path: root/src/lane.cpp

/*
===============================================================================

Copyright (C) 2024 Benoit Germain <bnt.germain@gmail.com>

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

===============================================================================
*/
#include "_pch.hpp"
#include "lane.hpp"

#include "debugspew.hpp"
#include "intercopycontext.hpp"
#include "threading.hpp"
#include "tools.hpp"

// #################################################################################################

// xxh64 of string "error" generated at https://www.pelock.com/products/hash-calculator
static constexpr UniqueKey kCachedError{ 0xD6F35DD608D0A203ull };
// xxh64 of string "tostring" generated at https://www.pelock.com/products/hash-calculator
static constexpr UniqueKey kCachedTostring{ 0xAB5EA23BCEA0C35Cull };

// #################################################################################################
// #################################################################################################
// ######################################### Lua API ###############################################
// #################################################################################################
// #################################################################################################

// lane:get_threadname()
static LUAG_FUNC(lane_get_threadname)
{
    Lane* const _lane{ ToLane(L_, StackIndex{ 1 }) };
    luaL_argcheck(L_, lua_gettop(L_) == 1, 2, "too many arguments");
    luaG_pushstring(L_, _lane->getDebugName());
    return 1;
}

// #################################################################################################

// void= finalizer( finalizer_func )
//
// finalizer_func( [err, stack_tbl] )
//
// Add a function that will be called when exiting the lane, either via
// normal return or an error.
//
static LUAG_FUNC(set_finalizer)
{
    luaL_argcheck(L_, lua_isfunction(L_, 1), 1, "finalizer should be a function");
    luaL_argcheck(L_, lua_gettop(L_) == 1, 1, "too many arguments");
    STACK_GROW(L_, 3);
    // Get the current finalizer table (if any), create one if it doesn't exist
    std::ignore = kFinalizerRegKey.getSubTable(L_, NArr{ 1 }, NRec{ 0 });                          // L_: finalizer {finalisers}
    // must cast to int, not lua_Integer, because LuaJIT signature of lua_rawseti is not the same as PUC-Lua.
    int const _idx{ static_cast<int>(lua_rawlen(L_, kIdxTop) + 1) };
    lua_pushvalue(L_, 1);                                                                          // L_: finalizer {finalisers} finalizer
    lua_rawseti(L_, -2, _idx);                                                                     // L_: finalizer {finalisers}
    // no need to adjust the stack, Lua does this for us
    return 0;
}

// #################################################################################################

// serves both to read and write the name from the inside of the lane
// upvalue #1 is the lane userdata
// this function is exported in a lane's state, therefore it is callable only from inside the Lane's state
static LUAG_FUNC(lane_threadname)
{
    // C s_lane structure is a light userdata upvalue
    Lane* const _lane{ luaG_tolightuserdata<Lane>(L_, StackIndex{ lua_upvalueindex(1) }) };
    LUA_ASSERT(L_, L_ == _lane->L); // this function is exported in a lane's state, therefore it is callable only from inside the Lane's state
    if (lua_gettop(L_) == 1) {
        lua_settop(L_, 1);
        STACK_CHECK_START_REL(L_, 0);
        _lane->storeDebugName(luaG_tostring(L_, kIdxTop));
        _lane->applyDebugName();
        STACK_CHECK(L_, 0);
        return 0;
    } else if (lua_gettop(L_) == 0) {
        luaG_pushstring(L_, _lane->getDebugName());
        return 1;
    } else {
        raise_luaL_error(L_, "Wrong number of arguments");
    }
}

// #################################################################################################

//---
// [...] | [nil, err_any, stack_tbl]= lane:join([wait_secs])
//
//  timeout:   returns nil
//  done:      returns return values (0..N)
//  error:     returns nil + error value [+ stack table]
//  cancelled: returns nil
//
static LUAG_FUNC(lane_join)
{
    Lane* const _lane{ ToLane(L_, StackIndex{ 1 }) };

    std::chrono::time_point<std::chrono::steady_clock> _until{ std::chrono::time_point<std::chrono::steady_clock>::max() };
    if (luaG_type(L_, StackIndex{ 2 }) == LuaType::NUMBER) { // we don't want to use lua_isnumber() because of autocoercion
        lua_Duration const duration{ lua_tonumber(L_, 2) };
        if (duration.count() >= 0.0) {
            _until = std::chrono::steady_clock::now() + std::chrono::duration_cast<std::chrono::steady_clock::duration>(duration);
        } else {
            raise_luaL_argerror(L_, StackIndex{ 2 }, "duration cannot be < 0");
        }

    } else if (!lua_isnoneornil(L_, 2)) {
        raise_luaL_argerror(L_, StackIndex{ 2 }, "incorrect duration type");
    }

    lua_settop(L_, 1);                                                                             // L_: lane
    bool const _done{ !_lane->thread.joinable() || _lane->waitForCompletion(_until) };

    if (!_done) {
        lua_pushnil(L_);                                                                           // L_: lane nil
        luaG_pushstring(L_, "timeout");                                                            // L_: lane nil "timeout"
        return 2;
    }

    STACK_CHECK_START_REL(L_, 0);                                                                  // L_: lane
    // Thread is Suspended or Done/Error/Cancelled; the Lane thread isn't working with it, therefore we can.

    int _ret{ 0 };
    int const _stored{ _lane->storeResults(L_) };
    STACK_GROW(L_, std::max(3, _stored + 1));
    switch (_lane->status.load(std::memory_order_acquire)) {
    case Lane::Suspended: // got yielded values
    case Lane::Done: // got regular return values
        {
            if (_stored == 0) {
                raise_luaL_error(L_, _lane->L ? "First return value must be non-nil when using join()" : "Can't join() more than once or after indexing");
            }
            lua_getiuservalue(L_, StackIndex{ 1 }, UserValueIndex{ 1 });                           // L_: lane {uv}
            for (int _i = 2; _i <= _stored; ++_i) {
                lua_rawgeti(L_, 2, _i);                                                            // L_: lane {uv} results2...N
            }
            lua_rawgeti(L_, 2, 1);                                                                 // L_: lane {uv} results2...N result1
            lua_replace(L_, 2);                                                                    // L_: lane results
            _ret = _stored;
        }
        break;

    case Lane::Error:
        {
            LUA_ASSERT(L_, _stored == 2 || _stored == 3);
            lua_getiuservalue(L_, StackIndex{ 1 }, UserValueIndex{ 1 });                           // L_: lane {uv}
            lua_rawgeti(L_, 2, 2);                                                                 // L_: lane {uv} <error>
            lua_rawgeti(L_, 2, 3);                                                                 // L_: lane {uv} <error> <trace>|nil
            if (lua_isnil(L_, -1)) {
                lua_replace(L_, 2);                                                                // L_: lane nil <error>
            } else {
                lua_rawgeti(L_, 2, 1);                                                             // L_: lane {uv} <error> <trace> nil
                lua_replace(L_, 2);                                                                // L_: lane nil <error> <trace>
            }
            _ret = lua_gettop(L_) - 1; // 2 or 3
        }
        break;

    case Lane::Cancelled:
        LUA_ASSERT(L_, _stored == 2);
        lua_getiuservalue(L_, StackIndex{ 1 }, UserValueIndex{ 1 });                              // L_: lane {uv}
        lua_rawgeti(L_, 2, 2);                                                                    // L_: lane {uv} cancel_error
        lua_rawgeti(L_, 2, 1);                                                                    // L_: lane {uv} cancel_error nil
        lua_replace(L_, -3);                                                                      // L_: lane nil cancel_error
        LUA_ASSERT(L_, lua_isnil(L_, -2) && kCancelError.equals(L_, kIdxTop));
        _ret = 2;
        break;

    default:
        DEBUGSPEW_CODE(DebugSpew(nullptr) << "Unknown Lane status: " << static_cast<int>(_lane->status.load(std::memory_order_relaxed)) << std::endl);
        LUA_ASSERT(L_, false);
        _ret = 0;
    }
    STACK_CHECK(L_, _ret);
    return _ret;
}

