os/signal: add func Ignored(sig Signal) bool

Ignored reports whether sig is currently ignored.

This implementation only works applies on Unix systems for now.  However, at
the moment that is also the case for Ignore() and several other signal
interaction methods, so that seems fair.

Fixes #22497

Change-Id: I7c1b1a5e12373ca5da44709500ff5acedc6f1316
Reviewed-on: https://go-review.googlesource.com/108376
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/signal/signal_test.go

@@ -192,6 +192,65 @@ func TestIgnore(t *testing.T) {
 	testCancel(t, true)
 }
 
+// Test that Ignored correctly detects changes to the ignored status of a signal.
+func TestIgnored(t *testing.T) {
+	// Ask to be notified on SIGWINCH.
+	c := make(chan os.Signal, 1)
+	Notify(c, syscall.SIGWINCH)
+
+	// If we're being notified, then the signal should not be ignored.
+	if Ignored(syscall.SIGWINCH) {
+		t.Errorf("expected SIGWINCH to not be ignored.")
+	}
+	Stop(c)
+	Ignore(syscall.SIGWINCH)
+
+	// We're no longer paying attention to this signal.
+	if !Ignored(syscall.SIGWINCH) {
+		t.Errorf("expected SIGWINCH to be ignored when explicitly ignoring it.")
+	}
+
+	Reset()
+}
+
+var checkSighupIgnored = flag.Bool("check_sighup_ignored", false, "if true, TestDetectNohup will fail if SIGHUP is not ignored.")
+
+// Test that Ignored(SIGHUP) correctly detects whether it is being run under nohup.
+func TestDetectNohup(t *testing.T) {
+	if *checkSighupIgnored {
+		if !Ignored(syscall.SIGHUP) {
+			t.Fatal("SIGHUP is not ignored.")
+		} else {
+			t.Log("SIGHUP is ignored.")
+		}
+	} else {
+		defer Reset()
+		// Ugly: ask for SIGHUP so that child will not have no-hup set
+		// even if test is running under nohup environment.
+		// We have no intention of reading from c.
+		c := make(chan os.Signal, 1)
+		Notify(c, syscall.SIGHUP)
+		if out, err := exec.Command(os.Args[0], "-test.run=TestDetectNohup", "-check_sighup_ignored").CombinedOutput(); err == nil {
+			t.Errorf("ran test with -check_sighup_ignored and it succeeded: expected failure.\nOutput:\n%s", out)
+		}
+		Stop(c)
+		// Again, this time with nohup, assuming we can find it.
+		_, err := os.Stat("/usr/bin/nohup")
+		if err != nil {
+			t.Skip("cannot find nohup; skipping second half of test")
+		}
+		Ignore(syscall.SIGHUP)
+		os.Remove("nohup.out")
+		out, err := exec.Command("/usr/bin/nohup", os.Args[0], "-test.run=TestDetectNohup", "-check_sighup_ignored").CombinedOutput()
+
+		data, _ := ioutil.ReadFile("nohup.out")
+		os.Remove("nohup.out")
+		if err != nil {
+			t.Errorf("ran test with -check_sighup_ignored under nohup and it failed: expected success.\nError: %v\nOutput:\n%s%s", err, out, data)
+		}
+	}
+}
+
 var sendUncaughtSighup = flag.Int("send_uncaught_sighup", 0, "send uncaught SIGHUP during TestStop")
 
 // Test that Stop cancels the channel's registrations.