runtime: fix timers crash

Timer callbacks occasionally crash
with "sched while holding locks" message.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5381043

src/pkg/runtime/time.goc

@@ -149,6 +149,8 @@ timerproc(void)
 {
 	int64 delta, now;
 	Timer *t;
+	void (*f)(int64, Eface);
+	Eface arg;
 
 	for(;;) {
 		runtime·lock(&timers);
@@ -173,7 +175,11 @@ timerproc(void)
 				siftdown(0);
 				t->i = -1;  // mark as removed
 			}
-			t->f(now, t->arg);
+			f = t->f;
+			arg = t->arg;
+			runtime·unlock(&timers);
+			f(now, arg);
+			runtime·lock(&timers);
 		}
 		if(delta < 0) {
 			// No timers left - put goroutine to sleep.

src/pkg/time/sleep_test.go

@@ -7,7 +7,9 @@ package time_test
 import (
 	"errors"
 	"fmt"
+	"runtime"
 	"sort"
+	"sync/atomic"
 	"testing"
 	. "time"
 )
@@ -47,6 +49,20 @@ func TestAfterFunc(t *testing.T) {
 	<-c
 }
 
+func TestAfterStress(t *testing.T) {
+	stop := uint32(0)
+	go func() {
+		for atomic.LoadUint32(&stop) == 0 {
+			runtime.GC()
+		}
+	}()
+	c := Tick(1)
+	for i := 0; i < 100; i++ {
+		<-c
+	}
+	atomic.StoreUint32(&stop, 1)
+}
+
 func BenchmarkAfterFunc(b *testing.B) {
 	i := b.N
 	c := make(chan bool)