runtime: Remove now unnecessary pad field from ParFor.

    
    It is not needed due to the removal of the ctx field.
    
    Reviewed-on: https://go-review.googlesource.com/16525

From-SVN: r229616

7 files changed, 275 insertions, 134 deletions

diff --git a/libgo/go/sync/atomic/atomic_test.go b/libgo/go/sync/atomic/atomic_test.go
index eaa3b6b..6dae0fd 100644
--- a/libgo/go/sync/atomic/atomic_test.go
+++ b/libgo/go/sync/atomic/atomic_test.go
@@ -164,7 +164,7 @@ func TestSwapPointer(t *testing.T) {
 	x.before = magicptr
 	x.after = magicptr
 	var j uintptr
-	for delta := uintptr(1); delta+delta > delta; delta += delta {
+	for delta := uintptr(1 << 16); delta+delta > delta; delta += delta {
 		k := SwapPointer(&x.i, unsafe.Pointer(delta))
 		if uintptr(x.i) != delta || uintptr(k) != j {
 			t.Fatalf("delta=%d i=%d j=%d k=%d", delta, x.i, j, k)
@@ -456,7 +456,7 @@ func TestCompareAndSwapPointer(t *testing.T) {
 	magicptr := uintptr(m)
 	x.before = magicptr
 	x.after = magicptr
-	for val := uintptr(1); val+val > val; val += val {
+	for val := uintptr(1 << 16); val+val > val; val += val {
 		x.i = unsafe.Pointer(val)
 		if !CompareAndSwapPointer(&x.i, unsafe.Pointer(val), unsafe.Pointer(val+1)) {
 			t.Fatalf("should have swapped %#x %#x", val, val+1)
@@ -595,7 +595,7 @@ func TestLoadPointer(t *testing.T) {
 	magicptr := uintptr(m)
 	x.before = magicptr
 	x.after = magicptr
-	for delta := uintptr(1); delta+delta > delta; delta += delta {
+	for delta := uintptr(1 << 16); delta+delta > delta; delta += delta {
 		k := LoadPointer(&x.i)
 		if k != x.i {
 			t.Fatalf("delta=%d i=%d k=%d", delta, x.i, k)
@@ -731,7 +731,7 @@ func TestStorePointer(t *testing.T) {
 	x.before = magicptr
 	x.after = magicptr
 	v := unsafe.Pointer(uintptr(0))
-	for delta := uintptr(1); delta+delta > delta; delta += delta {
+	for delta := uintptr(1 << 16); delta+delta > delta; delta += delta {
 		StorePointer(&x.i, unsafe.Pointer(v))
 		if x.i != v {
 			t.Fatalf("delta=%d i=%d v=%d", delta, x.i, v)
@@ -759,14 +759,12 @@ var hammer32 = map[string]func(*uint32, int){
 	"SwapInt32":             hammerSwapInt32,
 	"SwapUint32":            hammerSwapUint32,
 	"SwapUintptr":           hammerSwapUintptr32,
-	"SwapPointer":           hammerSwapPointer32,
 	"AddInt32":              hammerAddInt32,
 	"AddUint32":             hammerAddUint32,
 	"AddUintptr":            hammerAddUintptr32,
 	"CompareAndSwapInt32":   hammerCompareAndSwapInt32,
 	"CompareAndSwapUint32":  hammerCompareAndSwapUint32,
 	"CompareAndSwapUintptr": hammerCompareAndSwapUintptr32,
-	"CompareAndSwapPointer": hammerCompareAndSwapPointer32,
 }
 
 func init() {
@@ -818,20 +816,6 @@ func hammerSwapUintptr32(uaddr *uint32, count int) {
 	}
 }
 
-func hammerSwapPointer32(uaddr *uint32, count int) {
-	// only safe when uintptr is 32-bit.
-	// not called on 64-bit systems.
-	addr := (*unsafe.Pointer)(unsafe.Pointer(uaddr))
-	seed := int(uintptr(unsafe.Pointer(&count)))
-	for i := 0; i < count; i++ {
-		new := uintptr(seed+i)<<16 | uintptr(seed+i)<<16>>16
-		old := uintptr(SwapPointer(addr, unsafe.Pointer(new)))
-		if old>>16 != old<<16>>16 {
-			panic(fmt.Sprintf("SwapPointer is not atomic: %#08x", old))
-		}
-	}
-}
-
 func hammerAddInt32(uaddr *uint32, count int) {
 	addr := (*int32)(unsafe.Pointer(uaddr))
 	for i := 0; i < count; i++ {
@@ -891,20 +875,6 @@ func hammerCompareAndSwapUintptr32(uaddr *uint32, count int) {
 	}
 }
 
