ipn/localapi, cli, clientmetric: add ipnbus feature tag; fix omit.go stub

Add a new "ipnbus" build feature tag so the watch-ipn-bus LocalAPI
endpoint can be independently controlled, rather than being gated
behind HasDebug || HasServe. Minimal/embedded builds that omit both
debug and serve were getting 404s on watch-ipn-bus, breaking
"tailscale up --authkey=..." and other CLI flows that depend on
WatchIPNBus.

In the CLI, check buildfeatures.HasIPNBus before attempting to watch
the IPN bus in "tailscale up"/"tailscale login", and exit early with
an informational message when the feature is omitted.

Also add the missing NewCounterFunc stub to clientmetric/omit.go,
which caused compilation errors when building with
ts_omit_clientmetrics and netstack enabled.

Fixes #19240

Change-Id: I2e3c69a72fc50fa02542b91b8a54859618a463d1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/up.go

@@ -599,6 +599,11 @@ func runUp(ctx context.Context, cmd string, args []string, upArgs upArgsT) (retE
 		}
 	}()
 
+	if !buildfeatures.HasIPNBus {
+		fmt.Fprintln(Stderr, "binary built with ts_omit_ipnbus; not waiting for completion")
+		return nil
+	}
+
 	// Start watching the IPN bus before we call Start() or StartLoginInteractive(),
 	// or we could miss IPN notifications.
 	//

feature/buildfeatures/feature_ipnbus_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_ipnbus
+
+package buildfeatures
+
+// HasIPNBus is whether the binary was built with support for modular feature "IPN notification bus (watch-ipn-bus) support, used by GUIs, debugging, and nicer 'tailscale up' support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_ipnbus" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasIPNBus = false

feature/buildfeatures/feature_ipnbus_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_ipnbus
+
+package buildfeatures
+
+// HasIPNBus is whether the binary was built with support for modular feature "IPN notification bus (watch-ipn-bus) support, used by GUIs, debugging, and nicer 'tailscale up' support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_ipnbus" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasIPNBus = true

feature/featuretags/featuretags.go

@@ -168,6 +168,7 @@ var Features = map[FeatureTag]FeatureMeta{
 	"health":             {Sym: "Health", Desc: "Health checking support"},
 	"hujsonconf":         {Sym: "HuJSONConf", Desc: "HuJSON config file support"},
 	"identityfederation": {Sym: "IdentityFederation", Desc: "Auth key generation via identity federation support"},
+	"ipnbus":             {Sym: "IPNBus", Desc: "IPN notification bus (watch-ipn-bus) support, used by GUIs, debugging, and nicer 'tailscale up' support"},
 	"iptables":           {Sym: "IPTables", Desc: "Linux iptables support"},
 	"kube":               {Sym: "Kube", Desc: "Kubernetes integration"},
 	"lazywg":             {Sym: "LazyWG", Desc: "Lazy WireGuard configuration for memory-constrained devices with large netmaps"},

ipn/localapi/localapi.go

@@ -115,7 +115,7 @@ func init() {
 		Register("bugreport", (*Handler).serveBugReport)
 		Register("pprof", (*Handler).servePprof)
 	}
-	if buildfeatures.HasDebug || buildfeatures.HasServe {
+	if buildfeatures.HasIPNBus {
 		Register("watch-ipn-bus", (*Handler).serveWatchIPNBus)
 	}
 	if buildfeatures.HasDNS {

util/clientmetric/omit.go

@@ -26,6 +26,7 @@ func WritePrometheusExpositionFormat(any) {}
 
 var zeroMetric Metric
 
-func NewCounter(string) *Metric          { return &zeroMetric }
+func NewCounter(string) *Metric                   { return &zeroMetric }
-func NewGauge(string) *Metric            { return &zeroMetric }
+func NewGauge(string) *Metric                     { return &zeroMetric }
-func NewAggregateCounter(string) *Metric { return &zeroMetric }
+func NewAggregateCounter(string) *Metric          { return &zeroMetric }
+func NewCounterFunc(string, func() int64) *Metric { return &zeroMetric }