filter: lua: reduce allocation

Suppress one allocation, reducing their number from 5 to 4, by
pre-allocating the userdata we use to wrap the record passed to
the lua filter.

    name          old time/op    new time/op    delta
    LUAProcess-8     638ns ± 4%     559ns ± 6%  -12.31%  (p=0.000 n=10+10)

    name          old alloc/op   new alloc/op   delta
    LUAProcess-8      152B ± 0%      104B ± 0%  -31.58%  (p=0.000 n=10+10)

    name          old allocs/op  new allocs/op  delta
    LUAProcess-8      5.00 ± 0%      4.00 ± 0%  -20.00%  (p=0.000 n=10+10)

Author: arl

filter/lua.go

@@ -23,6 +23,7 @@ type LUAConfig struct {
 
 type LUA struct {
 	l       *lua.LState
+	ud      *lua.LUserData
 	luaFunc lua.LValue
 }
 
@@ -37,9 +38,14 @@ func NewLUA(cfg baker.FilterParams) (baker.Filter, error) {
 	// TODO: check function exists
 	luaFunc := l.GetGlobal(dcfg.FilterName)
 
+	// Preallocate the userdata we use to wrap the record passed to the filter.
+	ud := l.NewUserData()
+	l.SetMetatable(ud, l.GetTypeMetatable(luaRecordTypeName))
+
 	f := &LUA{
 		luaFunc: luaFunc,
 		l:       l,
+		ud:      ud,
 	}
 
 	runtime.SetFinalizer(f, func(f *LUA) { f.l.Close() })
@@ -56,11 +62,14 @@ func (t *LUA) Process(rec baker.Record, next func(baker.Record)) {
 		return 0
 	})
 
+	// Modify the record inside the pre-allocated user value
+	t.ud.Value = &luaRecord{r: rec}
+
 	err := t.l.CallByParam(lua.P{
 		Fn:      t.luaFunc,
 		NRet:    0,
 		Protect: true,
-	}, recordToLua(t.l, rec),
+	}, t.ud,
 		luaNext)
 
 	if err != nil {