-func hammerCompareAndSwapPointer32(uaddr *uint32, count int) {
-	// only safe when uintptr is 32-bit.
-	// not called on 64-bit systems.
-	addr := (*unsafe.Pointer)(unsafe.Pointer(uaddr))
-	for i := 0; i < count; i++ {
-		for {
-			v := LoadPointer(addr)
-			if CompareAndSwapPointer(addr, v, unsafe.Pointer(uintptr(v)+1)) {
-				break
-			}
-		}
-	}
-}
-
 func TestHammer32(t *testing.T) {
 	const p = 4
 	n := 100000
@@ -940,14 +910,12 @@ var hammer64 = map[string]func(*uint64, int){
 	"SwapInt64":             hammerSwapInt64,
 	"SwapUint64":            hammerSwapUint64,
 	"SwapUintptr":           hammerSwapUintptr64,
-	"SwapPointer":           hammerSwapPointer64,
 	"AddInt64":              hammerAddInt64,
 	"AddUint64":             hammerAddUint64,
 	"AddUintptr":            hammerAddUintptr64,
 	"CompareAndSwapInt64":   hammerCompareAndSwapInt64,
 	"CompareAndSwapUint64":  hammerCompareAndSwapUint64,
 	"CompareAndSwapUintptr": hammerCompareAndSwapUintptr64,
-	"CompareAndSwapPointer": hammerCompareAndSwapPointer64,
 }
 
 func init() {
@@ -999,20 +967,6 @@ func hammerSwapUintptr64(uaddr *uint64, count int) {
 	}
 }
 
-func hammerSwapPointer64(uaddr *uint64, count int) {
-	// only safe when uintptr is 64-bit.
-	// not called on 32-bit systems.
-	addr := (*unsafe.Pointer)(unsafe.Pointer(uaddr))
-	seed := int(uintptr(unsafe.Pointer(&count)))
-	for i := 0; i < count; i++ {
-		new := uintptr(seed+i)<<32 | uintptr(seed+i)<<32>>32
-		old := uintptr(SwapPointer(addr, unsafe.Pointer(new)))
-		if old>>32 != old<<32>>32 {
-			panic(fmt.Sprintf("SwapPointer is not atomic: %v", old))
-		}
-	}
-}
-
 func hammerAddInt64(uaddr *uint64, count int) {
 	addr := (*int64)(unsafe.Pointer(uaddr))
 	for i := 0; i < count; i++ {
@@ -1072,20 +1026,6 @@ func hammerCompareAndSwapUintptr64(uaddr *uint64, count int) {
 	}
 }
 
-func hammerCompareAndSwapPointer64(uaddr *uint64, count int) {
-	// only safe when uintptr is 64-bit.
-	// not called on 32-bit systems.
-	addr := (*unsafe.Pointer)(unsafe.Pointer(uaddr))
-	for i := 0; i < count; i++ {
-		for {
-			v := LoadPointer(addr)
-			if CompareAndSwapPointer(addr, v, unsafe.Pointer(uintptr(v)+1)) {
-				break
-			}
-		}
-	}
-}
-
 func TestHammer64(t *testing.T) {
 	if test64err != nil {
 		t.Skipf("Skipping 64-bit tests: %v", test64err)
@@ -1465,9 +1405,6 @@ func TestUnaligned64(t *testing.T) {
 }
 
