runtime: fix init scheduling bug.

if there is a goroutine waiting to run
and the init goroutine enters a system call,
entersyscall was trying to kick off a new
scheduler for the other goroutine, causing
a panic (new goroutines can't run until main.main).

R=r
DELTA=32  (32 added, 0 deleted, 0 changed)
OCL=31982
CL=31982

src/pkg/runtime/proc.c

@@ -478,6 +478,10 @@ sys·entersyscall(uint64 callerpc, int64 trap)
 	USED(callerpc, trap);
 
 	lock(&sched);
+	if(sched.predawn) {
+		unlock(&sched);
+		return;
+	}
 	g->status = Gsyscall;
 	// Leave SP around for gc and traceback.
 	// Do before notewakeup so that gc
@@ -502,6 +506,10 @@ void
 sys·exitsyscall(void)
 {
 	lock(&sched);
+	if(sched.predawn) {
+		unlock(&sched);
+		return;
+	}
 	g->status = Grunning;
 	sched.msyscall--;
 	sched.mcpu++;

test/initsyscall.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This used to crash because the scheduler
+// tried to kick off a new scheduling thread for f
+// when time.Nanoseconds went into the system call.
+// It's not okay to schedule new goroutines
+// until main has started.
+
+package main
+
+import "log"
+import "time"
+
+func f() {
+}
+
+func init() {
+	go f();
+	time.Nanoseconds();
+}
+
+func main() {
+}
+