// #################################################################################################

LUAG_FUNC(lane_resume)
{
    static constexpr StackIndex kIdxSelf{ 1 };
    Lane* const _lane{ ToLane(L_, kIdxSelf) };
    lua_State* const _L2{ _lane->L };

    // wait until the lane yields
    std::optional<Lane::Status> _hadToWait{}; // for debugging, if we ever raise the error just below
    {
        std::unique_lock _guard{ _lane->doneMutex };
        Lane::Status const _status{ _lane->status.load(std::memory_order_acquire) };
        if (_status == Lane::Pending || _status == Lane::Running || _status == Lane::Resuming) {
            _hadToWait = _status;
            _lane->doneCondVar.wait(_guard, [_lane]() { return _lane->status.load(std::memory_order_acquire) == Lane::Suspended; });
        }
    }
    if (_lane->status.load(std::memory_order_acquire) != Lane::Suspended) {
        if (_hadToWait) {
            raise_luaL_error(L_, "INTERNAL ERROR: Lane status is %s instead of 'suspended'", _lane->threadStatusString().data());
        } else {
            raise_luaL_error(L_, "Can't resume a non-suspended coroutine-type Lane");
        }
    }
    int const _nargs{ lua_gettop(L_) - 1 };
    int const _nresults{ lua_gettop(_L2) };
    STACK_CHECK_START_ABS(L_, 1 + _nargs);                                                         // L_: self args...                               _L2: results...
    STACK_CHECK_START_ABS(_L2, _nresults);

    // clear any fetched returned values that we might have stored previously
    _lane->resetResultsStorage(L_, kIdxSelf);

    // to retrieve the yielded value of the coroutine on our stack
    InterCopyContext _cin{ _lane->U, DestState{ L_ }, SourceState{ _L2 }, {}, {}, {}, {}, {} };
    if (_cin.interMove(_nresults) != InterCopyResult::Success) {                                   // L_: self args... results...                    _L2:
        raise_luaL_error(L_, "Failed to retrieve yielded values");
    }

    // to send our args on the coroutine stack
    InterCopyContext _cout{ _lane->U, DestState{ _L2 }, SourceState{ L_ }, {}, SourceIndex{ 2 }, {}, {}, {} };
    if (_cout.interCopy(_nargs) != InterCopyResult::Success) {                                     // L_: self args... results...                    _L2: args...
        raise_luaL_error(L_, "Failed to send resumed values");
    }

    STACK_CHECK(_L2, _nargs); // we should have removed everything from the lane's stack, and pushed our args
    STACK_CHECK(L_, 1 + _nargs + _nresults); // and the results of the coroutine are on top here
    std::unique_lock _guard{ _lane->doneMutex };
    _lane->status.store(Lane::Resuming, std::memory_order_release);
    _lane->doneCondVar.notify_one();
    return _nresults;
}

// #################################################################################################

// key is numeric, wait until the thread returns and populate the environment with the return values
// If the return values signal an error, propagate it
// Else If key is found in the environment, return it
static int lane_index_number(lua_State* L_)
{
    static constexpr StackIndex kIdxSelf{ 1 };

    Lane* const _lane{ ToLane(L_, kIdxSelf) };
    LUA_ASSERT(L_, lua_gettop(L_) == 2);                                                           // L_: lane n
    int const _key{ static_cast<int>(lua_tointeger(L_, 2)) };
    lua_pop(L_, 1);                                                                                // L_: lane

    std::chrono::time_point<std::chrono::steady_clock> _until{ std::chrono::time_point<std::chrono::steady_clock>::max() };
    if (!_lane->waitForCompletion(_until)) {
        raise_luaL_error(L_, "INTERNAL ERROR: Failed to join");
    }

    // make sure results are stored
    int const _stored{ _lane->storeResults(L_) };
    if (_key > _stored) {
        // get nil if indexing beyond the actual returned value count
        lua_pushnil(L_);                                                                           // L_: lane nil
    } else {
        _lane->pushIndexedResult(L_, _key);                                                        // L_: lane result
    }
    return 1;
}

// #################################################################################################

// If key is "status" return the thread status
// If key is found in the environment, return it
// Else raise an error
static int lane_index_string(lua_State* L_)
{
    static constexpr StackIndex kIdxSelf{ 1 };
    static constexpr StackIndex kIdxKey{ 2 };

    Lane* const _lane{ ToLane(L_, kIdxSelf) };
    LUA_ASSERT(L_, lua_gettop(L_) == 2);                                                           // L_: lane "key"

    std::string_view const _keystr{ luaG_tostring(L_, kIdxKey) };
    lua_settop(L_, 2); // keep only our original arguments on the stack

    // look in metatable first
    lua_getmetatable(L_, kIdxSelf);                                                                // L_: lane "key" mt
    lua_replace(L_, -3);                                                                           // L_: mt "key"
    if (luaG_rawget(L_, StackIndex{ -2 }) != LuaType::NIL) { // found something?                   // L_: mt value
        return 1; // done
    }

    lua_pop(L_, 2);                                                                                // L_:
    if (_keystr == "status") {
        _lane->pushStatusString(L_);                                                               // L_: lane "key" "<status>"
        return 1;
    }
    if (_keystr == "error_trace_level") {
        std::ignore = _lane->pushErrorTraceLevel(L_);                                              // L_: lane "key" "<level>"
        return 1;
    }
    raise_luaL_error(L_, "unknown field '%s'", _keystr.data());
}

// #################################################################################################

// lane:__index(key,usr) -> value
static LUAG_FUNC(lane_index)
{
    static constexpr StackIndex kIdxSelf{ 1 };
    static constexpr StackIndex kKey{ 2 };
    Lane* const _lane{ ToLane(L_, kIdxSelf) };
    LUA_ASSERT(L_, lua_gettop(L_) == 2);

    switch (luaG_type(L_, kKey)) {
    case LuaType::NUMBER:
        return lane_index_number(L_); // stack modification is undefined, returned value is at the top

    case LuaType::STRING:
        return lane_index_string(L_); // stack modification is undefined, returned value is at the top

    default: // unknown key
        lua_getmetatable(L_, kIdxSelf);                                                            // L_: mt
        kCachedError.pushKey(L_);                                                                  // L_: mt kCachedError
        if (luaG_rawget(L_, StackIndex{ -2 }) != LuaType::FUNCTION) {                              // L_: mt error()
            raise_luaL_error(L_, "INTERNAL ERROR: cached error() is a %s, not a function", luaG_typename(L_, kIdxTop).data());
        }
        luaG_pushstring(L_, "Unknown key: ");                                                      // L_: mt error() "Unknown key: "
        kCachedTostring.pushKey(L_);                                                               // L_: mt error() "Unknown key: " kCachedTostring
        if (luaG_rawget(L_, StackIndex{ -4 }) != LuaType::FUNCTION) {                              // L_: mt error() "Unknown key: " tostring()
            raise_luaL_error(L_, "INTERNAL ERROR: cached tostring() is a %s, not a function", luaG_typename(L_, kIdxTop).data());
        }
        lua_pushvalue(L_, kKey);                                                                   // L_: mt error() "Unknown key: " tostring() k
        lua_call(L_, 1, 1);                                                                        // L_: mt error() "Unknown key: " "k"
        lua_concat(L_, 2);                                                                         // L_: mt error() "Unknown key: <k>"
        lua_call(L_, 1, 0); // error( "Unknown key: " .. key) -> doesn't return                    // L_: mt
        raise_luaL_error(L_, "%s[%s]: should not get here!", _lane->getDebugName().data(), luaG_typename(L_, kKey).data());
    }
}

