runtime: clear locked bit when goroutine exits

Otherwise the next goroutine run on the m
can get inadvertently locked if it executes a cgo call
that turns on the internal lock.

While we're here, fix the cgo panic unwind to
decrement m->ncgo like the non-panic unwind does.

Fixes #4971.

R=golang-dev, iant, dvyukov
CC=golang-dev
https://golang.org/cl/7627043

src/pkg/runtime/cgocall.c

@@ -95,7 +95,8 @@ static void unwindm(void);
 
 // Call from Go to C.
 
-static FuncVal unlockOSThread = { runtime·unlockOSThread };
+static void endcgo(void);
+static FuncVal endcgoV = { endcgo };
 
 void
 runtime·cgocall(void (*fn)(void*), void *arg)
@@ -123,7 +124,7 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 	 * cgo callback. Add entry to defer stack in case of panic.
 	 */
 	runtime·lockOSThread();
-	d.fn = &unlockOSThread;
+	d.fn = &endcgoV;
 	d.siz = 0;
 	d.link = g->defer;
 	d.argp = (void*)-1;  // unused because unlockm never recovers
@@ -148,6 +149,16 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 	runtime·asmcgocall(fn, arg);
 	runtime·exitsyscall();
 
+	if(g->defer != &d || d.fn != &endcgoV)
+		runtime·throw("runtime: bad defer entry in cgocallback");
+	g->defer = d.link;
+	endcgo();
+}
+
+static void
+endcgo(void)
+{
+	runtime·unlockOSThread();
 	m->ncgo--;
 	if(m->ncgo == 0) {
 		// We are going back to Go and are not in a recursive
@@ -156,11 +167,6 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 		m->cgomal = nil;
 	}
 
-	if(g->defer != &d || d.fn != &unlockOSThread)
-		runtime·throw("runtime: bad defer entry in cgocallback");
-	g->defer = d.link;
-	runtime·unlockOSThread();
-
 	if(raceenabled)
 		runtime·raceacquire(&cgosync);
 }

src/pkg/runtime/proc.c

@@ -1173,6 +1173,11 @@ goexit0(G *gp)
 	gp->lockedm = nil;
 	m->curg = nil;
 	m->lockedg = nil;
+	if(m->locked & ~LockExternal) {
+		runtime·printf("invalid m->locked = %d", m->locked);
+		runtime·throw("internal lockOSThread error");
+	}	
+	m->locked = 0;
 	runtime·unwindstack(gp, nil);
 	gfput(m->p, gp);
 	schedule();

src/pkg/runtime/syscall_windows_test.go

@@ -172,9 +172,6 @@ func TestCallbackGC(t *testing.T) {
 }
 
 func TestCallbackPanic(t *testing.T) {
-	// TODO(brainman): http://golang.org/issue/4971
-	t.Skip("TestCallbackPanic disabled: http://golang.org/issue/4971")
-
 	// Make sure panic during callback unwinds properly.
 	if runtime.LockedOSThread() {
 		t.Fatal("locked OS thread on entry to TestCallbackPanic")