runtime: unblock special glibc signals on each thread

Glibc uses some special signals for special thread operations.  These
signals will be used in programs that use cgo and invoke certain glibc
functions, such as setgid.  In order for this to work, these signals
need to not be masked by any thread.  Before this change, they were
being masked by programs that used os/signal.Notify, because it
carefully masks all non-thread-specific signals in all threads so that a
dedicated thread will collect and report those signals (see ensureSigM
in signal1_unix.go).

This change adds the two glibc special signals to the set of signals
that are unmasked in each thread.

Fixes #12498.

Change-Id: I797d71a099a2169c186f024185d44a2e1972d4ad
Reviewed-on: https://go-review.googlesource.com/14297Reviewed-by: David Crawshaw <crawshaw@golang.org>

misc/cgo/test/setgid_linux.go

@@ -14,11 +14,14 @@ package cgotest
 import "C"
 
 import (
+	"os"
+	"os/signal"
+	"syscall"
 	"testing"
 	"time"
 )
 
-func testSetgid(t *testing.T) {
+func runTestSetgid() bool {
 	c := make(chan bool)
 	go func() {
 		C.setgid(0)
@@ -26,7 +29,21 @@ func testSetgid(t *testing.T) {
 	}()
 	select {
 	case <-c:
+		return true
 	case <-time.After(5 * time.Second):
+		return false
+	}
+
+}
+
+func testSetgid(t *testing.T) {
+	if !runTestSetgid() {
 		t.Error("setgid hung")
 	}
+
+	// Now try it again after using signal.Notify.
+	signal.Notify(make(chan os.Signal, 1), syscall.SIGINT)
+	if !runTestSetgid() {
+		t.Error("setgid hung after signal.Notify")
+	}
 }

src/runtime/signal_linux.go

@@ -44,8 +44,8 @@ var sigtable = [...]sigTabT{
 	/* 29 */ {_SigNotify, "SIGIO: i/o now possible"},
 	/* 30 */ {_SigNotify, "SIGPWR: power failure restart"},
 	/* 31 */ {_SigNotify, "SIGSYS: bad system call"},
-	/* 32 */ {_SigSetStack, "signal 32"}, /* SIGCANCEL; see issue 6997 */
-	/* 33 */ {_SigSetStack, "signal 33"}, /* SIGSETXID; see issue 3871, 9400 */
+	/* 32 */ {_SigSetStack + _SigUnblock, "signal 32"}, /* SIGCANCEL; see issue 6997 */
+	/* 33 */ {_SigSetStack + _SigUnblock, "signal 33"}, /* SIGSETXID; see issues 3871, 9400, 12498 */
 	/* 34 */ {_SigNotify, "signal 34"},
 	/* 35 */ {_SigNotify, "signal 35"},
 	/* 36 */ {_SigNotify, "signal 36"},