafe fn push_string(state: *mut ffi::lua_State, s: &str) {
    ffi::lua_pushlstring(state, s.as_ptr() as *const c_char, s.len());
}

pub unsafe extern "C" fn destructor<T>(state: *mut ffi::lua_State) -> c_int {
    match catch_unwind(|| {
        let obj = &mut *(ffi::lua_touserdata(state, 1) as *mut T);
        mem::replace(obj, mem::uninitialized());
        0
    }) {
        Ok(r) => r,
        Err(p) => {
            push_panic(state, p);
            ffi::lua_error(state)
        }
    }
}

// In the context of a lua callback, this will call the given function and if the given function
// returns an error, *or if the given function panics*, this will result in a call to lua_error (a
// longjmp).  The error or panic is wrapped in such a way that when calling pop_error back on
// the rust side, it will resume the panic.
pub unsafe fn callback_error<R, F>(state: *mut ffi::lua_State, f: F) -> R
where
    F: FnOnce() -> LuaResult<R> + UnwindSafe,
{
    match catch_unwind(f) {
        Ok(Ok(r)) => r,
        Ok(Err(err)) => {
            push_error(state, err);
            ffi::lua_error(state)
        }
        Err(p) => {
            push_panic(state, p);
            ffi::lua_error(state)
        }
    }
}

// Pushes a WrappedError::Error to the top of the stack
pub unsafe fn push_error(state: *mut ffi::lua_State, err: LuaError) {
    push_wrapped_error(state, WrappedError::Error(err));
}

// Pushes a WrappedError::Panic to the top of the stack
pub unsafe fn push_panic(state: *mut ffi::lua_State, panic: Box<Any + Send>) {
    push_wrapped_error(state, WrappedError::Panic(Some(panic)));
}

// Pops a WrappedError off of the top of the stack, if it is a WrappedError::Error, returns it, if
// it is a WrappedError::Panic, clears the current stack and panics.
pub unsafe fn pop_error(state: *mut ffi::lua_State) -> LuaError {
    assert!(
        ffi::lua_gettop(state) > 0,
        "pop_error called with nothing on the stack"
    );

    // Pop an error off of the lua stack and interpret it as a wrapped error, or as a string.  If
    // the error cannot be interpreted as a string simply print that it was an unprintable error.

    if is_wrapped_error(state, -1) {
        let userdata = ffi::lua_touserdata(state, -1);
        match &mut *(userdata as *mut WrappedError) {
            &mut WrappedError::Error(ref err) => {
                let err = err.clone();
                ffi::lua_pop(state, 1);
                err
            }
            &mut WrappedError::Panic(ref mut p) => {
                let p = p.take().unwrap_or_else(|| {
                    Box::new("internal error: panic error used twice")
                });
                ffi::lua_settop(state, 0);
                resume_unwind(p)
            }
        }

    } else if let Some(s) = ffi::lua_tolstring(state, -1, ptr::null_mut()).as_ref() {
        let error = CStr::from_ptr(s).to_str().unwrap().to_owned();
        ffi::lua_pop(state, 1);
        // This seems terrible, but as far as I can tell, this is exactly what the stock lua
        // repl does.
        if error.ends_with("<eof>") {
            LuaError::IncompleteStatement(error).into()
        } else {
            LuaError::ScriptError(error).into()
        }

    } else {
        ffi::lua_pop(state, 1);
        LuaError::ScriptError("<unprintable error>".to_owned()).into()
    }
}

// ffi::lua_pcall with a message handler that gives a nice traceback.  If the
// caught error is actually a LuaError, will simply pass the error along.  Does
// not call checkstack, and uses 2 extra stack spaces.
pub unsafe fn pcall_with_traceback(
    state: *mut ffi::lua_State,
    nargs: c_int,
    nresults: c_int,
) -> c_int {
    unsafe extern "C" fn message_handler(state: *mut ffi::lua_State) -> c_int {
        if is_wrapped_error(state, 1) {
            if !is_panic_error(state, 1) {
                let error = pop_error(state);
                ffi::luaL_traceback(state, state, ptr::null(), 0);
                let traceback = CStr::from_ptr(ffi::lua_tolstring(state, -1, ptr::null_mut()))
                    .to_str()
                    .unwrap()
                    .to_owned();
                push_error(state, LuaError::CallbackError(traceback, Arc::new(error)));
            }
        } else {
            let s = ffi::lua_tolstring(state, 1, ptr::null_mut());
            if !s.is_null() {
                ffi::luaL_traceback(state, state, s, 0);
            } else {
                ffi::luaL_traceback(state, state, cstr!("<unprintable lua error>"), 0);
            }
        }
        1
    }

