// 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 ipnserver

import (
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"net"
	"net/http"
	"os"
	"os/exec"
	"os/signal"
	"os/user"
	"path/filepath"
	"runtime"
	"strconv"
	"strings"
	"sync"
	"syscall"
	"time"

	"go4.org/mem"
	"inet.af/netaddr"
	"inet.af/peercred"
	"tailscale.com/control/controlclient"
	"tailscale.com/envknob"
	"tailscale.com/ipn"
	"tailscale.com/ipn/ipnlocal"
	"tailscale.com/ipn/localapi"
	"tailscale.com/ipn/store/aws"
	"tailscale.com/logtail/backoff"
	"tailscale.com/net/netstat"
	"tailscale.com/net/tsdial"
	"tailscale.com/paths"
	"tailscale.com/safesocket"
	"tailscale.com/smallzstd"
	"tailscale.com/types/logger"
	"tailscale.com/util/groupmember"
	"tailscale.com/util/pidowner"
	"tailscale.com/util/systemd"
	"tailscale.com/version"
	"tailscale.com/version/distro"
	"tailscale.com/wgengine"
	"tailscale.com/wgengine/monitor"
)

// Options is the configuration of the Tailscale node agent.
type Options struct {
	// VarRoot is the the Tailscale daemon's private writable
	// directory (usually "/var/lib/tailscale" on Linux) that
	// contains the "tailscaled.state" file, the "certs" directory
	// for TLS certs, and the "files" directory for incoming
	// Taildrop files before they're moved to a user directory.
	// If empty, Taildrop and TLS certs don't function.
	VarRoot string

	// AutostartStateKey, if non-empty, immediately starts the agent
	// using the given StateKey. If empty, the agent stays idle and
	// waits for a frontend to start it.
	AutostartStateKey ipn.StateKey

	// SurviveDisconnects specifies how the server reacts to its
	// frontend disconnecting. If true, the server keeps running on
	// its existing state, and accepts new frontend connections. If
	// false, the server dumps its state and becomes idle.
	//
	// This is effectively whether the platform is in "server
	// mode" by default. On Linux, it's true; on Windows, it's
	// false. But on some platforms (currently only Windows), the
	// "server mode" can be overridden at runtime with a change in
	// Prefs.ForceDaemon/WantRunning.
	//
	// To support CLI connections (notably, "tailscale status"),
	// the actual definition of "disconnect" is when the
	// connection count transitions from 1 to 0.
	SurviveDisconnects bool
}

// Server is an IPN backend and its set of 0 or more active localhost
// TCP or unix socket connections talking to that backend.
type Server struct {
	b            *ipnlocal.LocalBackend
	logf         logger.Logf
	backendLogID string
	// resetOnZero is whether to call bs.Reset on transition from
	// 1->0 connections.  That is, this is whether the backend is
	// being run in "client mode" that requires an active GUI
	// connection (such as on Windows by default).  Even if this
	// is true, the ForceDaemon pref can override this.
	resetOnZero       bool
	autostartStateKey ipn.StateKey

	bsMu sync.Mutex // lock order: bsMu, then mu
	bs   *ipn.BackendServer

	mu             sync.Mutex
	serverModeUser *user.User                   // or nil if not in server mode
	lastUserID     string                       // tracks last userid; on change, Reset state for paranoia
	allClients     map[net.Conn]connIdentity    // HTTP or IPN
	clients        map[net.Conn]bool            // subset of allClients; only IPN protocol
	disconnectSub  map[chan<- struct{}]struct{} // keys are subscribers of disconnects
}

// LocalBackend returns the server's LocalBackend.
func (s *Server) LocalBackend() *ipnlocal.LocalBackend { return s.b }

// connIdentity represents the owner of a localhost TCP or unix socket connection.
type connIdentity struct {
	Conn       net.Conn
	NotWindows bool // runtime.GOOS != "windows"

	// Fields used when NotWindows:
	IsUnixSock bool            // Conn is a *net.UnixConn
	Creds      *peercred.Creds // or nil

