sync: release Pool memory during second and later GCs

Pool memory was only being released during the first GC after the first Put.

Put assumes that p.local != nil means p is on the allPools list.
poolCleanup (called during each GC) removed each pool from allPools
but did not clear p.local, so each pool was cleared by exactly one GC
and then never cleared again.

This bug was introduced late in the Go 1.3 release cycle.

Fixes #8979.

LGTM=rsc
R=golang-codereviews, bradfitz, r, rsc
CC=golang-codereviews, khr
https://golang.org/cl/162980043

src/sync/pool.go

@@ -200,6 +200,8 @@ func poolCleanup() {
 			}
 			l.shared = nil
 		}
+		p.local = nil
+		p.localSize = 0
 	}
 	allPools = []*Pool{}
 }

src/sync/pool_test.go

@@ -69,32 +69,44 @@ func TestPoolNew(t *testing.T) {
 	}
 }
 
-// Test that Pool does not hold pointers to previously cached
-// resources
+// Test that Pool does not hold pointers to previously cached resources.
 func TestPoolGC(t *testing.T) {
+	testPool(t, true)
+}
+
+// Test that Pool releases resources on GC.
+func TestPoolRelease(t *testing.T) {
+	testPool(t, false)
+}
+
+func testPool(t *testing.T, drain bool) {
 	var p Pool
-	var fin uint32
 	const N = 100
-	for i := 0; i < N; i++ {
-		v := new(string)
-		runtime.SetFinalizer(v, func(vv *string) {
-			atomic.AddUint32(&fin, 1)
-		})
-		p.Put(v)
-	}
-	for i := 0; i < N; i++ {
-		p.Get()
-	}
-	for i := 0; i < 5; i++ {
-		runtime.GC()
-		time.Sleep(time.Duration(i*100+10) * time.Millisecond)
-		// 1 pointer can remain on stack or elsewhere
-		if atomic.LoadUint32(&fin) >= N-1 {
-			return
+loop:
+	for try := 0; try < 3; try++ {
+		var fin, fin1 uint32
+		for i := 0; i < N; i++ {
+			v := new(string)
+			runtime.SetFinalizer(v, func(vv *string) {
+				atomic.AddUint32(&fin, 1)
+			})
+			p.Put(v)
+		}
+		if drain {
+			for i := 0; i < N; i++ {
+				p.Get()
+			}
+		}
+		for i := 0; i < 5; i++ {
+			runtime.GC()
+			time.Sleep(time.Duration(i*100+10) * time.Millisecond)
+			// 1 pointer can remain on stack or elsewhere
+			if fin1 = atomic.LoadUint32(&fin); fin1 >= N-1 {
+				continue loop
+			}
 		}
+		t.Fatalf("only %v out of %v resources are finalized on try %v", fin1, N, try)
 	}
-	t.Fatalf("only %v out of %v resources are finalized",
-		atomic.LoadUint32(&fin), N)
 }
 
 func TestPoolStress(t *testing.T) {