    let msgh_position = ffi::lua_gettop(state) - nargs;
    ffi::lua_pushcfunction(state, message_handler);
    ffi::lua_insert(state, msgh_position);
    let ret = ffi::lua_pcall(state, nargs, nresults, msgh_position);
    ffi::lua_remove(state, msgh_position);
    ret
}

pub unsafe fn resume_with_traceback(
    state: *mut ffi::lua_State,
    from: *mut ffi::lua_State,
    nargs: c_int,
) -> c_int {
    let res = ffi::lua_resume(state, from, nargs);
    if res != ffi::LUA_OK && res != ffi::LUA_YIELD {
        if is_wrapped_error(state, 1) {
            if !is_panic_error(state, 1) {
                let error = pop_error(state);
                ffi::luaL_traceback(from, state, ptr::null(), 0);
                let traceback = CStr::from_ptr(ffi::lua_tolstring(from, -1, ptr::null_mut()))
                    .to_str()
                    .unwrap()
                    .to_owned();
                push_error(from, LuaError::CallbackError(traceback, Arc::new(error)));
            }
        } else {
            let s = ffi::lua_tolstring(state, 1, ptr::null_mut());
            if !s.is_null() {
                ffi::luaL_traceback(from, state, s, 0);
            } else {
                ffi::luaL_traceback(from, state, cstr!("<unprintable lua error>"), 0);
            }
        }
    }
    res
}

// A variant of pcall that does not allow lua to catch panic errors from callback_error
pub unsafe extern "C" fn safe_pcall(state: *mut ffi::lua_State) -> c_int {
    if ffi::lua_pcall(state, ffi::lua_gettop(state) - 1, ffi::LUA_MULTRET, 0) != ffi::LUA_OK {
        if is_panic_error(state, -1) {
            ffi::lua_error(state);
        }
    }
    ffi::lua_gettop(state)
}

// A variant of xpcall that does not allow lua to catch panic errors from callback_error
pub unsafe extern "C" fn safe_xpcall(state: *mut ffi::lua_State) -> c_int {
    unsafe extern "C" fn xpcall_msgh(state: *mut ffi::lua_State) -> c_int {
        if is_panic_error(state, -1) {
            1
        } else {
            ffi::lua_pushvalue(state, ffi::lua_upvalueindex(1));
            ffi::lua_insert(state, 1);
            ffi::lua_call(state, ffi::lua_gettop(state) - 1, ffi::LUA_MULTRET);
            ffi::lua_gettop(state)
        }
    }

    ffi::lua_pushvalue(state, 2);
    ffi::lua_pushcclosure(state, xpcall_msgh, 1);
    ffi::lua_copy(state, 1, 2);
    ffi::lua_insert(state, 1);

    let res = ffi::lua_pcall(state, ffi::lua_gettop(state) - 2, ffi::LUA_MULTRET, 1);
    if res != ffi::LUA_OK {
        if is_panic_error(state, -1) {
            ffi::lua_error(state);
        }
    }
    res
}

/// Does not call checkstack, uses 1 stack space
pub unsafe fn main_state(state: *mut ffi::lua_State) -> *mut ffi::lua_State {
    ffi::lua_rawgeti(state, ffi::LUA_REGISTRYINDEX, ffi::LUA_RIDX_MAINTHREAD);
    let state = ffi::lua_tothread(state, -1);
    ffi::lua_pop(state, 1);
    state
}

static ERROR_METATABLE_REGISTRY_KEY: u8 = 0;

enum WrappedError {
    Error(LuaError),
    Panic(Option<Box<Any + Send>>),
}

// Pushes the given error or panic as a wrapped error onto the stack
unsafe fn push_wrapped_error(state: *mut ffi::lua_State, err: WrappedError) {
    // Wrapped errors have a __tostring metamethod and a 'backtrace' normal
    // method.

