os/signal: make NotifyContext Cause match context.Canceled

Fixes #77639

Change-Id: I90e5d6f68c02749ac6e407f1c3565312efafb033
Reviewed-on: https://go-review.googlesource.com/c/go/+/779860
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

src/os/signal/signal.go

@@ -354,3 +354,10 @@ type signalError string
 func (s signalError) Error() string {
 	return string(s)
 }
+
+func (s signalError) Is(target error) bool {
+	if target == context.Canceled {
+		return true
+	}
+	return false
+}

src/os/signal/signal_test.go

@@ -924,3 +924,25 @@ func TestSignalTrace(t *testing.T) {
 	close(quit)
 	<-done
 }
+
+// #77639.
+func TestNotifyContextCause(t *testing.T) {
+	ctx, stop := NotifyContext(t.Context(), syscall.SIGINT)
+	defer stop()
+	syscall.Kill(syscall.Getpid(), syscall.SIGINT)
+	<-ctx.Done()
+
+	err := ctx.Err()
+	if err == nil {
+		t.Error("ctx.Err returned nil")
+	} else if !errors.Is(err, context.Canceled) {
+		t.Errorf("error %v is not context.Canceled", err)
+	}
+
+	err = context.Cause(ctx)
+	if err == nil {
+		t.Error("context.Cause returned nil")
+	} else if !errors.Is(err, context.Canceled) {
+		t.Errorf("cause %v is not context.Canceled", err)
+	}
+}