	// Used on Windows:
	// TODO(bradfitz): merge these into the peercreds package and
	// use that for all.
	Pid    int
	UserID string
	User   *user.User
}

// getConnIdentity returns the localhost TCP connection's identity information
// (pid, userid, user). If it's not Windows (for now), it returns a nil error
// and a ConnIdentity with NotWindows set true. It's only an error if we expected
// to be able to map it and couldn't.
func (s *Server) getConnIdentity(c net.Conn) (ci connIdentity, err error) {
	ci = connIdentity{Conn: c}
	if runtime.GOOS != "windows" { // for now; TODO: expand to other OSes
		ci.NotWindows = true
		_, ci.IsUnixSock = c.(*net.UnixConn)
		ci.Creds, _ = peercred.Get(c)
		return ci, nil
	}
	la, err := netaddr.ParseIPPort(c.LocalAddr().String())
	if err != nil {
		return ci, fmt.Errorf("parsing local address: %w", err)
	}
	ra, err := netaddr.ParseIPPort(c.RemoteAddr().String())
	if err != nil {
		return ci, fmt.Errorf("parsing local remote: %w", err)
	}
	if !la.IP().IsLoopback() || !ra.IP().IsLoopback() {
		return ci, errors.New("non-loopback connection")
	}
	tab, err := netstat.Get()
	if err != nil {
		return ci, fmt.Errorf("failed to get local connection table: %w", err)
	}
	pid := peerPid(tab.Entries, la, ra)
	if pid == 0 {
		return ci, errors.New("no local process found matching localhost connection")
	}
	ci.Pid = pid
	uid, err := pidowner.OwnerOfPID(pid)
	if err != nil {
		var hint string
		if runtime.GOOS == "windows" {
			hint = " (WSL?)"
		}
		return ci, fmt.Errorf("failed to map connection's pid to a user%s: %w", hint, err)
	}
	ci.UserID = uid
	u, err := lookupUserFromID(s.logf, uid)
	if err != nil {
		return ci, fmt.Errorf("failed to look up user from userid: %w", err)
	}
	ci.User = u
	return ci, nil
}

func lookupUserFromID(logf logger.Logf, uid string) (*user.User, error) {
	u, err := user.LookupId(uid)
	if err != nil && runtime.GOOS == "windows" && errors.Is(err, syscall.Errno(0x534)) {
		logf("[warning] issue 869: os/user.LookupId failed; ignoring")
		// Work around https://github.com/tailscale/tailscale/issues/869 for
		// now. We don't strictly need the username. It's just a nice-to-have.
		// So make up a *user.User if their machine is broken in this way.
		return &user.User{
			Uid:      uid,
			Username: "unknown-user-" + uid,
			Name:     "unknown user " + uid,
		}, nil
	}
	return u, err
}

// blockWhileInUse blocks while until either a Read from conn fails
// (i.e. it's closed) or until the server is able to accept ci as a
// user.
func (s *Server) blockWhileInUse(conn io.Reader, ci connIdentity) {
	s.logf("blocking client while server in use; connIdentity=%v", ci)
	connDone := make(chan struct{})
	go func() {
		io.Copy(ioutil.Discard, conn)
		close(connDone)
	}()
	ch := make(chan struct{}, 1)
	s.registerDisconnectSub(ch, true)
	defer s.registerDisconnectSub(ch, false)
	for {
		select {
		case <-connDone:
			s.logf("blocked client Read completed; connIdentity=%v", ci)
			return
		case <-ch:
			s.mu.Lock()
			err := s.checkConnIdentityLocked(ci)
			s.mu.Unlock()
			if err == nil {
				s.logf("unblocking client, server is free; connIdentity=%v", ci)
				// Server is now available again for a new user.
				// TODO(bradfitz): keep this connection alive. But for
				// now just return and have our caller close the connection
				// (which unblocks the io.Copy goroutine we started above)
				// and then the client (e.g. Windows) will reconnect and
				// discover that it works.
				return
			}
		}
	}
}