    unsafe extern "C" fn error_tostring(state: *mut ffi::lua_State) -> c_int {
        callback_error(state, || {
            if !is_wrapped_error(state, -1) {
                return Err(LuaError::ConversionError(
                    "not WrappedError in error method".to_owned(),
                ));
            }

            let userdata = ffi::lua_touserdata(state, -1);
            match &*(userdata as *const WrappedError) {
                &WrappedError::Error(ref error) => {
                    push_string(state, &error.to_string());
                    ffi::lua_remove(state, -2);
                }
                &WrappedError::Panic(_) => {
                    // This should be impossible, there should be no way for lua
                    // to catch a panic error.
                    push_string(state, "panic error");
                    ffi::lua_remove(state, -2);
                }
            }

            Ok(1)
        })
    }

    ffi::luaL_checkstack(state, 2, ptr::null());

    let err_userdata = ffi::lua_newuserdata(state, mem::size_of::<WrappedError>()) as
        *mut WrappedError;

    ptr::write(err_userdata, err);

    get_error_metatable(state);
    if ffi::lua_isnil(state, -1) != 0 {
        ffi::lua_pop(state, 1);

        ffi::luaL_checkstack(state, 7, ptr::null());

        ffi::lua_newtable(state);
        ffi::lua_pushlightuserdata(
            state,
            &ERROR_METATABLE_REGISTRY_KEY as *const u8 as *mut c_void,
        );
        ffi::lua_pushvalue(state, -2);

        push_string(state, "__gc");
        ffi::lua_pushcfunction(state, destructor::<WrappedError>);
        ffi::lua_settable(state, -3);

        push_string(state, "__tostring");
        ffi::lua_pushcfunction(state, error_tostring);
        ffi::lua_settable(state, -3);

        push_string(state, "__metatable");
        ffi::lua_pushboolean(state, 0);
        ffi::lua_settable(state, -3);

        ffi::lua_settable(state, ffi::LUA_REGISTRYINDEX);
    }

    ffi::lua_setmetatable(state, -2);
}

// Checks if the error at the given index is a WrappedError::Panic
unsafe fn is_panic_error(state: *mut ffi::lua_State, index: c_int) -> bool {
    assert_ne!(
        ffi::lua_checkstack(state, 2),
        0,
        "somehow not enough stack space to check if an error is panic"
    );

    let index = ffi::lua_absindex(state, index);

    let userdata = ffi::lua_touserdata(state, index);
    if userdata.is_null() {
        return false;
    }

    if ffi::lua_getmetatable(state, index) == 0 {
        return false;
    }

    get_error_metatable(state);
    if ffi::lua_rawequal(state, -1, -2) == 0 {
        ffi::lua_pop(state, 2);
        return false;
    }

    ffi::lua_pop(state, 2);

    match &*(userdata as *const WrappedError) {
        &WrappedError::Error(_) => false,
        &WrappedError::Panic(_) => true,
    }
}

unsafe fn is_wrapped_error(state: *mut ffi::lua_State, index: c_int) -> bool {
    assert_ne!(
        ffi::lua_checkstack(state, 2),
        0,
        "somehow not enough stack space to check if an error is rrapped"
    );

    let index = ffi::lua_absindex(state, index);

    let userdata = ffi::lua_touserdata(state, index);
    if userdata.is_null() {
        return false;
    }

    if ffi::lua_getmetatable(state, index) == 0 {
        return false;
    }

    get_error_metatable(state);
    let res = ffi::lua_rawequal(state, -1, -2) != 0;
    ffi::lua_pop(state, 2);
    res
}

unsafe fn get_error_metatable(state: *mut ffi::lua_State) -> c_int {
    ffi::lua_pushlightuserdata(
        state,
        &ERROR_METATABLE_REGISTRY_KEY as *const u8 as *mut c_void,
    );
    ffi::lua_gettable(state, ffi::LUA_REGISTRYINDEX)
}