script.go

// Package script provides lua scripting for skipper
package script

import (
	"bufio"
	"bytes"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"os"
	"strings"

	log "github.com/sirupsen/logrus"

	lua "github.com/yuin/gopher-lua"
	lua_parse "github.com/yuin/gopher-lua/parse"
	"github.com/zalando/skipper/filters"
	"github.com/zalando/skipper/script/base64"

	"github.com/cjoudrey/gluahttp"
	"github.com/cjoudrey/gluaurl"
	gjson "layeh.com/gopher-json"
)

var (
	// InitialPoolSize is the number of lua states created initially per route
	InitialPoolSize int = 3

	// MaxPoolSize is the number of lua states stored per route - there may be more parallel
	// requests, but only this number is cached.
	MaxPoolSize int = 10

	errLuaSourcesDisabled = errors.New("lua Sources are disabled")
)

type LuaOptions struct {
	// Modules configures enabled standard and additional (preloaded) Lua modules.
	// For standard Lua modules (see https://www.lua.org/manual/5.1/manual.html)
	// use "<module>.<symbol>" (e.g. "base.print") to selectively enable module symbols.
	// Additional modules are preloaded with all symbols.
	// Empty value enables all modules.
	Modules []string
	// Sources that are allowed as input sources. Valid sources
	// are "none", "file" and "inline". Empty slice will enable
	// both as default. To disable the use of lua filters use
	// "none".
	Sources []string
}

type luaScript struct {
	modules []string
	sources []string
}

// NewLuaScript creates a new filter spec
func NewLuaScript() filters.Spec {
	spec, _ := NewLuaScriptWithOptions(LuaOptions{})
	return spec
}

// NewLuaScriptWithOptions creates a new filter spec with options
func NewLuaScriptWithOptions(opts LuaOptions) (filters.Spec, error) {
	sources := opts.Sources
	if len(sources) == 0 {
		// backwards compatible default, allow all
		sources = append(sources, "file", "inline")
	} else if contains("none", opts.Sources) {
		sources = nil
	}
	return &luaScript{
		modules: opts.Modules,
		sources: sources,
	}, nil
}

// Name returns the name of the filter ("lua")
func (ls *luaScript) Name() string {
	return filters.LuaName
}

// CreateFilter creates the lua script filter.
func (ls *luaScript) CreateFilter(config []interface{}) (filters.Filter, error) {
	if len(ls.sources) == 0 {
		return nil, errLuaSourcesDisabled
	}
	if len(config) == 0 {
		return nil, filters.ErrInvalidFilterParameters
	}
	src, ok := config[0].(string)
	if !ok {
		return nil, filters.ErrInvalidFilterParameters
	}
	var params []string
	for _, p := range config[1:] {
		ps, ok := p.(string)
		if !ok {
			return nil, filters.ErrInvalidFilterParameters
		}
		params = append(params, ps)
	}

	s := &script{source: src, routeParams: params}
	if err := s.initScript(ls.modules, ls.sources); err != nil {
		return nil, err
	}
	return s, nil
}

type script struct {
	source      string
	routeParams []string

	pool        chan *lua.LState
	proto       *lua.FunctionProto
	load        []luaModule
	preload     []luaModule
	hasRequest  bool
	hasResponse bool
}

func (s *script) getState() (*lua.LState, error) {
	select {
	case L := <-s.pool:
		if L == nil {
			return nil, errors.New("pool closed")
		}
		return L, nil
	default:
		return s.newState()
	}
}

func (s *script) putState(L *lua.LState) {
	if s.pool == nil { // pool closed
		L.Close()
		return
	}
	select {
	case s.pool <- L:
	default: // pool full, close state
		L.Close()
	}
}

func (s *script) newState() (*lua.LState, error) {
	L := lua.NewState(lua.Options{SkipOpenLibs: true})

	for _, m := range s.load {
		m.load(L)
	}

	for _, m := range s.preload {
		m.preload(L)
	}

	L.Push(L.NewFunctionFromProto(s.proto))

