runtime: avoid concurrent use of synctest timer race context.

When running a timer in a synctest bubble, create a temporary
race context for the operation. We can't use the bubble timer's
context, because multiple timers can execute simultaneously within
a bubble.

Fixes #76691

Change-Id: If862dd63333c5b3b2e75d44a078acd0e9da77a8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/753200
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Commit-Queue: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/runtime/time.go

@@ -1126,13 +1126,7 @@ func (t *timer) unlockAndRun(now int64, bubble *synctestBubble) {
 		// Note that we are running on a system stack,
 		// so there is no chance of getg().m being reassigned
 		// out from under us while this function executes.
-		gp := getg()
-		var tsLocal *timers
-		if bubble == nil {
-			tsLocal = &gp.m.p.ptr().timers
-		} else {
-			tsLocal = &bubble.timers
-		}
+		tsLocal := &getg().m.p.ptr().timers
 		if tsLocal.raceCtx == 0 {
 			tsLocal.raceCtx = racegostart(abi.FuncPCABIInternal((*timers).run) + sys.PCQuantum)
 		}
@@ -1184,11 +1178,7 @@ func (t *timer) unlockAndRun(now int64, bubble *synctestBubble) {
 		if gp.racectx != 0 {
 			throw("unexpected racectx")
 		}
-		if bubble == nil {
-			gp.racectx = gp.m.p.ptr().timers.raceCtx
-		} else {
-			gp.racectx = bubble.timers.raceCtx
-		}
+		gp.racectx = gp.m.p.ptr().timers.raceCtx
 	}
 
 	if ts != nil {

src/testing/synctest/synctest_test.go

@@ -188,3 +188,18 @@ func TestNow(t *testing.T) {
 		}
 	})
 }
+
+func TestSynctestTimerRaceCtxCrash(t *testing.T) {
+	for range 20 {
+		t.Run("", func(t *testing.T) {
+			synctest.Test(t, func(t *testing.T) {
+				for range 100 {
+					time.AfterFunc(1*time.Second, func() {
+						time.AfterFunc(0, func() {})
+					})
+				}
+				time.Sleep(2 * time.Second)
+			})
+		})
+	}
+}