// #################################################################################################
// ######################################## Utilities ##############################################
// #################################################################################################

#if USE_DEBUG_SPEW()
namespace {
    // can't use direct LUA_x errcode indexing because the sequence is not the same between Lua 5.1 and 5.2 :-(
    // LUA_ERRERR doesn't have the same value
    struct errcode_name
    {
        LuaError code;
        std::string_view const name;
    };

    namespace local {
        static struct errcode_name sErrCodes[] = {
            { LuaError::OK, "LUA_OK" },
            { LuaError::YIELD, "LUA_YIELD" },
            { LuaError::ERRRUN, "LUA_ERRRUN" },
            { LuaError::ERRSYNTAX, "LUA_ERRSYNTAX" },
            { LuaError::ERRMEM, "LUA_ERRMEM" },
            { LuaError::ERRGCMM, "LUA_ERRGCMM" },
            { LuaError::ERRERR, "LUA_ERRERR" },
            { LuaError::ERRFILE, "LUA_ERRFILE" },
        };
    } // namespace local

    static std::string_view GetErrcodeName(LuaError _code) noexcept
    {
        for (errcode_name const& _entry : local::sErrCodes) {
            if (_entry.code == _code) {
                return _entry.name;
            }
        }
        return "<nullptr>";
    }
} // namespace
#endif // USE_DEBUG_SPEW()

// #################################################################################################

/*
 * str= lane_error( error_val|str )
 *
 * Called if there's an error in some lane; add call stack to error message
 * just like 'lua.c' normally does.
 *
 * ".. will be called with the error message and its return value will be the
 *     message returned on the stack by lua_pcall."
 *
 * Note: Rather than modifying the error message itself, it would be better
 *     to provide the call stack (as string) completely separated. This would
 *     work great with non-string error values as well (current system does not).
 *     (This is NOT possible with the Lua 5.1 'lua_pcall()'; we could of course
 *     implement a Lanes-specific 'pcall' of our own that does this). TBD!!! :)
 *       --AKa 22-Jan-2009
 */

// xxh64 of string "kStackTraceRegKey" generated at https://www.pelock.com/products/hash-calculator
static constexpr RegistryUniqueKey kStackTraceRegKey{ 0x3F327747CACAA904ull };

int Lane::LuaErrorHandler(lua_State* L_)
{
    // error message (any type)
    STACK_CHECK_START_ABS(L_, 1);                                                                  // L_: some_error

    // Don't do stack survey for cancelled lanes.
    //
    if (kCancelError.equals(L_, StackIndex{ 1 })) {
        return 1; // just pass on
    }

    STACK_GROW(L_, 4); // lua_setfield consumes a stack slot, so we have to account for it
    bool const _extended{ kExtendedStackTraceRegKey.readBoolValue(L_) };
    STACK_CHECK(L_, 1);

    // Place stack trace at 'registry[kStackTraceRegKey]' for the 'lua_pcall()'
    // caller to fetch. This bypasses the Lua 5.1 limitation of only one
    // return value from error handler to 'lua_pcall()' caller.

    // It's adequate to push stack trace as a table. This gives the receiver
    // of the stack best means to format it to their liking. Also, it allows
    // us to add more stack info later, if needed.
    //
    // table of { "sourcefile.lua:<line>", ... }
    //
    lua_newtable(L_);                                                                              // L_: some_error {}
    StackIndex const kIdxTraceTbl{ luaG_absindex(L_, kIdxTop) };

    // Best to start from level 1, but in some cases it might be a C function
    // and we don't get '.currentline' for that. It's okay - just keep level
    // and table index growing separate.    --AKa 22-Jan-2009
    //
    lua_Debug _ar;
    for (int _n = 1; lua_getstack(L_, _n, &_ar); ++_n) {
        lua_getinfo(L_, _extended ? "Sln" : "Sl", &_ar);
        if (_extended) {
            lua_newtable(L_);                                                                      // L_: some_error {} {}
            StackIndex const kIdxFrameTbl{ luaG_absindex(L_, kIdxTop) };
            lua_pushstring(L_, _ar.source);                                                        // L_: some_error {} {} source
            luaG_setfield(L_, kIdxFrameTbl, std::string_view{ "source" });                         // L_: some_error {} {}

            lua_pushinteger(L_, _ar.currentline);                                                  // L_: some_error {} {} currentline
            luaG_setfield(L_, kIdxFrameTbl, std::string_view{ "currentline" });                    // L_: some_error {} {}

            lua_pushstring(L_, _ar.name ? _ar.name : "<?>");                                       // L_: some_error {} {} name
            luaG_setfield(L_, kIdxFrameTbl, std::string_view{ "name" });                           // L_: some_error {} {}

            lua_pushstring(L_, _ar.namewhat);                                                      // L_: some_error {} {} namewhat
            luaG_setfield(L_, kIdxFrameTbl, std::string_view{ "namewhat" });                       // L_: some_error {} {}

            lua_pushstring(L_, _ar.what);                                                          // L_: some_error {} {} what
            luaG_setfield(L_, kIdxFrameTbl, std::string_view{ "what" });                           // L_: some_error {} {}
        } else if (_ar.currentline > 0) {
            luaG_pushstring(L_, "%s:%d", _ar.short_src, _ar.currentline);                          // L_: some_error {} "blah:blah"
        } else {
            luaG_pushstring(L_, "%s:?", _ar.short_src);                                            // L_: some_error {} "blah"
        }
        lua_rawseti(L_, kIdxTraceTbl, static_cast<lua_Integer>(_n));                               // L_: some_error {}
    }

    // store the stack trace table in the registry
    kStackTraceRegKey.setValue(L_, [](lua_State* L_) { lua_insert(L_, -2); });                     // L_: some_error

    STACK_CHECK(L_, 1);
    return 1; // the untouched error value
}

// #################################################################################################
// ########################################## Finalizer ############################################
// #################################################################################################

[[nodiscard]]
static int PushStackTrace(lua_State* const L_, Lane::ErrorTraceLevel const errorTraceLevel_, LuaError const rc_, [[maybe_unused]] StackIndex const stk_base_)
{
    // Lua 5.1 error handler is limited to one return value; it stored the stack trace in the registry
    StackIndex const _top{ lua_gettop(L_) };
    switch (rc_) {
    case LuaError::OK: // no error, body return values are on the stack
        break;

    case LuaError::ERRRUN: // cancellation or a runtime error
        if (errorTraceLevel_ != Lane::Minimal) { // when not Minimal, we installed a handler
            STACK_CHECK_START_REL(L_, 0);
            // fetch the call stack table from the registry where the handler stored it
            STACK_GROW(L_, 1);
            // yields nil if no stack was generated (in case of cancellation for example)
            kStackTraceRegKey.pushValue(L_);                                                       // L_: err trace|nil
            STACK_CHECK(L_, 1);