	err := L.PCall(0, lua.MultRet, nil)
	if err != nil {
		L.Close()
		return nil, err
	}
	return L, nil
}

func (s *script) initScript(modules, validSources []string) error {
	// Compile
	var reader io.Reader
	var name string

	if strings.HasSuffix(s.source, ".lua") {
		if !contains("file", validSources) {
			return fmt.Errorf(`invalid lua source referenced "file", allowed: "%v"`, validSources)
		}

		file, err := os.Open(s.source)
		if err != nil {
			return err
		}

		defer func() {
			if err = file.Close(); err != nil {
				log.Errorf("Failed to close lua file %s: %v", s.source, err)
			}
		}()

		reader = bufio.NewReader(file)
		name = s.source
	} else {
		if !contains("inline", validSources) {
			return fmt.Errorf(`invalid lua source referenced "inline", allowed: "%v"`, validSources)
		}
		reader = strings.NewReader(s.source)
		name = "<script>"
	}
	chunk, err := lua_parse.Parse(reader, name)
	if err != nil {
		return err
	}
	proto, err := lua.Compile(chunk, name)
	if err != nil {
		return err
	}
	s.proto = proto

	// Configure enabled modules and symbols
	moduleConfig := moduleConfig(modules)

	if len(moduleConfig) == 0 {
		s.load = append(s.load, standardModules...)
	} else {
		L := lua.NewState(lua.Options{SkipOpenLibs: true})
		defer L.Close()

		for _, m := range standardModules {
			name := m.name
			if m.name == lua.BaseLibName {
				name = "base" // alias for empty lua.BaseLibName
			}
			if symbols, ok := moduleConfig[name]; ok {
				if len(symbols) > 0 {
					m = m.withSymbols(L, symbols)
				}
				s.load = append(s.load, m)
			}
		}
	}

	// Configure additional modules
	additionalModules := []luaModule{
		{"base64", base64.Loader, nil},
		{"http", gluahttp.NewHttpModule(&http.Client{}).Loader, nil},
		{"url", gluaurl.Loader, nil},
		{"json", gjson.Loader, nil},
	}

	if len(moduleConfig) == 0 {
		s.preload = append(s.preload, additionalModules...)
	} else {
		for _, m := range additionalModules {
			// TODO: enable selected symbols for preloaded modules
			if _, ok := moduleConfig[m.name]; ok {
				s.preload = append(s.preload, m)
			}
		}
	}

	// Detect request and response functions
	// Note: use s.newState() instead of lua.NewState() to load only enabled modules
	L, err := s.newState()
	if err != nil {
		return err
	}
	defer L.Close()

	if fn := L.GetGlobal("request"); fn.Type() == lua.LTFunction {
		s.hasRequest = true
	}
	if fn := L.GetGlobal("response"); fn.Type() == lua.LTFunction {
		s.hasResponse = true
	}
	if !s.hasRequest && !s.hasResponse {
		return errors.New("at least one of `request` and `response` function must be present")
	}

	// Init state pool
	s.pool = make(chan *lua.LState, MaxPoolSize) // FIXME make configurable
	for i := 0; i < InitialPoolSize; i++ {
		L, err := s.newState()
		if err != nil {
			return err
		}
		s.putState(L)
	}
	return nil
}

func (s *script) Request(f filters.FilterContext) {
	if s.hasRequest {
		s.runFunc("request", f)
	}
}

func (s *script) Response(f filters.FilterContext) {
	if s.hasResponse {
		s.runFunc("response", f)
	}
}

func (s *script) runFunc(name string, f filters.FilterContext) {
	L, err := s.getState()
	if err != nil {
		log.Errorf("Error obtaining lua environment: %v", err)
		return
	}
	defer s.putState(L)

	pt := L.CreateTable(len(s.routeParams), len(s.routeParams))
	for i, p := range s.routeParams {
		k, v, _ := strings.Cut(p, "=")
		pt.RawSetString(k, lua.LString(v))
		pt.RawSetInt(i+1, lua.LString(p))
	}

