all: optimize for zero allocation scheduling

name     old time/op      new time/op     delta
Call-16  2.98µs ±0%       2.90µs ±2%     -2.61%  (p=0.000 n=9+10)

name     old alloc/op    new alloc/op     delta
Call-16   24.0B ±0%       0.0B         -100.00%  (p=0.000 n=10+10)

name     old allocs/op  new allocs/op     delta
Call-16    1.00 ±0%       0.00         -100.00%  (p=0.000 n=10+10)

Author: changkun

mainthread.go

@@ -9,12 +9,17 @@ import (
 	"sync"
 )
 
-var funcQ = make(chan func(), runtime.GOMAXPROCS(0))
+var funcQ = make(chan funcData, runtime.GOMAXPROCS(0))
 
 func init() {
 	runtime.LockOSThread()
 }
 
+type funcData struct {
+	fn   func()
+	done chan struct{}
+}
+
 // Init initializes the functionality for running arbitrary subsequent
 // functions on a main system thread.
 //
@@ -33,7 +38,12 @@ func Init(main func()) {
 	for {
 		select {
 		case f := <-funcQ:
-			f()
+			func() {
+				defer func() {
+					f.done <- struct{}{}
+				}()
+				f.fn()
+			}()
 		case <-done:
 			return
 		}
@@ -45,12 +55,7 @@ func Call(f func()) {
 	done := donePool.Get().(chan struct{})
 	defer donePool.Put(done)
 
-	funcQ <- func() {
-		defer func() {
-			done <- struct{}{}
-		}()
-		f()
-	}
+	funcQ <- funcData{fn: f, done: done}
 	<-done
 }