 func TestNilDeref(t *testing.T) {
-	if p := runtime.GOOS + "/" + runtime.GOARCH; p == "freebsd/arm" || p == "netbsd/arm" {
-		t.Skipf("issue 7338: skipping test on %q", p)
-	}
 	funcs := [...]func(){
 		func() { CompareAndSwapInt32(nil, 0, 0) },
 		func() { CompareAndSwapInt64(nil, 0, 0) },
diff --git a/libgo/go/sync/export_test.go b/libgo/go/sync/export_test.go
index fa5983a..6f49b3b 100644
--- a/libgo/go/sync/export_test.go
+++ b/libgo/go/sync/export_test.go
@@ -7,3 +7,5 @@ package sync
 // Export for testing.
 var Runtime_Semacquire = runtime_Semacquire
 var Runtime_Semrelease = runtime_Semrelease
+
+const RaceEnabled = raceenabled
diff --git a/libgo/go/sync/mutex.go b/libgo/go/sync/mutex.go
index 73b3377..3f280ad 100644
--- a/libgo/go/sync/mutex.go
+++ b/libgo/go/sync/mutex.go
@@ -48,15 +48,31 @@ func (m *Mutex) Lock() {
 	}
 
 	awoke := false
+	iter := 0
 	for {
 		old := m.state
 		new := old | mutexLocked
 		if old&mutexLocked != 0 {
+			if runtime_canSpin(iter) {
+				// Active spinning makes sense.
+				// Try to set mutexWoken flag to inform Unlock
+				// to not wake other blocked goroutines.
+				if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
+					atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
+					awoke = true
+				}
+				runtime_doSpin()
+				iter++
+				continue
+			}
 			new = old + 1<<mutexWaiterShift
 		}
 		if awoke {
 			// The goroutine has been woken from sleep,
 			// so we need to reset the flag in either case.
+			if new&mutexWoken == 0 {
+				panic("sync: inconsistent mutex state")
+			}
 			new &^= mutexWoken
 		}
 		if atomic.CompareAndSwapInt32(&m.state, old, new) {
@@ -65,6 +81,7 @@ func (m *Mutex) Lock() {
 			}
 			runtime_Semacquire(&m.sema)
 			awoke = true
+			iter = 0
 		}
 	}
 
diff --git a/libgo/go/sync/mutex_test.go b/libgo/go/sync/mutex_test.go
index 151b25c..91a4855 100644
--- a/libgo/go/sync/mutex_test.go
+++ b/libgo/go/sync/mutex_test.go
@@ -134,3 +134,58 @@ func BenchmarkMutexWork(b *testing.B) {
 func BenchmarkMutexWorkSlack(b *testing.B) {
 	benchmarkMutex(b, true, true)
 }
+
+func BenchmarkMutexNoSpin(b *testing.B) {
+	// This benchmark models a situation where spinning in the mutex should be
+	// non-profitable and allows to confirm that spinning does not do harm.
+	// To achieve this we create excess of goroutines most of which do local work.
+	// These goroutines yield during local work, so that switching from
+	// a blocked goroutine to other goroutines is profitable.
+	// As a matter of fact, this benchmark still triggers some spinning in the mutex.
+	var m Mutex
+	var acc0, acc1 uint64
+	b.SetParallelism(4)
+	b.RunParallel(func(pb *testing.PB) {
+		c := make(chan bool)
+		var data [4 << 10]uint64
+		for i := 0; pb.Next(); i++ {
+			if i%4 == 0 {
+				m.Lock()
+				acc0 -= 100
+				acc1 += 100
+				m.Unlock()
+			} else {
+				for i := 0; i < len(data); i += 4 {
+					data[i]++
+				}
+				// Elaborate way to say runtime.Gosched
+				// that does not put the goroutine onto global runq.
+				go func() {
+					c <- true
+				}()
+				<-c
+			}
+		}
+	})
+}
+
+func BenchmarkMutexSpin(b *testing.B) {
+	// This benchmark models a situation where spinning in the mutex should be
+	// profitable. To achieve this we create a goroutine per-proc.
+	// These goroutines access considerable amount of local data so that
+	// unnecessary rescheduling is penalized by cache misses.
+	var m Mutex
+	var acc0, acc1 uint64
+	b.RunParallel(func(pb *testing.PB) {
+		var data [16 << 10]uint64
+		for i := 0; pb.Next(); i++ {
+			m.Lock()
+			acc0 -= 100
+			acc1 += 100
+			m.Unlock()
+			for i := 0; i < len(data); i += 4 {
+				data[i]++
+			}
+		}
+	})
+}
diff --git a/libgo/go/sync/runtime.go b/libgo/go/sync/runtime.go
index 3b86630..c66d2de 100644
--- a/libgo/go/sync/runtime.go
+++ b/libgo/go/sync/runtime.go
@@ -38,3 +38,10 @@ func init() {
 	var s syncSema
 	runtime_Syncsemcheck(unsafe.Sizeof(s))
 }
+
+// Active spinning runtime support.
+// runtime_canSpin returns true is spinning makes sense at the moment.
+func runtime_canSpin(i int) bool
+
+// runtime_doSpin does active spinning.
+func runtime_doSpin()
diff --git a/libgo/go/sync/waitgroup.go b/libgo/go/sync/waitgroup.go
index 92cc57d..de399e6 100644
--- a/libgo/go/sync/waitgroup.go
+++ b/libgo/go/sync/waitgroup.go
@@ -15,23 +15,21 @@ import (
 // runs and calls Done when finished.  At the same time,
 // Wait can be used to block until all goroutines have finished.
 type WaitGroup struct {
-	m       Mutex
-	counter int32
-	waiters int32
-	sema    *uint32
+	// 64-bit value: high 32 bits are counter, low 32 bits are waiter count.
+	// 64-bit atomic operations require 64-bit alignment, but 32-bit
+	// compilers do not ensure it. So we allocate 12 bytes and then use
+	// the aligned 8 bytes in them as state.
+	state1 [12]byte
+	sema   uint32
 }
 