	err = L.CallByParam(
		lua.P{
			Fn:      L.GetGlobal(name),
			NRet:    0,
			Protect: true,
		},
		s.filterContextAsLuaTable(L, f),
		pt,
	)
	if err != nil {
		log.Errorf("Error calling %s from %s: %v", name, s.source, err)
	}
}

func (s *script) filterContextAsLuaTable(L *lua.LState, f filters.FilterContext) *lua.LTable {
	// this will be passed as parameter to the lua functions
	// add metatable to dynamically access fields in the context
	t := L.CreateTable(0, 0)
	mt := L.CreateTable(0, 1)
	mt.RawSetString("__index", L.NewFunction(getContextValue(f)))
	L.SetMetatable(t, mt)
	return t
}

func serveRequest(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		t := s.Get(-1)
		r, ok := t.(*lua.LTable)
		if !ok {
			// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
			// s.RaiseError("unsupported type %v, need a table", t.Type())
			// return 0
			s.Push(lua.LString("invalid type, need a table"))
			return 1
		}
		res := &http.Response{}
		r.ForEach(serveTableWalk(res))
		f.Serve(res)
		return 0
	}
}

func serveTableWalk(res *http.Response) func(lua.LValue, lua.LValue) {
	return func(k, v lua.LValue) {
		sk, ok := k.(lua.LString)
		if !ok {
			// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
			// s.RaiseError("unsupported key type %v, need a string", k.Type())
			return
		}
		switch string(sk) {
		case "status_code":
			n, ok := v.(lua.LNumber)
			if !ok {
				// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
				// s.RaiseError("unsupported status_code type %v, need a number", v.Type())
				return
			}
			res.StatusCode = int(n)

		case "header":
			t, ok := v.(*lua.LTable)
			if !ok {
				// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
				// s.RaiseError("unsupported header type %v, need a table", v.Type())
				return
			}
			h := make(http.Header)
			t.ForEach(serveHeaderWalk(h))
			res.Header = h

		case "body":
			var body []byte
			var err error
			switch v.Type() {
			case lua.LTString:
				data := string(v.(lua.LString))
				body = []byte(data)
			case lua.LTTable:
				body, err = gjson.Encode(v.(*lua.LTable))
				if err != nil {
					// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
					// s.RaiseError("%v", err)
					return
				}
			}
			res.Body = io.NopCloser(bytes.NewBuffer(body))
		}
	}
}

func serveHeaderWalk(h http.Header) func(lua.LValue, lua.LValue) {
	return func(k, v lua.LValue) {
		h.Set(k.String(), v.String())
	}
}

func getContextValue(f filters.FilterContext) func(*lua.LState) int {
	var request, response, state_bag, path_param *lua.LTable
	var serve *lua.LFunction
	return func(s *lua.LState) int {
		key := s.ToString(-1)
		var ret lua.LValue
		switch key {
		case "request":
			// initialize access to request on first use
			if request == nil {
				request = s.CreateTable(0, 0)
				mt := s.CreateTable(0, 2)
				mt.RawSetString("__index", s.NewFunction(getRequestValue(f)))
				mt.RawSetString("__newindex", s.NewFunction(setRequestValue(f)))
				s.SetMetatable(request, mt)
			}
			ret = request
		case "response":
			if response == nil {
				response = s.CreateTable(0, 0)
				mt := s.CreateTable(0, 2)
				mt.RawSetString("__index", s.NewFunction(getResponseValue(f)))
				mt.RawSetString("__newindex", s.NewFunction(setResponseValue(f)))
				s.SetMetatable(response, mt)
			}
			ret = response
		case "state_bag":
			if state_bag == nil {
				state_bag = s.CreateTable(0, 0)
				mt := s.CreateTable(0, 2)
				mt.RawSetString("__index", s.NewFunction(getStateBag(f)))
				mt.RawSetString("__newindex", s.NewFunction(setStateBag(f)))
				s.SetMetatable(state_bag, mt)
			}
			ret = state_bag
		case "path_param":
			if path_param == nil {
				path_param = s.CreateTable(0, 0)
				mt := s.CreateTable(0, 1)
				mt.RawSetString("__index", s.NewFunction(getPathParam(f)))
				s.SetMetatable(path_param, mt)
			}
			ret = path_param
		case "serve":
			if serve == nil {
				serve = s.NewFunction(serveRequest(f))
			}
			ret = serve
		default:
			return 0
		}
		s.Push(ret)
		return 1
	}
}

