os/signal: Stop restores original signal handling

Since Stop was introduced, it would revert to the system default for the
signal, rather than to the default Go behavior.  Change it to revert to
the default Go behavior.

Change-Id: I345467ece0e49e31b2806d6fce2f1937b17905a6
Reviewed-on: https://go-review.googlesource.com/18229Reviewed-by: Russ Cox <rsc@golang.org>

src/os/signal/signal_test.go

@@ -182,13 +182,14 @@ func TestStop(t *testing.T) {
 	sigs := []syscall.Signal{
 		syscall.SIGWINCH,
 		syscall.SIGHUP,
+		syscall.SIGUSR1,
 	}
 
 	for _, sig := range sigs {
 		// Send the signal.
 		// If it's SIGWINCH, we should not see it.
 		// If it's SIGHUP, maybe we'll die. Let the flag tell us what to do.
-		if sig != syscall.SIGHUP || *sendUncaughtSighup == 1 {
+		if sig == syscall.SIGWINCH || (sig == syscall.SIGHUP && *sendUncaughtSighup == 1) {
 			syscall.Kill(syscall.Getpid(), sig)
 		}
 		time.Sleep(100 * time.Millisecond)

src/runtime/signal1_unix.go

@@ -50,26 +50,12 @@ func initsig() {
 		}
 		fwdSig[i] = getsig(i)
 
-		// For some signals, we respect an inherited SIG_IGN handler
-		// rather than insist on installing our own default handler.
-		// Even these signals can be fetched using the os/signal package.
-		switch i {
-		case _SIGHUP, _SIGINT:
-			if fwdSig[i] == _SIG_IGN {
-				continue
+		if !sigInstallGoHandler(i) {
+			// Even if we are not installing a signal handler,
+			// set SA_ONSTACK if necessary.
+			if fwdSig[i] != _SIG_DFL && fwdSig[i] != _SIG_IGN {
+				setsigstack(i)
 			}
-		}
-
-		if t.flags&_SigSetStack != 0 {
-			setsigstack(i)
-			continue
-		}
-
-		// When built using c-archive or c-shared, only
-		// install signal handlers for synchronous signals.
-		// Set SA_ONSTACK for other signals if necessary.
-		if (isarchive || islibrary) && t.flags&_SigPanic == 0 {
-			setsigstack(i)
 			continue
 		}
 
@@ -78,6 +64,31 @@ func initsig() {
 	}
 }
 
+func sigInstallGoHandler(sig int32) bool {
+	// For some signals, we respect an inherited SIG_IGN handler
+	// rather than insist on installing our own default handler.
+	// Even these signals can be fetched using the os/signal package.
+	switch sig {
+	case _SIGHUP, _SIGINT:
+		if fwdSig[sig] == _SIG_IGN {
+			return false
+		}
+	}
+
+	t := &sigtable[sig]
+	if t.flags&_SigSetStack != 0 {
+		return false
+	}
+
+	// When built using c-archive or c-shared, only install signal
+	// handlers for synchronous signals.
+	if (isarchive || islibrary) && t.flags&_SigPanic == 0 {
+		return false
+	}
+
+	return true
+}
+
 func sigenable(sig uint32) {
 	if sig >= uint32(len(sigtable)) {
 		return
@@ -105,7 +116,11 @@ func sigdisable(sig uint32) {
 		ensureSigM()
 		disableSigChan <- sig
 		<-maskUpdatedChan
-		if t.flags&_SigHandling != 0 {
+
+		// If initsig does not install a signal handler for a
+		// signal, then to go back to the state before Notify
+		// we should remove the one we installed.
+		if !sigInstallGoHandler(int32(sig)) {
 			t.flags &^= _SigHandling
 			setsig(int32(sig), fwdSig[sig], true)
 		}