runtime: handle runtime.Goexit during init

Fixes #5963.

R=golang-dev, dsymonds, dvyukov
CC=golang-dev
https://golang.org/cl/11879045

src/pkg/runtime/proc.c

@@ -160,10 +160,14 @@ extern void main·main(void);
 
 static FuncVal scavenger = {runtime·MHeap_Scavenger};
 
+static FuncVal initDone = { runtime·unlockOSThread };
+
 // The main goroutine.
 void
 runtime·main(void)
 {
+	Defer d;
+
 	newm(sysmon, nil);
 
 	// Lock the main goroutine onto this, the main OS thread,
@@ -173,10 +177,24 @@ runtime·main(void)
 	// by calling runtime.LockOSThread during initialization
 	// to preserve the lock.
 	runtime·lockOSThread();
+	
+	// Defer unlock so that runtime.Goexit during init does the unlock too.
+	d.fn = &initDone;
+	d.siz = 0;
+	d.link = g->defer;
+	d.argp = (void*)-1;
+	d.special = true;
+	d.free = false;
+	g->defer = &d;
+
 	if(m != &runtime·m0)
 		runtime·throw("runtime·main not on m0");
 	runtime·newproc1(&scavenger, nil, 0, 0, runtime·main);
 	main·init();
+
+	if(g->defer != &d || d.fn != &initDone)
+		runtime·throw("runtime: bad defer entry after init");
+	g->defer = d.link;
 	runtime·unlockOSThread();
 
 	main·main();

test/fixedbugs/issue5963.go

+// run
+
+// Copyright 2013 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.
+
+// Used to die in runtime due to init goroutine exiting while
+// locked to main thread.
+
+package main
+
+import (
+	"os"
+	"runtime"
+)
+
+func init() {
+	c := make(chan int, 1)
+	defer func() {
+		c <- 0
+	}()
+	go func() {
+		os.Exit(<-c)
+	}()
+	runtime.Goexit()
+}
+
+func main() {
+}
+
+/* Before fix:
+
+invalid m->locked = 2
+fatal error: internal lockOSThread error
+
+goroutine 2 [runnable]:
+runtime.MHeap_Scavenger()
+	/Users/rsc/g/go/src/pkg/runtime/mheap.c:438
+runtime.goexit()
+	/Users/rsc/g/go/src/pkg/runtime/proc.c:1313
+created by runtime.main
+	/Users/rsc/g/go/src/pkg/runtime/proc.c:165
+
+goroutine 3 [runnable]:
+main.func·002()
+	/Users/rsc/g/go/test/fixedbugs/issue5963.go:22
+created by main.init·1
+	/Users/rsc/g/go/test/fixedbugs/issue5963.go:24 +0xb9
+exit status 2
+*/