            // For cancellation the error message is kCancelError, and a stack trace isn't placed
            // For other errors, the message can be whatever was thrown, and we should have a stack trace table
            LUA_ASSERT(L_, luaG_type(L_, StackIndex{ 1 + stk_base_ }) == (kCancelError.equals(L_, stk_base_) ? LuaType::NIL : LuaType::TABLE));
            // Just leaving the stack trace table on the stack is enough to get it through to the master.
        } else {
            // any kind of error can be thrown with error(), or through a lane/linda cancellation
            LUA_ASSERT(L_, lua_gettop(L_) == stk_base_);
        }
        break;

    case LuaError::ERRMEM: // memory allocation error (handler not called)
    case LuaError::ERRERR: // error while running the error handler (if any, for example an out-of-memory condition)
    default:
        // the Lua core provides a string error message in those situations
        LUA_ASSERT(L_, (lua_gettop(L_) == stk_base_) && (luaG_type(L_, stk_base_) == LuaType::STRING));
        break;
    }
    return lua_gettop(L_) - _top; // either 0 or 1
}

// #################################################################################################
//---
// Run finalizers - if any - with the given arguments
//
// If 'rc' is nonzero, error message and stack index (the latter only when errorTraceLevel_ == 1) are available as:
//      [-1]: stack trace (table)
//      [-2]: error message (any type)
//
// Returns:
//      0 if finalizers were run without error (or there were none)
//      LUA_ERRxxx return code if any of the finalizers failed
//
// TBD: should we add stack trace on failing finalizer, wouldn't be hard..
//

[[nodiscard]]
static LuaError run_finalizers(Lane* const lane_, Lane::ErrorTraceLevel const errorTraceLevel_, LuaError const lua_rc_)
{
    // if we are a coroutine, we can't run the finalizers in the coroutine state!
    lua_State* const _L{ lane_->S };
    kFinalizerRegKey.pushValue(_L);                                                                // _L: ... finalizers|nil
    if (lua_isnil(_L, -1)) {
        lua_pop(_L, 1);
        return LuaError::OK; // no finalizers
    }

    STACK_GROW(_L, 5);

    StackIndex const _finalizers{ lua_gettop(_L) };
    // always push something as error handler, to have the same stack structure
    int const _error_handler{ (errorTraceLevel_ != Lane::Minimal)
        ? (lua_pushcfunction(_L, Lane::LuaErrorHandler), lua_gettop(_L))
        : (lua_pushnil(_L), 0)
    };                                                                                             // _L: ... finalizers error_handler()

    LuaError _rc{ LuaError::OK };
    int _finalizer_pushed{};
    for (int _n = static_cast<int>(lua_rawlen(_L, _finalizers)); _n > 0; --_n) {
        int _args{ 0 };
        lua_rawgeti(_L, _finalizers, _n);                                                          // _L: ... finalizers error_handler() finalizer()
        LUA_ASSERT(_L, lua_isfunction(_L, -1));
        if (lua_rc_ != LuaError::OK) { // we have <error>, [trace] on the thread stack
            LUA_ASSERT(_L, lane_->nresults == 1 || lane_->nresults == 2);
            //std::string_view const _err_msg{ luaG_tostring(_L, 1) };
            if (lane_->isCoroutine()) {
                // transfer them on the main state
                lua_pushvalue(lane_->L, 1);
                // note we don't always have a stack trace for example when kCancelError, or when we got an error that doesn't call our handler, such as LUA_ERRMEM
                if (lane_->nresults == 2) {
                    lua_pushvalue(lane_->L, 2);
                }
                lua_xmove(lane_->L, _L, lane_->nresults);                                          // _L: ... finalizers error_handler() finalizer <error> [trace]
            } else {
                lua_pushvalue(_L, 1);                                                              // _L: ... finalizers error_handler() finalizer <error>
                // note we don't always have a stack trace for example when kCancelError, or when we got an error that doesn't call our handler, such as LUA_ERRMEM
                if (lane_->nresults == 2) {
                    lua_pushvalue(_L, 2);                                                          // _L: ... finalizers error_handler() finalizer <error> trace
                }
            }
            _args = lane_->nresults;
        }

        // if no error from the main body, finalizer doesn't receive any argument, else it gets the error message and optional stack trace
        _rc = ToLuaError(lua_pcall(_L, _args, 0, _error_handler));                                 // _L: ... finalizers error_handler() err_msg2?
        if (_rc != LuaError::OK) {
            StackIndex const _top{ lua_gettop(_L) };
            _finalizer_pushed = 1 + PushStackTrace(_L, errorTraceLevel_, _rc, _top);               // _L: ... finalizers error_handler() err_msg2? trace
            // If one finalizer fails, don't run the others. Return this
            // as the 'real' error, replacing what we could have had (or not)
            // from the actual code.
            break;
        }
        // no error, proceed to next finalizer                                                     // _L: ... finalizers error_handler()
    }

    if (_rc != LuaError::OK) {
        lane_->nresults = _finalizer_pushed;
        if (lane_->isCoroutine()) {                                                                // _L: ... finalizers error_handler() <error2> trace2
            // put them back in the thread state stack, as if the finalizer was run from there
            lua_settop(lane_->L, 0);
            lua_xmove(_L, lane_->L, _finalizer_pushed);                                            // _L: ... finalizers error_handler()             L: <error2> [trace2]
            lua_pop(_L, 2);                                                                        // _L: ...                                        L: <error2> [trace2]
        } else {                                                                                   // _L: ... finalizers error_handler() <error2> [trace2]
            // adjust the stack to keep only the error data from the finalizer
            for (int const _n : std::ranges::reverse_view{ std::ranges::iota_view{ 1, _finalizer_pushed + 1 } }) {
                lua_replace(_L, _n);
            }
            lua_settop(_L, _finalizer_pushed);                                                     // _L: <error2> [trace2]
        }
    } else { // no error from the finalizers, make sure only the original return values from the lane body remain on the stack
        lua_settop(_L, _finalizers - 1);
    }

    if (lane_->isCoroutine()) {
        // only the coroutine thread should remain on the master state when we are done
        LUA_ASSERT(_L, lua_gettop(_L) == 1 && luaG_type(_L, StackIndex{ 1 }) == LuaType::THREAD);
    }

    return _rc;
}

// #################################################################################################

/*
 * Add the lane to selfdestruct chain; the ones still running at the end of the
 * whole process will be cancelled.
 */
void Lane::selfdestructAdd()
{
    std::lock_guard<std::mutex> _guard{ U->selfdestructMutex };
    assert(selfdestruct_next == nullptr);

    selfdestruct_next = U->selfdestructFirst;
    assert(selfdestruct_next);

    U->selfdestructFirst = this;
}

// #################################################################################################

// A free-running lane has ended; remove it from selfdestruct chain
[[nodiscard]]
bool Lane::selfdestructRemove()
{
    bool _found{ false };
    std::lock_guard<std::mutex> _guard{ U->selfdestructMutex };
    // Make sure (within the MUTEX) that we actually are in the chain
    // still (at process exit they will remove us from chain and then
    // cancel/kill).
    //
    if (selfdestruct_next != nullptr) {
        Lane** _ref{ &U->selfdestructFirst };

        while (*_ref != SELFDESTRUCT_END) {
            if (*_ref == this) {
                *_ref = selfdestruct_next;
                selfdestruct_next = nullptr;
                // the terminal shutdown should wait until the lane is done with its lua_close()
                U->selfdestructingCount.fetch_add(1, std::memory_order_release);
                _found = true;
                break;
            }
            _ref = &((*_ref)->selfdestruct_next);
        }
        assert(_found);
    }
    return _found;
}

// #################################################################################################
// ########################################## Main #################################################
// #################################################################################################

static void PrepareLaneHelpers(Lane* const lane_)
{
    lua_State* const _L{ lane_->L };
    // Tie "set_finalizer()" to the state
    lua_pushcfunction(_L, LG_set_finalizer);
    tools::PopulateFuncLookupTable(_L, kIdxTop, "set_finalizer");
    lua_setglobal(_L, "set_finalizer");

