runtime: add fake time support back.

Revived from CL 15690048.

Fixes #5356.

LGTM=rsc
R=adg, dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/101400043

src/runtime/proc.c

@@ -2758,6 +2758,8 @@ static void
 checkdead(void)
 {
 	G *gp;
+	P *p;
+	M *mp;
 	int32 run, grunning, s;
 	uintptr i;
 
@@ -2799,6 +2801,24 @@ checkdead(void)
 	runtime·unlock(&runtime·allglock);
 	if(grunning == 0)  // possible if main goroutine calls runtime·Goexit()
 		runtime·throw("no goroutines (main called runtime.Goexit) - deadlock!");
+
+	// Maybe jump time forward for playground.
+	if((gp = runtime·timejump()) != nil) {
+		runtime·casgstatus(gp, Gwaiting, Grunnable);
+		globrunqput(gp);
+ 		p = pidleget();
+ 		if(p == nil)
+ 			runtime·throw("checkdead: no p for timer");
+ 		mp = mget();
+ 		if(mp == nil)
+ 			newm(nil, p);
+ 		else {
+ 			mp->nextp = p;
+ 			runtime·notewakeup(&mp->park);
+ 		}
+ 		return;
+ 	}
+
 	g->m->throwing = -1;  // do not dump full stacks
 	runtime·throw("all goroutines are asleep - deadlock!");
 }

src/runtime/rt0_nacl_amd64p32.s

@@ -25,6 +25,6 @@ TEXT _rt0_amd64p32_nacl(SB),NOSPLIT,$16
 
 TEXT main(SB),NOSPLIT,$0
 	// Uncomment for fake time like on Go Playground.
-	//MOVQ	$1257894000000000000, AX
+	MOVQ	$1257894000000000000, AX
-	//MOVQ	AX, runtime·timens(SB)
+	MOVQ	AX, runtime·faketime(SB)
 	JMP	runtime·rt0_go(SB)

src/runtime/sys_nacl_amd64p32.s

@@ -60,7 +60,7 @@ TEXT syscall·naclWrite(SB), NOSPLIT, $24-20
 TEXT runtime·write(SB),NOSPLIT,$16-20
 	// If using fake time and writing to stdout or stderr,
 	// emit playback header before actual data.
-	MOVQ runtime·timens(SB), AX
+	MOVQ runtime·faketime(SB), AX
 	CMPQ AX, $0
 	JEQ write
 	MOVL fd+0(FP), DI
@@ -242,7 +242,7 @@ TEXT runtime·mmap(SB),NOSPLIT,$8
 	RET
 
 TEXT time·now(SB),NOSPLIT,$16
-	MOVQ runtime·timens(SB), AX
+	MOVQ runtime·faketime(SB), AX
 	CMPQ AX, $0
 	JEQ realtime
 	MOVQ $0, DX
@@ -277,7 +277,7 @@ TEXT runtime·nacl_clock_gettime(SB),NOSPLIT,$0
 	RET
 
 TEXT runtime·nanotime(SB),NOSPLIT,$16
-	MOVQ runtime·timens(SB), AX
+	MOVQ runtime·faketime(SB), AX
 	CMPQ AX, $0
 	JEQ 3(PC)
 	MOVQ	AX, ret+0(FP)

src/runtime/time.go

@@ -35,8 +35,8 @@ var timers struct {
 	t            []*timer
 }
 
-// nacl fake time support.
+// nacl fake time support - time in nanoseconds since 1970
-var timens int64
+var faketime int64
 
 // Package time APIs.
 // Godoc uses the comments in package time, not these.
@@ -194,7 +194,7 @@ func timerproc() {
 			f(arg, seq)
 			lock(&timers.lock)
 		}
-		if delta < 0 {
+		if delta < 0 || faketime > 0 {
 			// No timers left - put goroutine to sleep.
 			timers.rescheduling = true
 			goparkunlock(&timers.lock, "timer goroutine (idle)")
@@ -208,6 +208,29 @@ func timerproc() {
 	}
 }
 
+func timejump() *g {
+	if faketime == 0 {
+		return nil
+	}
+
+	lock(&timers.lock)
+	if !timers.created || len(timers.t) == 0 {
+		unlock(&timers.lock)
+		return nil
+	}
+
+	var gp *g
+	if faketime < timers.t[0].when {
+		faketime = timers.t[0].when
+		if timers.rescheduling {
+			timers.rescheduling = false
+			gp = timers.gp
+		}
+	}
+	unlock(&timers.lock)
+	return gp
+}
+
 // Heap maintenance algorithms.
 
 func siftupTimer(i int) {