func getRequestValue(f filters.FilterContext) func(*lua.LState) int {
	var header, cookie, url_query *lua.LTable
	return func(s *lua.LState) int {
		key := s.ToString(-1)
		var ret lua.LValue
		switch key {
		case "header":
			if header == nil {
				header = s.CreateTable(0, 2)
				header.RawSetString("add", s.NewFunction(addRequestHeader(f)))
				header.RawSetString("values", s.NewFunction(requestHeaderValues(f)))

				mt := s.CreateTable(0, 3)
				mt.RawSetString("__index", s.NewFunction(getRequestHeader(f)))
				mt.RawSetString("__newindex", s.NewFunction(setRequestHeader(f)))
				mt.RawSetString("__call", s.NewFunction(iterateRequestHeader(f)))
				s.SetMetatable(header, mt)
			}
			ret = header
		case "cookie":
			if cookie == nil {
				cookie = s.CreateTable(0, 0)
				mt := s.CreateTable(0, 3)
				mt.RawSetString("__index", s.NewFunction(getRequestCookie(f)))
				mt.RawSetString("__newindex", s.NewFunction(unsupported("setting cookie is not supported")))
				mt.RawSetString("__call", s.NewFunction(iterateRequestCookie(f)))
				s.SetMetatable(cookie, mt)
			}
			ret = cookie
		case "outgoing_host":
			ret = lua.LString(f.OutgoingHost())
		case "backend_url":
			ret = lua.LString(f.BackendUrl())
		case "host":
			ret = lua.LString(f.Request().Host)
		case "remote_addr":
			ret = lua.LString(f.Request().RemoteAddr)
		case "content_length":
			ret = lua.LNumber(f.Request().ContentLength)
		case "proto":
			ret = lua.LString(f.Request().Proto)
		case "method":
			ret = lua.LString(f.Request().Method)
		case "url":
			ret = lua.LString(f.Request().URL.String())
		case "url_path":
			ret = lua.LString(f.Request().URL.Path)
		case "url_query":
			if url_query == nil {
				url_query = s.CreateTable(0, 0)
				mt := s.CreateTable(0, 3)
				mt.RawSetString("__index", s.NewFunction(getRequestURLQuery(f)))
				mt.RawSetString("__newindex", s.NewFunction(setRequestURLQuery(f)))
				mt.RawSetString("__call", s.NewFunction(iterateRequestURLQuery(f)))
				s.SetMetatable(url_query, mt)
			}
			ret = url_query
		case "url_raw_query":
			ret = lua.LString(f.Request().URL.RawQuery)
		default:
			return 0
		}
		s.Push(ret)
		return 1
	}
}

func setRequestValue(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		key := s.ToString(-2)
		switch key {
		case "outgoing_host":
			f.SetOutgoingHost(s.ToString(-1))
		case "url":
			u, err := url.Parse(s.ToString(-1))
			if err != nil {
				// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
				// s.RaiseError("%v", err)
				return 0
			}
			f.Request().URL = u
		case "url_path":
			f.Request().URL.Path = s.ToString(-1)
		case "url_raw_query":
			f.Request().URL.RawQuery = s.ToString(-1)
		default:
			// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
			// s.RaiseError("unsupported request field %s", key)
			// do nothing for now
		}
		return 0
	}
}

func getResponseValue(f filters.FilterContext) func(*lua.LState) int {
	var header *lua.LTable
	return func(s *lua.LState) int {
		key := s.ToString(-1)
		var ret lua.LValue
		switch key {
		case "header":
			if header == nil {
				header = s.CreateTable(0, 2)
				header.RawSetString("add", s.NewFunction(addResponseHeader(f)))
				header.RawSetString("values", s.NewFunction(responseHeaderValues(f)))