    // Tie "lane_threadname()" to the state
    // But don't register it in the lookup database because of the Lane pointer upvalue
    lua_pushlightuserdata(_L, lane_);
    lua_pushcclosure(_L, LG_lane_threadname, 1);
    lua_setglobal(_L, "lane_threadname");

    // Tie "cancel_test()" to the state
    lua_pushcfunction(_L, LG_cancel_test);
    tools::PopulateFuncLookupTable(_L, kIdxTop, "cancel_test");
    lua_setglobal(_L, "cancel_test");
}

// #################################################################################################

static void lane_main(Lane* const lane_)
{
    // wait until the launching thread has finished preparing L
#ifndef __PROSPERO__
    lane_->ready.wait();
#else // __PROSPERO__
    while (!lane_->ready.test()) {
        std::this_thread::yield();
    }
#endif // __PROSPERO__

    lane_->applyDebugName();
    lua_State* const _L{ lane_->L };
    LuaError _rc{ LuaError::ERRRUN };
    if (lane_->status.load(std::memory_order_acquire) == Lane::Pending) { // nothing wrong happened during preparation, we can work
        // At this point, the lane function and arguments are on the stack, possibly preceded by the error handler
        int const _errorHandlerCount{ lane_->errorHandlerCount() };
        int _nargs{ lua_gettop(_L) - 1 - _errorHandlerCount };
        {
            std::unique_lock _guard{ lane_->doneMutex };
            lane_->status.store(Lane::Running, std::memory_order_release); // Pending -> Running
        }

        PrepareLaneHelpers(lane_);
        if (lane_->S == lane_->L) {                                                                // L: eh? f args...
            _rc = ToLuaError(lua_pcall(_L, _nargs, LUA_MULTRET, _errorHandlerCount));              // L: eh? retvals|err
            lane_->nresults = lua_gettop(_L) - _errorHandlerCount;
        } else {
            // S and L are different: we run as a coroutine in Lua thread L created in state S
            do {
                // starting with Lua 5.4, lua_resume can leave more stuff on the stack below the actual yielded values.
                // that's why we have lane_->nresults
                _rc = luaG_resume(_L, nullptr, _nargs, &lane_->nresults);                          // L: eh? ... retvals|err...
                if (_rc == LuaError::YIELD) {
                    // change our status to suspended, and wait until someone wants us to resume
                    std::unique_lock _guard{ lane_->doneMutex };
                    lane_->status.store(Lane::Suspended, std::memory_order_release); // Running -> Suspended
                    lane_->doneCondVar.notify_one();
                    // wait until the user wants us to resume
                    // TODO: do I update waiting_on or not, so that the lane can be woken by cancellation requests here?
                    // lane_->waiting_on = &lane_->doneCondVar;
                    lane_->doneCondVar.wait(_guard, [lane_]() { return lane_->status.load(std::memory_order_acquire) == Lane::Resuming; });
                    // here lane_->doneMutex is locked again
                    // lane_->waiting_on = nullptr;
                    lane_->status.store(Lane::Running, std::memory_order_release); // Resuming -> Running
                    // on the stack we find the values pushed by lane:resume()
                    _nargs = lua_gettop(_L);
                }
            } while (_rc == LuaError::YIELD);
            if (_rc != LuaError::OK) {                                                             // : err...
                // for some reason, in my tests with Lua 5.4, when the coroutine raises an error, I have 3 copies of it on the stack
                // or false + the error message when running Lua 5.1
                // since the rest of our code wants only the error message, let us keep only the latter.
                lane_->nresults = 1;
                lua_replace(_L, 1);                                                                // L: err...
                lua_settop(_L, 1);                                                                 // L: err
                // now we build the stack trace table if the error trace level requests it
                std::ignore = Lane::LuaErrorHandler(_L);                                           // L: err
            }
        }

        if (_errorHandlerCount) {
            lua_remove(_L, 1);                                                                     // L: retvals|error
        }

        // in case of error and if it exists, fetch stack trace from registry and push it
        lane_->nresults += PushStackTrace(_L, lane_->errorTraceLevel, _rc, StackIndex{ 1 });       // L: retvals|error [trace]

        DEBUGSPEW_CODE(DebugSpew(lane_->U) << "Lane " << _L << " body: " << GetErrcodeName(_rc) << " (" << (kCancelError.equals(_L, StackIndex{ 1 }) ? "cancelled" : luaG_typename(_L, StackIndex{ 1 })) << ")" << std::endl);
        // Call finalizers, if the script has set them up.
        // If the lane is not a coroutine, there is only a regular state, so everything is the same whether we use S or L.
        // If the lane is a coroutine, this has to be done from the master state (S), not the thread (L), because we can't lua_pcall in a thread state
        LuaError const _rc2{ run_finalizers(lane_, lane_->errorTraceLevel, _rc) };
        DEBUGSPEW_CODE(DebugSpew(lane_->U) << "Lane " << _L << " finalizer: " << GetErrcodeName(_rc2) << std::endl);
        if (_rc2 != LuaError::OK) { // Error within a finalizer!
            // the finalizer generated an error, and left its own error message [and stack trace] on the stack
            _rc = _rc2; // we're overruling the earlier script error or normal return
        }
        lane_->waiting_on = nullptr;  // just in case
        if (lane_->selfdestructRemove()) { // check and remove (under lock!)
            // We're a free-running thread and no-one is there to clean us up.
            lane_->closeState();
            lane_->U->selfdestructMutex.lock();
            // done with lua_close(), terminal shutdown sequence may proceed
            lane_->U->selfdestructingCount.fetch_sub(1, std::memory_order_release);
            lane_->U->selfdestructMutex.unlock();

            // we destroy ourselves, therefore our thread member too, from inside the thread body
            // detach so that we don't try to join, as this doesn't seem a good idea
            lane_->thread.detach();
            delete lane_;
            return;
        }
    }

    // leave results (1..top) or error message + stack trace (1..2) on the stack - master will copy them
    Lane::Status const _st{ (_rc == LuaError::OK) ? Lane::Done : kCancelError.equals(_L, StackIndex{ 1 }) ? Lane::Cancelled : Lane::Error };
    // 'doneMutex' protects the -> Done|Error|Cancelled state change, and the Running|Suspended|Resuming state change too
    std::lock_guard _guard{ lane_->doneMutex };
    lane_->status.store(_st, std::memory_order_release);
    lane_->doneCondVar.notify_one(); // wake up master (while 'lane_->doneMutex' is on)
}

// #################################################################################################

#if LUA_VERSION_NUM >= 504
static LUAG_FUNC(lane_close)
{
    [[maybe_unused]] Lane* const _lane{ ToLane(L_, StackIndex{ 1 }) };                             // L_: lane err|nil
    // drop the error if any
    lua_settop(L_, 1);                                                                             // L_: lane

    // no error if the lane body doesn't return a non-nil first value
    luaG_pushstring(L_, "close");                                                                  // L_: lane "close"
    lua_pushcclosure(L_, LG_lane_join, 1);                                                         // L_: lane join()
    lua_insert(L_, 1);                                                                             // L_: join() lane
    lua_call(L_, 1, LUA_MULTRET);                                                                  // L_: join() results
    return lua_gettop(L_);
}
#endif // LUA_VERSION_NUM >= 504

// #################################################################################################

// = lane_gc( lane_ud )
//
// Cleanup for a thread userdata. If the thread is still executing, leave it
// alive as a free-running thread (will clean up itself).
//
// * Why NOT cancel/kill a loose thread: 
//
// At least timer system uses a free-running thread, they should be handy
// and the issue of canceling/killing threads at gc is not very nice, either
// (would easily cause waits at gc cycle, which we don't want).
//
static LUAG_FUNC(lane_gc)
{
    bool _have_gc_cb{ false };
    Lane* const _lane{ ToLane(L_, StackIndex{ 1 }) };                                              // L_: ud

