// Copyright (c) 2020 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 router

import (
	"bufio"
	"context"
	"fmt"
	"io"
	"os"
	"os/exec"
	"strings"
	"sync"
	"syscall"
	"time"

	"github.com/tailscale/wireguard-go/tun"
	"golang.org/x/sys/windows"
	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
	"inet.af/netaddr"
	"tailscale.com/logtail/backoff"
	"tailscale.com/net/dns"
	"tailscale.com/types/logger"
)

type winRouter struct {
	logf                func(fmt string, args ...interface{})
	nativeTun           *tun.NativeTun
	routeChangeCallback *winipcfg.RouteChangeCallback
	dns                 *dns.Manager
	firewall            *firewallTweaker

	// firewallSubproc is a subprocess that runs a tweaked version of
	// wireguard-windows's "default route killswitch" code. We run it
	// as a subprocess because it does unsafe callouts to the WFP API,
	// and we want to defend against memory corruption in our main
	// process. Owned and mutated only by Set, and doesn't need a lock
	// because Set is only called with wgengine's lock held,
	// preventing concurrent reconfigs.
	firewallSubproc *exec.Cmd
}

func newUserspaceRouter(logf logger.Logf, tundev tun.Device) (Router, error) {
	nativeTun := tundev.(*tun.NativeTun)
	luid := winipcfg.LUID(nativeTun.LUID())
	guid, err := luid.GUID()
	if err != nil {
		return nil, err
	}

	return &winRouter{
		logf:      logf,
		nativeTun: nativeTun,
		dns:       dns.NewManager(logf, guid.String()),
		firewall: &firewallTweaker{
			logf:    logger.WithPrefix(logf, "firewall: "),
			tunGUID: *guid,
		},
	}, nil
}

func (r *winRouter) Up() error {
	r.firewall.clear()

	var err error
	t0 := time.Now()
	r.routeChangeCallback, err = monitorDefaultRoutes(r.nativeTun)
	d := time.Since(t0).Round(time.Millisecond)
	if err != nil {
		return fmt.Errorf("monitorDefaultRoutes, after %v: %v", d, err)
	}
	r.logf("monitorDefaultRoutes done after %v", d)
	return nil
}

func (r *winRouter) Set(cfg *Config) error {
	if cfg == nil {
		cfg = &shutdownConfig
	}

	var localAddrs []string
	for _, la := range cfg.LocalAddrs {
		localAddrs = append(localAddrs, la.String())
	}
	r.firewall.set(localAddrs, cfg.Routes)

	err := configureInterface(cfg, r.nativeTun)
	if err != nil {
		r.logf("ConfigureInterface: %v", err)
		return err
	}

	if err := r.dns.Set(cfg.DNS); err != nil {
		return fmt.Errorf("dns set: %w", err)
	}

	// Flush DNS on router config change to clear cached DNS entries (solves #1430)
	if err := dns.Flush(); err != nil {
		r.logf("flushdns error: %v", err)
	}

	return nil
}

func hasDefaultRoute(routes []netaddr.IPPrefix) bool {
	for _, route := range routes {
		if route.Bits == 0 {
			return true
		}
	}
	return false
}

func (r *winRouter) Close() error {
	r.firewall.clear()

	if err := r.dns.Down(); err != nil {
		return fmt.Errorf("dns down: %w", err)
	}
	if r.routeChangeCallback != nil {
		r.routeChangeCallback.Unregister()
	}

	return nil
}

func cleanup(logf logger.Logf, interfaceName string) {
	// Nothing to do here.
}

// firewallTweaker changes the Windows firewall. Normally this wouldn't be so complicated,
// but it can be REALLY SLOW to change the Windows firewall for reasons not understood.
// Like 4 minutes slow. But usually it's tens of milliseconds.
// See https://github.com/tailscale/tailscale/issues/785.
// So this tracks the desired state and runs the actual adjusting code asynchrounsly.
type firewallTweaker struct {
	logf    logger.Logf
	tunGUID windows.GUID

	mu             sync.Mutex
	didProcRule    bool
	running        bool     // doAsyncSet goroutine is running
	known          bool     // firewall is in known state (in lastVal)
	wantLocal      []string // next value we want, or "" to delete the firewall rule
	lastLocal      []string // last set value, if known
	wantKillswitch bool
	lastKillswitch bool

	// Only touched by doAsyncSet, so mu doesn't need to be held.

	// fwProc is a subprocess that runs the wireguard-windows firewall
	// killswitch code. It is only non-nil when the default route
	// killswitch is active, and may go back and forth between nil and
	// non-nil any number of times during the process's lifetime.
	fwProc *exec.Cmd
	// stop makes fwProc exit when closed.
	stop io.Closer
}

func (ft *firewallTweaker) clear() { ft.set(nil, nil) }

// set takes CIDRs to allow, and the routes that point into the Tailscale tun interface.
// Empty slices remove firewall rules.
//
// set takes ownership of cidrs, but not routes.
func (ft *firewallTweaker) set(cidrs []string, routes []netaddr.IPPrefix) {
	ft.mu.Lock()
	defer ft.mu.Unlock()

	if len(cidrs) == 0 {
		ft.logf("marking for removal")
	} else {
		ft.logf("marking allowed %v", cidrs)
	}
	ft.wantLocal = cidrs
	ft.wantKillswitch = hasDefaultRoute(routes)
	if ft.running {
		// The doAsyncSet goroutine will check ft.wantLocal/wantKillswitch
		// before returning.
		return
	}
	ft.logf("starting netsh goroutine")
	ft.running = true
	go ft.doAsyncSet()
}

