runtime: work around NetBSD bug in thread creation

Block signals explicitly during lwp_create since blocking via
the context does not work.

This was originally added in pkgsrc as
http://cvsweb.netbsd.org/bsdweb.cgi/pkgsrc/lang/go/patches/patch-src_runtime_os__netbsd.go?rev=1.1

Fixes #19295.

Change-Id: I431e70c047cff446e9cfc20d356f45df98c63487
Reviewed-on: https://go-review.googlesource.com/41350Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/os_netbsd.go

@@ -167,13 +167,23 @@ func newosproc(mp *m, stk unsafe.Pointer) {
 	var uc ucontextt
 	getcontext(unsafe.Pointer(&uc))
 
+	// _UC_SIGMASK does not seem to work here.
+	// It would be nice if _UC_SIGMASK and _UC_STACK
+	// worked so that we could do all the work setting
+	// the sigmask and the stack here, instead of setting
+	// the mask here and the stack in netbsdMstart.
+	// For now do the blocking manually.
 	uc.uc_flags = _UC_SIGMASK | _UC_CPU
 	uc.uc_link = nil
 	uc.uc_sigmask = sigset_all
 
+	var oset sigset
+	sigprocmask(_SIG_SETMASK, &sigset_all, &oset)
+
 	lwp_mcontext_init(&uc.uc_mcontext, stk, mp, mp.g0, funcPC(netbsdMstart))
 
 	ret := lwp_create(unsafe.Pointer(&uc), 0, unsafe.Pointer(&mp.procid))
+	sigprocmask(_SIG_SETMASK, &oset, nil)
 	if ret < 0 {
 		print("runtime: failed to create new OS thread (have ", mcount()-1, " already; errno=", -ret, ")\n")
 		if ret == -_EAGAIN {