os/signal: make TestStop resilient to initially-blocked signals

For reasons unknown, SIGUSR1 appears to be blocked at process start
for tests on the android-arm-corellium and android-arm64-corellium
builders. (This has been observed before, too: see CL 203957.)
Make the test resilient to blocked signals by always calling Notify
and waiting for potential signal delivery after sending any signal
that is not known to be unblocked.

Also remove the initial SIGWINCH signal from testCancel. The behavior
of an unhandled SIGWINCH is already tested in TestStop, so we don't
need to re-test that same case: waiting for an unhandled signal takes
a comparatively long time (because we necessarily don't know when it
has been delivered), so this redundancy makes the overall test binary
needlessly slow, especially since it is called from both TestReset and
TestIgnore.

Since each signal is always unblocked while we have a notification
channel registered for it, we don't need to modify any other tests:
TestStop and testCancel are the only functions that send signals
without a registered channel.

Fixes #38165
Updates #33174
Updates #15661

Change-Id: I215880894e954b62166024085050d34323431b63
Reviewed-on: https://go-review.googlesource.com/c/go/+/226461
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: Bryan C. Mills

src/os/signal/signal_test.go

@@ -152,10 +152,6 @@ func TestStress(t *testing.T) {
 }
 
 func testCancel(t *testing.T, ignore bool) {
-	// Send SIGWINCH. By default this signal should be ignored.
-	syscall.Kill(syscall.Getpid(), syscall.SIGWINCH)
-	quiesce()
-
 	// Ask to be notified on c1 when a SIGWINCH is received.
 	c1 := make(chan os.Signal, 1)
 	Notify(c1, syscall.SIGWINCH)
@@ -175,25 +171,16 @@ func testCancel(t *testing.T, ignore bool) {
 	waitSig(t, c2, syscall.SIGHUP)
 
 	// Ignore, or reset the signal handlers for, SIGWINCH and SIGHUP.
+	// Either way, this should undo both calls to Notify above.
 	if ignore {
 		Ignore(syscall.SIGWINCH, syscall.SIGHUP)
+		// Don't bother deferring a call to Reset: it is documented to undo Notify,
+		// but its documentation says nothing about Ignore, and (as of the time of
+		// writing) it empirically does not undo an Ignore.
 	} else {
 		Reset(syscall.SIGWINCH, syscall.SIGHUP)
 	}
 
-	// At this point we do not expect any further signals on c1.
-	// However, it is just barely possible that the initial SIGWINCH
-	// at the start of this function was delivered after we called
-	// Notify on c1. In that case the waitSig for SIGWINCH may have
-	// picked up that initial SIGWINCH, and the second SIGWINCH may
-	// then have been delivered on the channel. This sequence of events
-	// may have caused issue 15661.
-	// So, read any possible signal from the channel now.
-	select {
-	case <-c1:
-	default:
-	}
-
 	// Send this process a SIGWINCH. It should be ignored.
 	syscall.Kill(syscall.Getpid(), syscall.SIGWINCH)
 
@@ -206,20 +193,24 @@ func testCancel(t *testing.T, ignore bool) {
 
 	select {
 	case s := <-c1:
-		t.Fatalf("unexpected signal %v", s)
+		t.Errorf("unexpected signal %v", s)
 	default:
 		// nothing to read - good
 	}
 
 	select {
 	case s := <-c2:
-		t.Fatalf("unexpected signal %v", s)
+		t.Errorf("unexpected signal %v", s)
 	default:
 		// nothing to read - good
 	}
 
-	// Reset the signal handlers for all signals.
-	Reset()
+	// One or both of the signals may have been blocked for this process
+	// by the calling process.
+	// Discard any queued signals now to avoid interfering with other tests.
+	Notify(c1, syscall.SIGWINCH)
+	Notify(c2, syscall.SIGHUP)
+	quiesce()
 }
 
 // Test that Reset cancels registration for listed signals on all channels.
@@ -313,61 +304,66 @@ func TestStop(t *testing.T) {
 			// Test the three different signals concurrently.
 			t.Parallel()
 
-			// Send the signal.
+			// If the signal is not ignored, send the signal before registering a
+			// channel to verify the behavior of the default Go handler.
 			// If it's SIGWINCH or SIGUSR1 we should not see it.
 			// If it's SIGHUP, maybe we'll die. Let the flag tell us what to do.
-			switch sig {
-			case syscall.SIGHUP:
-				if *sendUncaughtSighup == 1 {
-					syscall.Kill(syscall.Getpid(), sig)
-					for *dieFromSighup {
-						quiesce()
-					}
-				}
-			default:
+			mayHaveBlockedSignal := false
+			if !Ignored(sig) && (sig != syscall.SIGHUP || *sendUncaughtSighup == 1) {
 				syscall.Kill(syscall.Getpid(), sig)
+				quiesce()
+
+				// We don't know whether sig is blocked for this process; see
+				// https://golang.org/issue/38165. Assume that it could be.
+				mayHaveBlockedSignal = true
 			}
-			quiesce()
 
 			// Ask for signal
 			c := make(chan os.Signal, 1)
 			Notify(c, sig)
 
-			// Send this process that signal
+			// Send this process the signal again.
 			syscall.Kill(syscall.Getpid(), sig)
 			waitSig(t, c, sig)
 
+			if mayHaveBlockedSignal {
+				// We may have received a queued initial signal in addition to the one
+				// that we sent after Notify. If so, waitSig may have observed that
+				// initial signal instead of the second one, and we may need to wait for
+				// the second signal to clear. Do that now.
+				quiesce()
+				select {
+				case <-c:
+				default:
+				}
+			}
+
 			// Stop watching for the signal and send it again.
 			// If it's SIGHUP, maybe we'll die. Let the flag tell us what to do.
 			Stop(c)
-			switch sig {
-			case syscall.SIGHUP:
-				if *sendUncaughtSighup == 2 {
-					syscall.Kill(syscall.Getpid(), sig)
-					for *dieFromSighup {
-						quiesce()
-					}
-				}
-			default:
+			if sig != syscall.SIGHUP || *sendUncaughtSighup == 2 {
 				syscall.Kill(syscall.Getpid(), sig)
-			}
+				quiesce()
 
-			quiesce()
-			select {
-			case s := <-c:
-				if sig == syscall.SIGUSR1 && s == syscall.SIGUSR1 && runtime.GOOS == "android" {
-					testenv.SkipFlaky(t, 38165)
+				select {
+				case s := <-c:
+					t.Errorf("unexpected signal %v", s)
+				default:
+					// nothing to read - good
 				}
-				t.Fatalf("unexpected signal %v", s)
-			default:
-				// nothing to read - good
+
+				// If we're going to receive a signal, it has almost certainly been
+				// received by now. However, it may have been blocked for this process —
+				// we don't know. Explicitly unblock it and wait for it to clear now.
+				Notify(c, sig)
+				quiesce()
+				Stop(c)
 			}
 		})
 	}
 }
 
-// Test that when run under nohup, an uncaught SIGHUP does not kill the program,
-// but a
+// Test that when run under nohup, an uncaught SIGHUP does not kill the program.
 func TestNohup(t *testing.T) {
 	// Ugly: ask for SIGHUP so that child will not have no-hup set
 	// even if test is running under nohup environment.