-// WaitGroup creates a new semaphore each time the old semaphore
-// is released. This is to avoid the following race:
-//
-// G1: Add(1)
-// G1: go G2()
-// G1: Wait() // Context switch after Unlock() and before Semacquire().
-// G2: Done() // Release semaphore: sema == 1, waiters == 0. G1 doesn't run yet.
-// G3: Wait() // Finds counter == 0, waiters == 0, doesn't block.
-// G3: Add(1) // Makes counter == 1, waiters == 0.
-// G3: go G4()
-// G3: Wait() // G1 still hasn't run, G3 finds sema == 1, unblocked! Bug.
+func (wg *WaitGroup) state() *uint64 {
+	if uintptr(unsafe.Pointer(&wg.state1))%8 == 0 {
+		return (*uint64)(unsafe.Pointer(&wg.state1))
+	} else {
+		return (*uint64)(unsafe.Pointer(&wg.state1[4]))
+	}
+}
 
 // Add adds delta, which may be negative, to the WaitGroup counter.
 // If the counter becomes zero, all goroutines blocked on Wait are released.
@@ -43,10 +41,13 @@ type WaitGroup struct {
 // at any time.
 // Typically this means the calls to Add should execute before the statement
 // creating the goroutine or other event to be waited for.
+// If a WaitGroup is reused to wait for several independent sets of events,
+// new Add calls must happen after all previous Wait calls have returned.
 // See the WaitGroup example.
 func (wg *WaitGroup) Add(delta int) {
+	statep := wg.state()
 	if raceenabled {
-		_ = wg.m.state // trigger nil deref early
+		_ = *statep // trigger nil deref early
 		if delta < 0 {
 			// Synchronize decrements with Wait.
 			raceReleaseMerge(unsafe.Pointer(wg))
@@ -54,7 +55,9 @@ func (wg *WaitGroup) Add(delta int) {
 		raceDisable()
 		defer raceEnable()
 	}
-	v := atomic.AddInt32(&wg.counter, int32(delta))
+	state := atomic.AddUint64(statep, uint64(delta)<<32)
+	v := int32(state >> 32)
+	w := uint32(state)
 	if raceenabled {
 		if delta > 0 && v == int32(delta) {
 			// The first increment must be synchronized with Wait.
@@ -66,18 +69,25 @@ func (wg *WaitGroup) Add(delta int) {
 	if v < 0 {
 		panic("sync: negative WaitGroup counter")
 	}
-	if v > 0 || atomic.LoadInt32(&wg.waiters) == 0 {
+	if w != 0 && delta > 0 && v == int32(delta) {
+		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
+	}
+	if v > 0 || w == 0 {
 		return
 	}
-	wg.m.Lock()
-	if atomic.LoadInt32(&wg.counter) == 0 {
-		for i := int32(0); i < wg.waiters; i++ {
-			runtime_Semrelease(wg.sema)
-		}
-		wg.waiters = 0
-		wg.sema = nil
+	// This goroutine has set counter to 0 when waiters > 0.
+	// Now there can't be concurrent mutations of state:
+	// - Adds must not happen concurrently with Wait,
+	// - Wait does not increment waiters if it sees counter == 0.
+	// Still do a cheap sanity check to detect WaitGroup misuse.
+	if *statep != state {
+		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
+	}
+	// Reset waiters count to 0.
+	*statep = 0
+	for ; w != 0; w-- {
+		runtime_Semrelease(&wg.sema)
 	}
-	wg.m.Unlock()
 }
 