    // if there a gc callback?
    lua_getiuservalue(L_, StackIndex{ 1 }, UserValueIndex{ 1 });                                   // L_: ud uservalue
    kLaneGC.pushKey(L_);                                                                           // L_: ud uservalue __gc
    if (luaG_rawget(L_, StackIndex{ -2 }) != LuaType::NIL) {                                       // L_: ud uservalue gc_cb|nil
        lua_remove(L_, -2);                                                                        // L_: ud gc_cb|nil
        luaG_pushstring(L_, _lane->getDebugName());                                                // L_: ud gc_cb name
        _have_gc_cb = true;
    } else {
        lua_pop(L_, 2);                                                                            // L_: ud
    }

    // We can read 'lane->status' without locks, but not wait for it
    if (_lane->status.load(std::memory_order_acquire) < Lane::Done) {
        // still running: will have to be cleaned up later
        _lane->selfdestructAdd();
        if (_have_gc_cb) {
            luaG_pushstring(L_, "selfdestruct");                                                   // L_: ud gc_cb name status
            lua_call(L_, 2, 0);                                                                    // L_: ud
        }
        return 0;
    } else if (_lane->L) {
        // no longer accessing the Lua VM: we can close right now
        _lane->securizeDebugName(L_);
        _lane->closeState();
    }

    // Clean up after a (finished) thread
    delete _lane;

    // do this after lane cleanup in case the callback triggers an error
    if (_have_gc_cb) {
        luaG_pushstring(L_, "closed");                                                             // L_: ud gc_cb name status
        lua_call(L_, 2, 0);                                                                        // L_: ud
    }
    return 0;
}

// #################################################################################################
// #################################### Lane implementation ########################################
// #################################################################################################

Lane::Lane(Universe* const U_, lua_State* const L_, ErrorTraceLevel const errorTraceLevel_, bool const asCoroutine_)
: U{ U_ }
, S{ L_ }
, L{ L_ }
, errorTraceLevel{ errorTraceLevel_ }
{
    STACK_CHECK_START_REL(S, 0);
    assert(errorTraceLevel == ErrorTraceLevel::Minimal || errorTraceLevel == ErrorTraceLevel::Basic || errorTraceLevel == ErrorTraceLevel::Extended);
    kExtendedStackTraceRegKey.setValue(S, [yes = errorTraceLevel == ErrorTraceLevel::Extended ? 1 : 0](lua_State* L_) { lua_pushboolean(L_, yes); });
    U->tracker.tracking_add(this);
    if (asCoroutine_) {
        L = lua_newthread(S);                                                                      // S: thread
        //kCoroutineRegKey.setValue(S, [](lua_State* const L_) { lua_insert(L_, -2); });             // S:
    }
    STACK_CHECK(S, asCoroutine_ ? 1 : 0);
}

// #################################################################################################

Lane::~Lane()
{
    // not necessary when using a jthread
    if (thread.joinable()) {
        thread.join();
    }
    // no longer tracked
    std::ignore = U->tracker.tracking_remove(this);
}

// #################################################################################################

void Lane::applyDebugName() const
{
    if constexpr (HAVE_DECODA_SUPPORT()) {
        // to see VM name in Decoda debugger Virtual Machine window
        luaG_pushstring(L, debugName);                                                             // L: ... "name"
        lua_setglobal(L, "decoda_name");                                                           // L: ...
    }
    // and finally set the OS thread name
    THREAD_SETNAME(debugName);
}

// #################################################################################################

[[nodiscard]]
CancelResult Lane::cancel(CancelOp const op_, std::chrono::time_point<std::chrono::steady_clock> const until_, WakeLane const wakeLane_, int const hookCount_)
{
    // this is a hook installed with lua_sethook: can't capture anything to be convertible to lua_Hook
    static constexpr lua_Hook _cancelHook{
        +[](lua_State* const L_, [[maybe_unused]] lua_Debug* const ar_) {
            DEBUGSPEW_CODE(DebugSpew(nullptr) << "cancel_hook" << std::endl);
            if (CheckCancelRequest(L_) != CancelRequest::None) {
                lua_sethook(L_, nullptr, 0, 0);
                raise_cancel_error(L_);
            }
        }
    };

    // remember that lanes are not transferable: only one thread can cancel a lane, so no multithreading issue here
    // We can read status without locks, but not wait for it (if Posix no PTHREAD_TIMEDJOIN)
    if (status.load(std::memory_order_acquire) >= Lane::Done) {
        // say "ok" by default, including when lane is already done
        return CancelResult::Cancelled;
    }

    // signal the linda to wake up the thread so that it can react to the cancel query
    // let us hope we never land here with a pointer on a linda that has been destroyed...
    if (op_.mode == CancelRequest::Soft) {
        return internalCancel(CancelRequest::Soft, until_, wakeLane_);
    } else if (op_.hookMask != LuaHookMask::None) {
        lua_sethook(L, _cancelHook, static_cast<int>(op_.hookMask), hookCount_);
        // TODO: maybe we should wake the lane here too, because the hook won't do much if the lane is blocked on a linda
    }

    return internalCancel(CancelRequest::Hard, until_, wakeLane_);
}

// #################################################################################################

[[nodiscard]]
CancelResult Lane::internalCancel(CancelRequest const rq_, std::chrono::time_point<std::chrono::steady_clock> const until_, WakeLane const wakeLane_)
{
    cancelRequest.store(rq_, std::memory_order_relaxed); // it's now signaled to stop
    if (rq_ == CancelRequest::Hard) {
        // lane_->thread.get_stop_source().request_stop();
    }
    if (wakeLane_ == WakeLane::Yes) { // wake the thread so that execution returns from any pending linda operation if desired
        if (status.load(std::memory_order_acquire) == Lane::Waiting) { // waiting_on is updated under control of status acquire/release semantics
            if (std::condition_variable* const _waiting_on{ waiting_on }) {
                _waiting_on->notify_all();
            }
        }
    }
    // wait until the lane stops working with its state (either Suspended or Done+)
    CancelResult const result{ waitForCompletion(until_) ? CancelResult::Cancelled : CancelResult::Timeout };
    return result;
}

// #################################################################################################

//---
// str= thread_status( lane )
//
// Returns: "pending"   not started yet
//          -> "running"   started, doing its work..
//             <-> "waiting"   blocked in a receive()
//                -> "done"     finished, results are there
//                   / "error"     finished at an error, error value is there
//                   / "cancelled"   execution cancelled by M (state gone)
//
[[nodiscard]]
std::string_view Lane::errorTraceLevelString() const
{
    std::string_view const _str{
        (errorTraceLevel == ErrorTraceLevel::Minimal) ? "minimal" :
        (errorTraceLevel == ErrorTraceLevel::Basic) ? "basic" :
        (errorTraceLevel == ErrorTraceLevel::Extended) ? "extended" :
        ""
    };
    return _str;
}

// #################################################################################################

namespace {
    namespace local {
        static struct luaL_Reg const sLaneFunctions[] = {
#if LUA_VERSION_NUM >= 504
            { "__close", LG_lane_close },
#endif // LUA_VERSION_NUM >= 504
            { "__gc", LG_lane_gc },
            { "__index", LG_lane_index },
            { "cancel", LG_lane_cancel },
            { "get_threadname", LG_lane_get_threadname },
            { "join", LG_lane_join },
            { "resume", LG_lane_resume },
            { nullptr, nullptr }
        };
    } // namespace local
} // namespace

