wgengine,cmd/tailscaled,feature/bird: move BIRD integration to ./feature

The BIRD (BGP) integration previously lived half in cmd/tailscaled
(which created a chirp client via a build-tag-gated file on some
platforms) and half in wgengine (which carried the client in its
Config and toggled the "tailscale" protocol as the node gained or
lost primary subnet router duty).

Move it all to a new feature/bird package, installed on the engine
via the new wgengine.HookNewBird hook, like other feature/* packages.
wgengine.Config.BIRDClient (and the wgengine.BIRDClient interface)
are replaced by a BIRDSocket path from which the engine constructs
the feature's Bird handle at startup. The subnet router overlap
detection and protocol toggling move into feature/bird, preserving
the previous ordering: state is recomputed before Reconfig's
ErrNoChanges early return and applied after the router is configured.

tailscaled keeps BIRD support by default on the platforms that
previously had it (linux, darwin, freebsd, openbsd) via
feature/condregister.

Also, add an integration test, as this feature lacked much test
coverage previously.

Updates #12614

Change-Id: I7866a50779e454c87933b358735f7dcd9e2b126f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -258,7 +258,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com                                                from tailscale.com/version
         tailscale.com/appc                                           from tailscale.com/ipn/ipnlocal+
      💣 tailscale.com/atomicfile                                     from tailscale.com/ipn+
-  LD    tailscale.com/chirp                                          from tailscale.com/cmd/tailscaled
+  LD    tailscale.com/chirp                                          from tailscale.com/feature/bird
         tailscale.com/client/local                                   from tailscale.com/client/web+
         tailscale.com/client/tailscale/apitype                       from tailscale.com/client/local+
         tailscale.com/client/web                                     from tailscale.com/ipn/ipnlocal
@@ -292,6 +292,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/feature/ace                                    from tailscale.com/feature/condregister
         tailscale.com/feature/acme                                   from tailscale.com/feature/condregister
         tailscale.com/feature/appconnectors                          from tailscale.com/feature/condregister
+  LD    tailscale.com/feature/bird                                   from tailscale.com/feature/condregister
         tailscale.com/feature/buildfeatures                          from tailscale.com/wgengine/magicsock+
         tailscale.com/feature/c2n                                    from tailscale.com/feature/condregister
         tailscale.com/feature/capture                                from tailscale.com/feature/condregister

cmd/tailscaled/deps_test.go

@@ -137,6 +137,20 @@ func TestOmitCaptivePortal(t *testing.T) {
 	}.Check(t)
 }
 
+func TestOmitBird(t *testing.T) {
+	deptest.DepChecker{
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		Tags:   "ts_omit_bird,ts_include_cli",
+		OnDep: func(dep string) {
+			switch dep {
+			case "tailscale.com/chirp", "tailscale.com/feature/bird":
+				t.Errorf("unexpected dep with ts_omit_bird: %q", dep)
+			}
+		},
+	}.Check(t)
+}
+
 func TestOmitAuth(t *testing.T) {
 	deptest.DepChecker{
 		GOOS:   "linux",

cmd/tailscaled/tailscaled.go

@@ -142,9 +142,8 @@ var args struct {
 }
 
 var (
-	installSystemDaemon   func([]string) error                      // non-nil on some platforms
-	uninstallSystemDaemon func([]string) error                      // non-nil on some platforms
-	createBIRDClient      func(string) (wgengine.BIRDClient, error) // non-nil on some platforms
+	installSystemDaemon   func([]string) error // non-nil on some platforms
+	uninstallSystemDaemon func([]string) error // non-nil on some platforms
 )
 
 // Note - we use function pointers for subcommands so that subcommands like
@@ -281,7 +280,7 @@ store state on filesystem.`)
 		log.Fatalf("--socket is required")
 	}
 
-	if buildfeatures.HasBird && args.birdSocketPath != "" && createBIRDClient == nil {
+	if buildfeatures.HasBird && args.birdSocketPath != "" && !wgengine.HookNewBird.IsSet() {
 		log.SetFlags(0)
 		log.Fatalf("--bird-socket is not supported on %s", runtime.GOOS)
 	}
@@ -804,12 +803,8 @@ func tryEngine(logf logger.Logf, sys *tsd.System, name string) (onlyNetstack boo
 	netstackSubnetRouter := onlyNetstack // but mutated later on some platforms
 	netns.SetEnabled(!onlyNetstack)
 
-	if args.birdSocketPath != "" && createBIRDClient != nil {
-		log.Printf("Connecting to BIRD at %s ...", args.birdSocketPath)
-		conf.BIRDClient, err = createBIRDClient(args.birdSocketPath)
-		if err != nil {
-			return false, fmt.Errorf("createBIRDClient: %w", err)
-		}
+	if buildfeatures.HasBird && args.birdSocketPath != "" {
+		conf.BIRDSocket = args.birdSocketPath
 	}
 	if onlyNetstack {
 		if runtime.GOOS == "linux" && distro.Get() == distro.Synology {

cmd/tailscaled/tailscaled_bird.go

@@ -1,17 +0,0 @@
-// Copyright (c) Tailscale Inc & contributors
-// SPDX-License-Identifier: BSD-3-Clause
-
-//go:build go1.19 && (linux || darwin || freebsd || openbsd) && !ts_omit_bird
-
-package main
-
-import (
-	"tailscale.com/chirp"
-	"tailscale.com/wgengine"
-)
-
-func init() {
-	createBIRDClient = func(ctlSocket string) (wgengine.BIRDClient, error) {
-		return chirp.New(ctlSocket)
-	}
-}