cmd/tailscale: test for new flags in tailscale up

`tailscale set` was created to set preferences, which used to be
overloaded into `tailscale up`. To move people over to the new
command, `up` was supposed to be frozen and no new preference flags
would be added. But people forgot, there was no test to warn them, and
so new flags were added anyway.

TestUpFlagSetIsFrozen complains when new flags are added to
`tailscale up`. It doesn’t try all combinations of GOOS, but since
the CI builds in every OS, the pull-request tests should cover this.

Updates #15460

Signed-off-by: Simon Law <sfllaw@sfllaw.ca>

cmd/tailscale/cli/up_test.go

@@ -0,0 +1,55 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package cli
+
+import (
+	"flag"
+	"testing"
+
+	"tailscale.com/util/set"
+)
+
+// validUpFlags are the only flags that are valid for tailscale up. The up
+// command is frozen: no new preferences can be added. Instead, add them to
+// tailscale set.
+// See tailscale/tailscale#15460.
+var validUpFlags = set.Of(
+	"accept-dns",
+	"accept-risk",
+	"accept-routes",
+	"advertise-connector",
+	"advertise-exit-node",
+	"advertise-routes",
+	"advertise-tags",
+	"auth-key",
+	"exit-node",
+	"exit-node-allow-lan-access",
+	"force-reauth",
+	"host-routes",
+	"hostname",
+	"json",
+	"login-server",
+	"netfilter-mode",
+	"nickname",
+	"operator",
+	"posture-checking",
+	"qr",
+	"reset",
+	"shields-up",
+	"snat-subnet-routes",
+	"ssh",
+	"stateful-filtering",
+	"timeout",
+	"unattended",
+)
+
+// TestUpFlagSetIsFrozen complains when new flags are added to tailscale up.
+func TestUpFlagSetIsFrozen(t *testing.T) {
+	upFlagSet.VisitAll(func(f *flag.Flag) {
+		name := f.Name
+		if !validUpFlags.Contains(name) {
+			t.Errorf("--%s flag added to tailscale up, new prefs go in tailscale set: see tailscale/tailscale#15460", name)
+		}
+	})
+}