 // Done decrements the WaitGroup counter.
@@ -87,51 +97,41 @@ func (wg *WaitGroup) Done() {
 
 // Wait blocks until the WaitGroup counter is zero.
 func (wg *WaitGroup) Wait() {
+	statep := wg.state()
 	if raceenabled {
-		_ = wg.m.state // trigger nil deref early
+		_ = *statep // trigger nil deref early
 		raceDisable()
 	}
-	if atomic.LoadInt32(&wg.counter) == 0 {
-		if raceenabled {
-			raceEnable()
-			raceAcquire(unsafe.Pointer(wg))
-		}
-		return
-	}
-	wg.m.Lock()
-	w := atomic.AddInt32(&wg.waiters, 1)
-	// This code is racing with the unlocked path in Add above.
-	// The code above modifies counter and then reads waiters.
-	// We must modify waiters and then read counter (the opposite order)
-	// to avoid missing an Add.
-	if atomic.LoadInt32(&wg.counter) == 0 {
-		atomic.AddInt32(&wg.waiters, -1)
-		if raceenabled {
-			raceEnable()
-			raceAcquire(unsafe.Pointer(wg))
-			raceDisable()
+	for {
+		state := atomic.LoadUint64(statep)
+		v := int32(state >> 32)
+		w := uint32(state)
+		if v == 0 {
+			// Counter is 0, no need to wait.
+			if raceenabled {
+				raceEnable()
+				raceAcquire(unsafe.Pointer(wg))
+			}
+			return
 		}
-		wg.m.Unlock()
-		if raceenabled {
-			raceEnable()
+		// Increment waiters count.
+		if atomic.CompareAndSwapUint64(statep, state, state+1) {
+			if raceenabled && w == 0 {
+				// Wait must be synchronized with the first Add.
+				// Need to model this is as a write to race with the read in Add.
+				// As a consequence, can do the write only for the first waiter,
+				// otherwise concurrent Waits will race with each other.
+				raceWrite(unsafe.Pointer(&wg.sema))
+			}
+			runtime_Semacquire(&wg.sema)
+			if *statep != 0 {
+				panic("sync: WaitGroup is reused before previous Wait has returned")
+			}
+			if raceenabled {
+				raceEnable()
+				raceAcquire(unsafe.Pointer(wg))
+			}
+			return
 		}
-		return
-	}
-	if raceenabled && w == 1 {
-		// Wait must be synchronized with the first Add.
-		// Need to model this is as a write to race with the read in Add.
-		// As a consequence, can do the write only for the first waiter,
-		// otherwise concurrent Waits will race with each other.
-		raceWrite(unsafe.Pointer(&wg.sema))
-	}
-	if wg.sema == nil {
-		wg.sema = new(uint32)
-	}
-	s := wg.sema
-	wg.m.Unlock()
-	runtime_Semacquire(s)
-	if raceenabled {
-		raceEnable()
-		raceAcquire(unsafe.Pointer(wg))
 	}
 }
diff --git a/libgo/go/sync/waitgroup_test.go b/libgo/go/sync/waitgroup_test.go
index 4c0a043..3e3e3bf 100644
--- a/libgo/go/sync/waitgroup_test.go
+++ b/libgo/go/sync/waitgroup_test.go
@@ -5,6 +5,7 @@
 package sync_test
 
 import (
+	"runtime"
 	. "sync"
 	"sync/atomic"
 	"testing"
@@ -46,6 +47,12 @@ func TestWaitGroup(t *testing.T) {
 	}
 }
 