				mt := s.CreateTable(0, 3)
				mt.RawSetString("__index", s.NewFunction(getResponseHeader(f)))
				mt.RawSetString("__newindex", s.NewFunction(setResponseHeader(f)))
				mt.RawSetString("__call", s.NewFunction(iterateResponseHeader(f)))
				s.SetMetatable(header, mt)
			}
			ret = header
		case "status_code":
			ret = lua.LNumber(f.Response().StatusCode)
		default:
			return 0
		}
		s.Push(ret)
		return 1
	}
}

func setResponseValue(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		key := s.ToString(-2)
		switch key {
		case "status_code":
			v := s.Get(-1)
			n, ok := v.(lua.LNumber)
			if !ok {
				s.RaiseError("unsupported status_code type %v, need a number", v.Type())
				return 0
			}
			f.Response().StatusCode = int(n)
		default:
			s.RaiseError("unsupported response field %s", key)
		}
		return 0
	}
}

func getStateBag(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		fld := s.ToString(-1)
		res, ok := f.StateBag()[fld]
		if !ok {
			return 0
		}
		switch res := res.(type) {
		case string:
			s.Push(lua.LString(res))
		case int:
			s.Push(lua.LNumber(res))
		case int64:
			s.Push(lua.LNumber(res))
		case float64:
			s.Push(lua.LNumber(res))
		case *lua.LTable:
			s.Push(res) // load *lua.LTable as is
		default:
			return 0
		}
		return 1
	}
}

func setStateBag(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		fld := s.ToString(-2)
		val := s.Get(-1)
		var res interface{}
		switch val.Type() {
		case lua.LTString:
			res = string(val.(lua.LString))
		case lua.LTNumber:
			res = float64(val.(lua.LNumber))
		case lua.LTTable:
			res = val // store *lua.LTable as is
		default:
			// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
			// s.RaiseError("unsupported state bag value type %v, need a string or a number", val.Type())
			return 0
		}
		f.StateBag()[fld] = res
		return 0
	}
}

func getRequestHeader(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		hdr := s.ToString(-1)
		res := f.Request().Header.Get(hdr)
		// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
		// if res != "" {
		//	s.Push(lua.LString(res))
		//	return 1
		// }
		// return 0
		s.Push(lua.LString(res))
		return 1
	}
}

func setRequestHeader(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		lv := s.Get(-1)
		hdr := s.ToString(-2)
		switch lv.Type() {
		case lua.LTNil:
			f.Request().Header.Del(hdr)
		case lua.LTString:
			str := string(lv.(lua.LString))
			if str == "" {
				f.Request().Header.Del(hdr)
			} else {
				f.Request().Header.Set(hdr, str)
			}
		default:
			val := s.ToString(-1)
			f.Request().Header.Set(hdr, val)
		}
		return 0
	}
}

func addRequestHeader(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		value := s.ToString(-1)
		name := s.ToString(-2)
		if name != "" && value != "" {
			f.Request().Header.Add(name, value)
		}
		return 0
	}
}

func requestHeaderValues(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		name := s.ToString(-1)
		values := f.Request().Header.Values(name)
		res := s.CreateTable(len(values), 0)
		for _, v := range values {
			res.Append(lua.LString(v))
		}
		s.Push(res)
		return 1
	}
}

func iterateRequestHeader(f filters.FilterContext) func(*lua.LState) int {
	// https://www.lua.org/pil/7.2.html
	return func(s *lua.LState) int {
		s.Push(s.NewFunction(nextHeader(f.Request().Header)))
		return 1
	}
}

func getRequestCookie(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		k := s.ToString(-1)
		c, err := f.Request().Cookie(k)
		if err == nil {
			s.Push(lua.LString(c.Value))
			return 1
		}
		return 0
	}
}

func iterateRequestCookie(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		s.Push(s.NewFunction(nextCookie(f.Request().Cookies())))
		return 1
	}
}