  // contains keys: { __close, __gc, __index, kCachedError, kCachedTostring, cancel, get_threadname, join }
void Lane::PushMetatable(lua_State* const L_)
{
    STACK_CHECK_START_REL(L_, 0);
    if (luaL_newmetatable(L_, kLaneMetatableName.data())) {                                        // L_: mt
        luaG_registerlibfuncs(L_, local::sLaneFunctions);
        // cache error() and tostring()
        kCachedError.pushKey(L_);                                                                  // L_: mt kCachedError
        lua_getglobal(L_, "error");                                                                // L_: mt kCachedError error()
        lua_rawset(L_, -3);                                                                        // L_: mt
        kCachedTostring.pushKey(L_);                                                               // L_: mt kCachedTostring
        lua_getglobal(L_, "tostring");                                                             // L_: mt kCachedTostring tostring()
        lua_rawset(L_, -3);                                                                        // L_: mt
        // hide the actual metatable from getmetatable()
        luaG_pushstring(L_, kLaneMetatableName);                                                   // L_: mt "Lane"
        lua_setfield(L_, -2, "__metatable");                                                       // L_: mt
    }
    STACK_CHECK(L_, 1);
}

// #################################################################################################

void Lane::pushStatusString(lua_State* const L_) const
{
    std::string_view const _str{ threadStatusString() };
    LUA_ASSERT(L_, !_str.empty());

    luaG_pushstring(L_, _str);
}

// #################################################################################################

void Lane::pushIndexedResult(lua_State* const L_, int const key_) const
{
    static constexpr StackIndex kIdxSelf{ 1 };
    LUA_ASSERT(L_, ToLane(L_, kIdxSelf) == this);                                                  // L_: lane ...
    STACK_GROW(L_, 3);

    lua_getiuservalue(L_, kIdxSelf, UserValueIndex{ 1 });                                          // L_: lane ... {uv}
    if (status.load(std::memory_order_acquire) != Lane::Error) {
        lua_rawgeti(L_, -1, key_);                                                                 // L_: lane ... {uv} uv[i]
        lua_remove(L_, -2);                                                                        // L_: lane ... uv[i]
        return;
    }

    // [1] = nil [2] = <error> [3] = [stack_trace]                                                 // L_: lane ... {uv}
    lua_rawgeti(L_, -1, 2);                                                                        // L_: lane ... {uv} <error>

    // any negative index gives just the error message without propagation
    if (key_ < 0) {
        lua_remove(L_, -2);                                                                        // L_: lane ... <error>
        return;
    }
    lua_getmetatable(L_, kIdxSelf);                                                                // L_: lane ... {uv} <error> {mt}
    lua_replace(L_, -3);                                                                           // L_: lane ... {mt} <error>
    // Note: Lua 5.1 interpreter is not prepared to show
    //       non-string errors, so we use 'tostring()' here
    //       to get meaningful output.  --AKa 22-Jan-2009
    //
    //       Also, the stack dump we get is no good; it only
    //       lists our internal Lanes functions. There seems
    //       to be no way to switch it off, though.
    //
    // Level 2 should show the line where 'h[x]' was read
    // but this only seems to work for string messages
    // (Lua 5.1.4). No idea, why.   --AKa 22-Jan-2009
    if constexpr (LUA_VERSION_NUM == 501) {
        if (!lua_isstring(L_, -1)) {
            // tostring() and error() are hidden in the lane metatable
            kCachedTostring.pushKey(L_);                                                           // L_: lane ... {mt} <error> kCachedTostring
            lua_rawget(L_, -3);                                                                    // L_: lane ... {mt} <error> tostring()
            lua_insert(L_, -2);                                                                    // L_: lane ... {mt} tostring() <error>
            lua_call(L_, 1, 1); // tostring(errstring)                                             // L_: lane ... {mt} "error"
        }
    }
    kCachedError.pushKey(L_);                                                                      // L_: lane ... {mt} "error" kCachedError
    lua_rawget(L_, -3);                                                                            // L_: lane ... {mt} "error" error()
    lua_replace(L_, -3);                                                                           // L_: lane ... error() "error"
    lua_pushinteger(L_, 2);                                                                        // L_: lane ... error() "error" 2
    lua_call(L_, 2, 0); // error(tostring(errstring), 3) -> doesn't return                         // L_: lane ...
    raise_luaL_error(L_, "%s: should not get here!", getDebugName().data());
}

// #################################################################################################

[[nodiscard]]
std::string_view Lane::pushErrorTraceLevel(lua_State* L_) const
{
    std::string_view const _str{ errorTraceLevelString() };
    LUA_ASSERT(L_, !_str.empty());

    return luaG_pushstring(L_, _str);
}

// #################################################################################################

// replace the current uservalue (a table holding the returned values of the lane body)
// by a new empty one, but transfer the gc_cb that is stored in there so that it is not lost
void Lane::resetResultsStorage(lua_State* const L_, StackIndex const self_idx_)
{
    STACK_GROW(L_, 4);
    STACK_CHECK_START_REL(L_, 0);
    StackIndex const _self_idx{ luaG_absindex(L_, self_idx_) };
    LUA_ASSERT(L_, ToLane(L_, _self_idx) == this);                                                 // L_: ... self ...
    // create the new table
    lua_newtable(L_);                                                                              // L_: ... self ... {}
    // get the current table
    lua_getiuservalue(L_, _self_idx, UserValueIndex{ 1 });                                         // L_: ... self ... {} {uv}
    LUA_ASSERT(L_, lua_istable(L_, -1));
    // read gc_cb from the current table
    kLaneGC.pushKey(L_);                                                                           // L_: ... self ... {} {uv} kLaneGC
    kLaneGC.pushKey(L_);                                                                           // L_: ... self ... {} {uv} kLaneGC kLaneGC
    lua_rawget(L_, -3);                                                                            // L_: ... self ... {} {uv} kLaneGC gc_cb|nil
    // store it in the new table
    lua_rawset(L_, -4);                                                                            // L_: ... self ... {} {uv}
    // we can forget the old table
    lua_pop(L_, 1);                                                                                // L_: ... self ... {}
    // and store the new one
    lua_setiuservalue(L_, _self_idx, UserValueIndex{ 1 });                                         // L_: ... self ...
    STACK_CHECK(L_, 0);
}

// #################################################################################################

// intern the debug name in the caller lua state so that the pointer remains valid after the lane's state is closed
void Lane::securizeDebugName(lua_State* const L_)
{
    STACK_CHECK_START_REL(L_, 0);
    STACK_GROW(L_, 3);
    // a Lane's uservalue should be a table
    lua_getiuservalue(L_, StackIndex{ 1 }, UserValueIndex{ 1 });                                   // L_: lane ... {uv}
    LUA_ASSERT(L_, lua_istable(L_, -1));
    // we don't care about the actual key, so long as it's unique and can't collide with anything.
    lua_newtable(L_);                                                                              // L_: lane ... {uv} {}
    {
        std::lock_guard<std::mutex> _guard{ debugNameMutex };
        debugName = luaG_pushstring(L_, debugName);                                                // L_: lane ... {uv} {} name
    }
    lua_rawset(L_, -3);                                                                            // L_: lane ... {uv}
    lua_pop(L_, 1);                                                                                // L_: lane
    STACK_CHECK(L_, 0);
}

// #################################################################################################

void Lane::startThread(int const priority_)
{
    thread = std::thread([this]() { lane_main(this); });
    if (priority_ != kThreadPrioDefault) {
        THREAD_SET_PRIORITY(thread, priority_, U->sudo);
    }
}

