cmd/tailscaled/childproc: add be-child registration mechanism

For ssh and maybe windows service babysitter later.

Updates #3802

Change-Id: I7492b98df98971b3fb72d148ba92c2276cca491f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/childproc/childproc.go

@@ -0,0 +1,20 @@
+// Copyright (c) 2022 Tailscale Inc & 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 childproc allows other packages to register "tailscaled be-child"
+// child process hook code. This avoids duplicating build tags in the
+// tailscaled package. Instead, the code that needs to fork/exec the self
+// executable (when it's tailscaled) can instead register the code
+// they want to run.
+package childproc
+
+var Code = map[string]func([]string) error{}
+
+// Add registers code f to run as 'tailscaled be-child <typ> [args]'.
+func Add(typ string, f func(args []string) error) {
+	if _, dup := Code[typ]; dup {
+		panic("dup hook " + typ)
+	}
+	Code[typ] = f
+}

cmd/tailscaled/depaware.txt

@@ -168,6 +168,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
   LD    tailscale.com/chirp                                          from tailscale.com/cmd/tailscaled
         tailscale.com/client/tailscale                               from tailscale.com/derp
         tailscale.com/client/tailscale/apitype                       from tailscale.com/client/tailscale+
+        tailscale.com/cmd/tailscaled/childproc                       from tailscale.com/cmd/tailscaled+
         tailscale.com/control/controlclient                          from tailscale.com/ipn/ipnlocal+
         tailscale.com/control/controlknobs                           from tailscale.com/control/controlclient+
         tailscale.com/derp                                           from tailscale.com/derp/derphttp+

cmd/tailscaled/tailscaled.go

@@ -28,6 +28,7 @@
 	"time"
 
 	"inet.af/netaddr"
+	"tailscale.com/cmd/tailscaled/childproc"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/envknob"
 	"tailscale.com/ipn"
@@ -105,6 +106,7 @@ func defaultTunName() string {
 	"install-system-daemon":   &installSystemDaemon,
 	"uninstall-system-daemon": &uninstallSystemDaemon,
 	"debug":                   &debugModeFunc,
+	"be-child":                &beChildFunc,
 }
 
 func main() {
@@ -601,3 +603,17 @@ func mustStartProxyListeners(socksAddr, httpAddr string) (socksListener, httpLis
 
 	return socksListener, httpListener
 }
+
+var beChildFunc = beChild
+
+func beChild(args []string) error {
+	if len(args) == 0 {
+		return errors.New("missing mode argument")
+	}
+	typ := args[0]
+	f, ok := childproc.Code[typ]
+	if !ok {
+		return fmt.Errorf("unknown be-child mode %q", typ)
+	}
+	return f(args[1:])
+}

ssh/tailssh/tailssh.go

@@ -27,6 +27,7 @@
 	"github.com/creack/pty"
 	"github.com/gliderlabs/ssh"
 	"inet.af/netaddr"
+	"tailscale.com/cmd/tailscaled/childproc"
 	"tailscale.com/envknob"
 	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/net/tsaddr"
@@ -34,6 +35,15 @@
 	"tailscale.com/types/logger"
 )
 
+func init() {
+	childproc.Add("ssh", sshChild)
+}
+
+func sshChild([]string) error {
+	fmt.Println("TODO(maisem): ssh dbus stuff")
+	return nil
+}
+
 // TODO(bradfitz): this is all very temporary as code is temporarily
 // being moved around; it will be restructured and documented in
 // following commits.

tstest/integration/tailscaled_deps_test_darwin.go

@@ -13,6 +13,7 @@
 	// process and can cache a prior success when a dependency changes.
 	_ "inet.af/netaddr"
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"

tstest/integration/tailscaled_deps_test_freebsd.go

@@ -13,6 +13,7 @@
 	// process and can cache a prior success when a dependency changes.
 	_ "inet.af/netaddr"
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"

tstest/integration/tailscaled_deps_test_linux.go

@@ -13,6 +13,7 @@
 	// process and can cache a prior success when a dependency changes.
 	_ "inet.af/netaddr"
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"

tstest/integration/tailscaled_deps_test_openbsd.go

@@ -13,6 +13,7 @@
 	// process and can cache a prior success when a dependency changes.
 	_ "inet.af/netaddr"
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"

tstest/integration/tailscaled_deps_test_windows.go

@@ -16,6 +16,7 @@
 	_ "golang.org/x/sys/windows/svc/mgr"
 	_ "golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	_ "inet.af/netaddr"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"