os/signal: completely ignore bogus signals

signal.Notify should ignore bogus signals (syscall.Signals with invalid
signal numbers). Ignoring means Notify/Stop/Reset all work fine, but the
channel will never receive any events.

Today, Stop hangs if Notify has only ever been called with bogus signals
because Stop assumes that runtime.signal_recv is running if a handler is
present. Notify unconditionally registers the handler, but only starts
signal_recv if the signal was valid.

Address this by avoiding registering the handler at all if the signal is
bogus.

We additionally add a bogus signal check to cancel (used by
Ignore/Reset). Currently those are calling into the runtime
signal_ignore and signal_disable, which do ignore bogus signals, but is
now inconsistent with the rest of the package.

For #77076.

Change-Id: I6a6a636c27c41a158e203bbf470be5f1f3f631bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/735040
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

src/os/signal/signal.go

@@ -54,6 +54,10 @@ func cancel(sigs []os.Signal, action func(int)) {
 	defer handlers.Unlock()
 
 	remove := func(n int) {
+		if n < 0 {
+			return
+		}
+
 		var zerohandler handler
 
 		for c, h := range handlers.m {
@@ -127,19 +131,27 @@ func Notify(c chan<- os.Signal, sig ...os.Signal) {
 	handlers.Lock()
 	defer handlers.Unlock()
 
-	h := handlers.m[c]
-	if h == nil {
-		if handlers.m == nil {
-			handlers.m = make(map[chan<- os.Signal]*handler)
+	// Lazily create the handler. If all of the signals are bogus there is
+	// no need to install a handler at all.
+	getHandler := func() *handler {
+		h := handlers.m[c]
+		if h == nil {
+			if handlers.m == nil {
+				handlers.m = make(map[chan<- os.Signal]*handler)
+			}
+			h = new(handler)
+			handlers.m[c] = h
 		}
-		h = new(handler)
-		handlers.m[c] = h
+
+		return h
 	}
 
 	add := func(n int) {
 		if n < 0 {
 			return
 		}
+
+		h := getHandler()
 		if !h.want(n) {
 			h.set(n)
 			if handlers.ref[n] == 0 {

src/runtime/signal_test.go

@@ -0,0 +1,26 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestSignalBogus(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("No syscall.Signal on plan9")
+	}
+
+	// This test more properly belongs in os/signal, but it depends on
+	// containing the only call to signal.Notify in the process, so it must
+	// run as an isolated subprocess, which is simplest with testprog.
+	t.Parallel()
+	output := runTestProg(t, "testprog", "SignalBogus")
+	want := "OK\n"
+	if output != want {
+		t.Fatalf("output is not %q\n%s", want, output)
+	}
+}

src/runtime/testdata/testprog/signal_bogus.go

@@ -0,0 +1,29 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !plan9
+
+package main
+
+import (
+	"os"
+	"os/signal"
+	"syscall"
+)
+
+func init() {
+	register("SignalBogus", SignalBogus)
+}
+
+// signal.Notify should effectively ignore bogus signal numbers. Never writing
+// to the channel, but otherwise allowing Notify/Stop as normal.
+//
+// This is a regression test for https://go.dev/issue/77076, where bogus
+// signals used to make Stop hang if there were no real signals installed.
+func SignalBogus() {
+	ch := make(chan os.Signal, 1)
+	signal.Notify(ch, syscall.Signal(0xdead))
+	signal.Stop(ch)
+	println("OK")
+}