runtime/cgo: fix deadlock involving signals on darwin

sigprocmask() is process-wide on darwin, so two concurrent
libcgo_sys_thread_start() can result in all signals permanently
blocked, which in particular blocks handling of nil derefs.
Fixes #4833.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/7324058

src/pkg/runtime/crash_test.go

@@ -99,6 +99,14 @@ func TestLockedDeadlock2(t *testing.T) {
 	testDeadlock(t, lockedDeadlockSource2)
 }
 
+func TestCgoSignalDeadlock(t *testing.T) {
+	got := executeTest(t, cgoSignalDeadlockSource, nil)
+	want := "OK\n"
+	if got != want {
+		t.Fatalf("expected %q, but got %q", want, got)
+	}
+}
+
 const crashSource = `
 package main
 
@@ -183,3 +191,68 @@ func main() {
 	select {}
 }
 `
+
+const cgoSignalDeadlockSource = `
+package main
+
+import "C"
+
+import (
+	"fmt"
+	"runtime"
+	"time"
+)
+
+func main() {
+	runtime.GOMAXPROCS(100)
+	ping := make(chan bool)
+	go func() {
+		for i := 0; ; i++ {
+			runtime.Gosched()
+			select {
+			case done := <-ping:
+				if done {
+					ping <- true
+					return
+				}
+				ping <- true
+			default:
+			}
+			func() {
+				defer func() {
+					recover()
+				}()
+				var s *string
+				*s = ""
+			}()
+		}
+	}()
+	time.Sleep(time.Millisecond)
+	for i := 0; i < 64; i++ {
+		go func() {
+			runtime.LockOSThread()
+			select {}
+		}()
+		go func() {
+			runtime.LockOSThread()
+			select {}
+		}()
+		time.Sleep(time.Millisecond)
+		ping <- false
+		select {
+		case <-ping:
+		case <-time.After(time.Second):
+			fmt.Printf("HANG\n")
+			return
+		}
+	}
+	ping <- true
+	select {
+	case <-ping:
+	case <-time.After(time.Second):
+		fmt.Printf("HANG\n")
+		return
+	}
+	fmt.Printf("OK\n")
+}
+`