+func knownRacy(t *testing.T) {
+	if RaceEnabled {
+		t.Skip("skipping known-racy test under the race detector")
+	}
+}
+
 func TestWaitGroupMisuse(t *testing.T) {
 	defer func() {
 		err := recover()
@@ -60,6 +67,95 @@ func TestWaitGroupMisuse(t *testing.T) {
 	t.Fatal("Should panic")
 }
 
+func TestWaitGroupMisuse2(t *testing.T) {
+	knownRacy(t)
+	if testing.Short() {
+		t.Skip("skipping flaky test in short mode; see issue 11443")
+	}
+	if runtime.NumCPU() <= 2 {
+		t.Skip("NumCPU<=2, skipping: this test requires parallelism")
+	}
+	defer func() {
+		err := recover()
+		if err != "sync: negative WaitGroup counter" &&
+			err != "sync: WaitGroup misuse: Add called concurrently with Wait" &&
+			err != "sync: WaitGroup is reused before previous Wait has returned" {
+			t.Fatalf("Unexpected panic: %#v", err)
+		}
+	}()
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
+	done := make(chan interface{}, 2)
+	// The detection is opportunistically, so we want it to panic
+	// at least in one run out of a million.
+	for i := 0; i < 1e6; i++ {
+		var wg WaitGroup
+		wg.Add(1)
+		go func() {
+			defer func() {
+				done <- recover()
+			}()
+			wg.Wait()
+		}()
+		go func() {
+			defer func() {
+				done <- recover()
+			}()
+			wg.Add(1) // This is the bad guy.
+			wg.Done()
+		}()
+		wg.Done()
+		for j := 0; j < 2; j++ {
+			if err := <-done; err != nil {
+				panic(err)
+			}
+		}
+	}
+	t.Fatal("Should panic")
+}
+
+func TestWaitGroupMisuse3(t *testing.T) {
+	knownRacy(t)
+	if runtime.NumCPU() <= 1 {
+		t.Skip("NumCPU==1, skipping: this test requires parallelism")
+	}
+	defer func() {
+		err := recover()
+		if err != "sync: negative WaitGroup counter" &&
+			err != "sync: WaitGroup misuse: Add called concurrently with Wait" &&
+			err != "sync: WaitGroup is reused before previous Wait has returned" {
+			t.Fatalf("Unexpected panic: %#v", err)
+		}
+	}()
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
+	done := make(chan interface{}, 1)
+	// The detection is opportunistically, so we want it to panic
+	// at least in one run out of a million.
+	for i := 0; i < 1e6; i++ {
+		var wg WaitGroup
+		wg.Add(1)
+		go func() {
+			wg.Done()
+		}()
+		go func() {
+			defer func() {
+				done <- recover()
+			}()
+			wg.Wait()
+			// Start reusing the wg before waiting for the Wait below to return.
+			wg.Add(1)
+			go func() {
+				wg.Done()
+			}()
+			wg.Wait()
+		}()
+		wg.Wait()
+		if err := <-done; err != nil {
+			panic(err)
+		}
+	}
+	t.Fatal("Should panic")
+}
+
 func TestWaitGroupRace(t *testing.T) {
 	// Run this test for about 1ms.
 	for i := 0; i < 1000; i++ {
@@ -85,6 +181,19 @@ func TestWaitGroupRace(t *testing.T) {
 	}
 }
 
+func TestWaitGroupAlign(t *testing.T) {
+	type X struct {
+		x  byte
+		wg WaitGroup
+	}
+	var x X
+	x.wg.Add(1)
+	go func(x *X) {
+		x.wg.Done()
+	}(&x)
+	x.wg.Wait()
+}
+
 func BenchmarkWaitGroupUncontended(b *testing.B) {
 	type PaddedWaitGroup struct {
 		WaitGroup
@@ -146,3 +255,17 @@ func BenchmarkWaitGroupWait(b *testing.B) {
 func BenchmarkWaitGroupWaitWork(b *testing.B) {
 	benchmarkWaitGroupWait(b, 100)
 }
+
+func BenchmarkWaitGroupActuallyWait(b *testing.B) {
+	b.ReportAllocs()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			var wg WaitGroup
+			wg.Add(1)
+			go func() {
+				wg.Done()
+			}()
+			wg.Wait()
+		}
+	})
+}