func (ft *firewallTweaker) runFirewall(args ...string) (time.Duration, error) {
	t0 := time.Now()
	args = append([]string{"advfirewall", "firewall"}, args...)
	cmd := exec.Command("netsh", args...)
	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
	err := cmd.Run()
	return time.Since(t0).Round(time.Millisecond), err
}

func (ft *firewallTweaker) doAsyncSet() {
	bo := backoff.NewBackoff("win-firewall", ft.logf, time.Minute)
	ctx := context.Background()

	ft.mu.Lock()
	for { // invariant: ft.mu must be locked when beginning this block
		val := ft.wantLocal
		if ft.known && strsEqual(ft.lastLocal, val) && ft.wantKillswitch == ft.lastKillswitch {
			ft.running = false
			ft.logf("ending netsh goroutine")
			ft.mu.Unlock()
			return
		}
		wantKillswitch := ft.wantKillswitch
		needClear := !ft.known || len(ft.lastLocal) > 0 || len(val) == 0
		needProcRule := !ft.didProcRule
		ft.mu.Unlock()

		err := ft.doSet(val, wantKillswitch, needClear, needProcRule)
		bo.BackOff(ctx, err)

		ft.mu.Lock()
		ft.lastLocal = val
		ft.lastKillswitch = wantKillswitch
		ft.known = (err == nil)
	}
}

// doSet creates and deletes firewall rules to make the system state
// match the values of local, killswitch, clear and procRule.
//
// local is the list of local Tailscale addresses (formatted as CIDR
// prefixes) to allow through the Windows firewall.
// killswitch, if true, enables the wireguard-windows based internet
// killswitch to prevent use of non-Tailscale default routes.
// clear, if true, removes all tailscale address firewall rules before
// adding local.
// procRule, if true, installs a firewall rule that permits the Tailscale
// process to dial out as it pleases.
//
// Must only be invoked from doAsyncSet.
func (ft *firewallTweaker) doSet(local []string, killswitch bool, clear bool, procRule bool) error {
	if clear {
		ft.logf("clearing Tailscale-In firewall rules...")
		// We ignore the error here, because netsh returns an error for
		// deleting something that doesn't match.
		// TODO(bradfitz): care? That'd involve querying it before/after to see
		// whether it was necessary/worked. But the output format is localized,
		// so can't rely on parsing English. Maybe need to use OLE, not netsh.exe?
		d, _ := ft.runFirewall("delete", "rule", "name=Tailscale-In", "dir=in")
		ft.logf("cleared Tailscale-In firewall rules in %v", d)
	}
	if procRule {
		ft.logf("deleting any prior Tailscale-Process rule...")
		d, err := ft.runFirewall("delete", "rule", "name=Tailscale-Process", "dir=in") // best effort
		if err == nil {
			ft.logf("removed old Tailscale-Process rule in %v", d)
		}
		var exe string
		exe, err = os.Executable()
		if err != nil {
			ft.logf("failed to find Executable for Tailscale-Process rule: %v", err)
		} else {
			ft.logf("adding Tailscale-Process rule to allow UDP for %q ...", exe)
			d, err = ft.runFirewall("add", "rule", "name=Tailscale-Process",
				"dir=in",
				"action=allow",
				"edge=yes",
				"program="+exe,
				"protocol=udp",
				"profile=any",
				"enable=yes",
			)
			if err != nil {
				ft.logf("error adding Tailscale-Process rule: %v", err)
			} else {
				ft.mu.Lock()
				ft.didProcRule = true
				ft.mu.Unlock()
				ft.logf("added Tailscale-Process rule in %v", d)
			}
		}
	}
	for _, cidr := range local {
		ft.logf("adding Tailscale-In rule to allow %v ...", cidr)
		var d time.Duration
		d, err := ft.runFirewall("add", "rule", "name=Tailscale-In", "dir=in", "action=allow", "localip="+cidr, "profile=private", "enable=yes")
		if err != nil {
			ft.logf("error adding Tailscale-In rule to allow %v: %v", cidr, err)
			return err
		}
		ft.logf("added Tailscale-In rule to allow %v in %v", cidr, d)
	}

	if killswitch && ft.fwProc == nil {
		exe, err := os.Executable()
		if err != nil {
			return err
		}
		proc := exec.Command(exe, "/firewall", ft.tunGUID.String())
		var (
			out io.ReadCloser
			in  io.WriteCloser
		)
		out, err = proc.StdoutPipe()
		if err != nil {
			return err
		}
		proc.Stderr = proc.Stdout
		in, err = proc.StdinPipe()
		if err != nil {
			out.Close()
			return err
		}

		go func(out io.ReadCloser) {
			b := bufio.NewReaderSize(out, 1<<10)
			for {
				line, err := b.ReadString('\n')
				if err != nil {
					return
				}
				line = strings.TrimSpace(line)
				if line != "" {
					ft.logf("fw-child: %s", line)
				}
			}
		}(out)

		if err := proc.Start(); err != nil {
			return err
		}
		ft.stop = in
		ft.fwProc = proc
	} else if !killswitch && ft.fwProc != nil {
		ft.stop.Close()
		ft.stop = nil
		ft.fwProc.Wait()
		ft.fwProc = nil
	}

	return nil
}

func strsEqual(a, b []string) bool {
	if len(a) != len(b) {
		return false
	}
	for i := range a {
		if a[i] != b[i] {
			return false
		}
	}
	return true
}