// #################################################################################################

void Lane::storeDebugName(std::string_view const& name_)
{
    STACK_CHECK_START_REL(L, 0);
    // store a hidden reference in the registry to make sure the string is kept around even if a lane decides to manually change the "decoda_name" global...
    kLaneNameRegKey.setValue(L, [name = name_](lua_State* L_) { luaG_pushstring(L_, name); });
    STACK_CHECK(L, 0);
    kLaneNameRegKey.pushValue(L);                                                                       // L: ... "name" ...
    // keep a direct view on the stored string
    {
        std::lock_guard<std::mutex> _guard{ debugNameMutex };
        debugName = luaG_tostring(L, kIdxTop);
    }
    lua_pop(L, 1);
    STACK_CHECK(L, 0);
}

// #################################################################################################

// take the results on the lane state stack and store them in our uservalue table, at numeric indices:
// t[0] = nresults
// t[i] = result #i
int Lane::storeResults(lua_State* const L_)
{
    static constexpr StackIndex kIdxSelf{ 1 };
    LUA_ASSERT(L_, ToLane(L_, kIdxSelf) == this);

    STACK_CHECK_START_REL(L_, 0);
    lua_getiuservalue(L_, kIdxSelf, UserValueIndex{ 1 });                                          // L_: lane ... {uv}
    StackIndex const _tidx{ lua_gettop(L_) };

    int _stored{};
    if (nresults == 0) {
        lua_rawgeti(L_, -1, 0);                                                                    // L_: lane ... {uv} nresults
        _stored = static_cast<int>(lua_tointeger(L_, -1));
        lua_pop(L_, 2);
        STACK_CHECK(L_, 0);
        return _stored;
    }

    switch (status.load(std::memory_order_acquire)) {
    default:
        // this is an internal error, we probably never get here
        lua_settop(L_, 0);                                                                         // L_:
        luaG_pushstring(L_, "Unexpected status: ");                                                // L_: "Unexpected status: "
        pushStatusString(L_);                                                                      // L_: "Unexpected status: " "<status>"
        lua_concat(L_, 2);                                                                         // L_: "Unexpected status: <status>"
        raise_lua_error(L_);

    case Lane::Error: // got 1 or 2 error values: <error> <trace>?
        // error can be anything (even nil!)
        if (errorTraceLevel == Lane::Minimal) {
            LUA_ASSERT(L_, (lua_gettop(L) == nresults) && (nresults == 1));
        } else {
            LUA_ASSERT(L_, (lua_gettop(L) == nresults) && (nresults == 2));
            LUA_ASSERT(L_, lua_istable(L, 2));
        }
        // convert this to nil,<error>,<trace>
        lua_pushnil(L);                                                                            //                                                L: <error> <trace> nil
        lua_insert(L, 1);                                                                          //                                                L: nil <error> <trace>
        ++nresults;
        [[fallthrough]];

    case Lane::Suspended: // got yielded values
    case Lane::Done: // got regular return values
        {
            InterCopyResult const _r{ InterCopyContext{ U, DestState{ L_ }, SourceState{ L }, {}, {}, {}, {}, {} }.interMove(nresults) };
            lua_settop(L, 0);                                                                      // L_: lane ... {uv} results                      L:
            _stored = nresults;
            nresults = 0;
            if (_r != InterCopyResult::Success) {
                raise_luaL_error(L_, "Tried to copy unsupported types");
            }
            for (int _i = _stored; _i > 0; --_i) {
                // pop the last element of the stack, to store it in the uservalue at its proper index
                lua_rawseti(L_, _tidx, _i);                                                        // L_: lane ... {uv} results...
            }                                                                                      // L_: lane ... {uv}
            // results[0] = nresults
            lua_pushinteger(L_, _stored);                                                          // L_: lane ... {uv} nresults
            lua_rawseti(L_, _tidx, 0);                                                             // L_: lane ... {uv}
            lua_pop(L_, 1);                                                                        // L_: lane ...
        }
        break;

    case Lane::Cancelled:
        _stored = 2;
        // we should have a single value, kCancelError, in the stack of L
        LUA_ASSERT(L_, nresults == 1 && lua_gettop(L) == 1 && kCancelError.equals(L, StackIndex{ 1 }));
        // store nil, cancel_error in the results
        lua_pushnil(L_);                                                                           // L_: lane ... {uv} nil
        lua_rawseti(L_, _tidx, 1);                                                                 // L_: lane ... {uv}
        kCancelError.pushKey(L_);                                                                  // L_: lane ... {uv} cancel_error
        lua_rawseti(L_, _tidx, 2);                                                                 // L_: lane ... {uv}
        // results[0] = nresults
        lua_pushinteger(L_, _stored);                                                              // L_: lane ... {uv} nresults
        lua_rawseti(L_, _tidx, 0);                                                                 // L_: lane ... {uv}
        lua_pop(L_, 1);                                                                            // L_: lane ...
        lua_settop(L, 0);                                                                          //                                                L:
        nresults = 0;
        break;
    }
    STACK_CHECK(L_, 0);
    LUA_ASSERT(L_, lua_gettop(L) == 0 && nresults == 0);
    // if we are suspended, all we want to do is gather the current yielded values
    if (status.load(std::memory_order_acquire) != Lane::Suspended) {
        // debugName is a pointer to string possibly interned in the lane's state, that no longer exists when the state is closed
        // so store it in the userdata uservalue at a key that can't possibly collide
        securizeDebugName(L_);
        closeState();
    }

    return _stored;
}

// #################################################################################################

//---
// str= thread_status( lane )
//
// "pending" -> | ("running" <-> "waiting") <-> "suspended" <-> "resuming" | -> "done"/"error"/"cancelled"

// "pending"   not started yet
// "running"   started, doing its work..
// "suspended" returned from a lua_resume
// "resuming"  told by its parent state to resume
// "waiting"   blocked in a send()/receive()
// "done"      finished, results are there
// "error"     finished at an error, error value is there
// "cancelled" execution cancelled (state gone)
//
[[nodiscard]]
std::string_view Lane::threadStatusString() const
{
    static constexpr std::string_view kStrs[] = {
        "pending",
        "running", "suspended", "resuming",
        "waiting",
        "done", "error", "cancelled"
    };
    static_assert(0 == static_cast<std::underlying_type_t<Lane::Status>>(Pending));
    static_assert(1 == static_cast<std::underlying_type_t<Lane::Status>>(Running));
    static_assert(2 == static_cast<std::underlying_type_t<Lane::Status>>(Suspended));
    static_assert(3 == static_cast<std::underlying_type_t<Lane::Status>>(Resuming));
    static_assert(4 == static_cast<std::underlying_type_t<Lane::Status>>(Waiting));
    static_assert(5 == static_cast<std::underlying_type_t<Lane::Status>>(Done));
    static_assert(6 == static_cast<std::underlying_type_t<Lane::Status>>(Error));
    static_assert(7 == static_cast<std::underlying_type_t<Lane::Status>>(Cancelled));
    auto const _status{ static_cast<std::underlying_type_t<Lane::Status>>(status.load(std::memory_order_acquire)) };
    if (_status < 0 || _status > 7) { // should never happen, but better safe than sorry
        return "";
    }
    return kStrs[_status];
}

// #################################################################################################

bool Lane::waitForCompletion(std::chrono::time_point<std::chrono::steady_clock> until_)
{
    std::unique_lock _guard{ doneMutex };
    // std::stop_token token{ thread.get_stop_token() };
    // return doneCondVar.wait_until(lock, token, secs_, [this](){ return status >= Lane::Done; });

    // wait until the lane stops working with its state (either Suspended or Done+)
    return doneCondVar.wait_until(_guard, until_, [this]()
        {
            auto const _status{ status.load(std::memory_order_acquire) };
            return _status == Lane::Suspended || _status >= Lane::Done;
        }
    );
}