runtime: run init on main thread

Fixes #3125.

R=golang-dev, r, minux.ma
CC=golang-dev
https://golang.org/cl/5714049

src/pkg/runtime/mheap.c

@@ -326,7 +326,7 @@ MHeap_FreeLocked(MHeap *h, MSpan *s)
 }
 
 // Release (part of) unused memory to OS.
-// Goroutine created in runtime·schedinit.
+// Goroutine created at startup.
 // Loop forever.
 void
 runtime·MHeap_Scavenger(void)

src/pkg/runtime/proc.c

@@ -209,8 +209,6 @@ runtime·schedinit(void)
 
 	mstats.enablegc = 1;
 	m->nomemprof--;
-
-	scvg = runtime·newproc1((byte*)runtime·MHeap_Scavenger, nil, 0, 0, runtime·schedinit);
 }
 
 extern void main·init(void);
@@ -228,6 +226,7 @@ runtime·main(void)
 	// to preserve the lock.
 	runtime·LockOSThread();
 	runtime·sched.init = true;
+	scvg = runtime·newproc1((byte*)runtime·MHeap_Scavenger, nil, 0, 0, runtime·main);
 	main·init();
 	runtime·sched.init = false;
 	if(!runtime·sched.lockmain)
@@ -587,10 +586,11 @@ top:
 		mput(m);
 	}
 
-	// Look for deadlock situation: one single active g which happens to be scvg.
-	if(runtime·sched.grunning == 1 && runtime·sched.gwait == 0) {
-		if(scvg->status == Grunning || scvg->status == Gsyscall)
-			runtime·throw("all goroutines are asleep - deadlock!");
+	// Look for deadlock situation.
+	if((scvg == nil && runtime·sched.grunning == 0) ||
+	   (scvg != nil && runtime·sched.grunning == 1 && runtime·sched.gwait == 0 &&
+	    (scvg->status == Grunning || scvg->status == Gsyscall))) {
+		runtime·throw("all goroutines are asleep - deadlock!");
 	}
 
 	m->nextg = nil;

src/pkg/runtime/runtime_linux_test.go

@@ -0,0 +1,29 @@
+// Copyright 2012 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.
+
+package runtime_test
+
+import (
+	. "runtime"
+	"syscall"
+	"testing"
+)
+
+var pid, tid int
+
+func init() {
+	// Record pid and tid of init thread for use during test.
+	// The call to LockOSThread is just to exercise it;
+	// we can't test that it does anything.
+	// Instead we're testing that the conditions are good
+	// for how it is used in init (must be on main thread).
+	pid, tid = syscall.Getpid(), syscall.Gettid()
+	LockOSThread()
+}
+
+func TestLockOSThread(t *testing.T) {
+	if pid != tid {
+		t.Fatalf("pid=%d but tid=%d", pid, tid)
+	}
+}