func nextCookie(cookies []*http.Cookie) func(*lua.LState) int {
	return func(s *lua.LState) int {
		if len(cookies) > 0 {
			c := cookies[0]
			s.Push(lua.LString(c.Name))
			s.Push(lua.LString(c.Value))
			cookies[0] = nil // mind peace
			cookies = cookies[1:]
			return 2
		}
		return 0
	}
}

func getResponseHeader(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		hdr := s.ToString(-1)
		res := f.Response().Header.Get(hdr)
		// TODO(sszuecs): https://github.com/zalando/skipper/issues/1487
		// if res != "" {
		//	s.Push(lua.LString(res))
		//	return 1
		// }
		// return 0
		s.Push(lua.LString(res))
		return 1
	}
}

func setResponseHeader(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		lv := s.Get(-1)
		hdr := s.ToString(-2)
		switch lv.Type() {
		case lua.LTNil:
			f.Response().Header.Del(hdr)
		case lua.LTString:
			str := string(lv.(lua.LString))
			if str == "" {
				f.Response().Header.Del(hdr)
			} else {
				f.Response().Header.Set(hdr, str)
			}
		default:
			val := s.ToString(-1)
			f.Response().Header.Set(hdr, val)
		}
		return 0
	}
}

func addResponseHeader(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		value := s.ToString(-1)
		name := s.ToString(-2)
		if name != "" && value != "" {
			f.Response().Header.Add(name, value)
		}
		return 0
	}
}

func responseHeaderValues(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		name := s.ToString(-1)
		values := f.Response().Header.Values(name)
		res := s.CreateTable(len(values), 0)
		for _, v := range values {
			res.Append(lua.LString(v))
		}
		s.Push(res)
		return 1
	}
}

func iterateResponseHeader(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		s.Push(s.NewFunction(nextHeader(f.Response().Header)))
		return 1
	}
}

func nextHeader(h http.Header) func(*lua.LState) int {
	keys := make([]string, 0, len(h))
	for k := range h {
		keys = append(keys, k)
	}
	return func(s *lua.LState) int {
		if len(keys) > 0 {
			k := keys[0]
			s.Push(lua.LString(k))
			s.Push(lua.LString(h.Get(k)))
			keys[0] = "" // mind peace
			keys = keys[1:]
			return 2
		}
		return 0
	}
}

func getRequestURLQuery(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		k := s.ToString(-1)
		res := f.Request().URL.Query().Get(k)
		if res != "" {
			s.Push(lua.LString(res))
			return 1
		}
		return 0
	}
}

func setRequestURLQuery(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		lv := s.Get(-1)
		k := s.ToString(-2)
		q := f.Request().URL.Query()
		switch lv.Type() {
		case lua.LTNil:
			q.Del(k)
		case lua.LTString:
			str := string(lv.(lua.LString))
			if str == "" {
				q.Del(k)
			} else {
				q.Set(k, str)
			}
		default:
			val := s.ToString(-1)
			q.Set(k, val)
		}
		f.Request().URL.RawQuery = q.Encode()
		return 0
	}
}

func iterateRequestURLQuery(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		s.Push(s.NewFunction(nextQuery(f.Request().URL.Query())))
		return 1
	}
}

func nextQuery(v url.Values) func(*lua.LState) int {
	keys := make([]string, 0, len(v))
	for k := range v {
		keys = append(keys, k)
	}
	return func(s *lua.LState) int {
		if len(keys) > 0 {
			k := keys[0]
			s.Push(lua.LString(k))
			s.Push(lua.LString(v.Get(k)))
			keys[0] = "" // mind peace
			keys = keys[1:]
			return 2
		}
		return 0
	}
}

func getPathParam(f filters.FilterContext) func(*lua.LState) int {
	return func(s *lua.LState) int {
		n := s.ToString(-1)
		p := f.PathParam(n)
		if p != "" {
			s.Push(lua.LString(p))
			return 1
		}
		return 0
	}
}

func unsupported(message string) func(*lua.LState) int {
	return func(s *lua.LState) int {
		s.RaiseError(message)
		return 0
	}
}

func contains(s string, a []string) bool {
	for _, w := range a {
		if s == w {
			return true
		}
	}
	return false
}