// bufferHasHTTPRequest reports whether br looks like it has an HTTP
// request in it, without reading any bytes from it.
func bufferHasHTTPRequest(br *bufio.Reader) bool {
	peek, _ := br.Peek(br.Buffered())
	return mem.HasPrefix(mem.B(peek), mem.S("GET ")) ||
		mem.HasPrefix(mem.B(peek), mem.S("POST ")) ||
		mem.Contains(mem.B(peek), mem.S(" HTTP/"))
}

// bufferIsConnect reports whether br looks like it's likely an HTTP
// CONNECT request.
//
// Invariant: br has already had at least 4 bytes Peek'ed.
func bufferIsConnect(br *bufio.Reader) bool {
	peek, _ := br.Peek(br.Buffered())
	return mem.HasPrefix(mem.B(peek), mem.S("CONN"))
}

func (s *Server) serveConn(ctx context.Context, c net.Conn, logf logger.Logf) {
	// First see if it's an HTTP request.
	br := bufio.NewReader(c)
	c.SetReadDeadline(time.Now().Add(time.Second))
	br.Peek(4)
	c.SetReadDeadline(time.Time{})

	// Handle logtail CONNECT requests early. (See docs on handleProxyConnectConn)
	if bufferIsConnect(br) {
		s.handleProxyConnectConn(ctx, br, c, logf)
		return
	}

	isHTTPReq := bufferHasHTTPRequest(br)

	ci, err := s.addConn(c, isHTTPReq)
	if err != nil {
		if isHTTPReq {
			fmt.Fprintf(c, "HTTP/1.0 500 Nope\r\nContent-Type: text/plain\r\nX-Content-Type-Options: nosniff\r\n\r\n%s\n", err.Error())
			c.Close()
			return
		}
		defer c.Close()
		bs := ipn.NewBackendServer(logf, nil, jsonNotifier(c, s.logf))
		_, occupied := err.(inUseOtherUserError)
		if occupied {
			bs.SendInUseOtherUserErrorMessage(err.Error())
			s.blockWhileInUse(c, ci)
		} else {
			bs.SendErrorMessage(err.Error())
			time.Sleep(time.Second)
		}
		return
	}

	// Tell the LocalBackend about the identity we're now running as.
	s.b.SetCurrentUserID(ci.UserID)

	if isHTTPReq {
		httpServer := &http.Server{
			// Localhost connections are cheap; so only do
			// keep-alives for a short period of time, as these
			// active connections lock the server into only serving
			// that user. If the user has this page open, we don't
			// want another switching user to be locked out for
			// minutes. 5 seconds is enough to let browser hit
			// favicon.ico and such.
			IdleTimeout: 5 * time.Second,
			ErrorLog:    logger.StdLogger(logf),
			Handler:     s.localhostHandler(ci),
		}
		httpServer.Serve(&oneConnListener{&protoSwitchConn{s: s, br: br, Conn: c}})
		return
	}

	defer s.removeAndCloseConn(c)
	logf("[v1] incoming control connection")

	if isReadonlyConn(ci, s.b.OperatorUserID(), logf) {
		ctx = ipn.ReadonlyContextOf(ctx)
	}

	for ctx.Err() == nil {
		msg, err := ipn.ReadMsg(br)
		if err != nil {
			if errors.Is(err, io.EOF) {
				logf("[v1] ReadMsg: %v", err)
			} else if ctx.Err() == nil {
				logf("ReadMsg: %v", err)
			}
			return
		}
		s.bsMu.Lock()
		if err := s.bs.GotCommandMsg(ctx, msg); err != nil {
			logf("GotCommandMsg: %v", err)
		}
		gotQuit := s.bs.GotQuit
		s.bsMu.Unlock()
		if gotQuit {
			return
		}
	}
}

func isReadonlyConn(ci connIdentity, operatorUID string, logf logger.Logf) bool {
	if runtime.GOOS == "windows" {
		// Windows doesn't need/use this mechanism, at least yet. It
		// has a different last-user-wins auth model.
		return false
	}
	const ro = true
	const rw = false
	if !safesocket.PlatformUsesPeerCreds() {
		return rw
	}
	creds := ci.Creds
	if creds == nil {
		logf("connection from unknown peer; read-only")
		return ro
	}
	uid, ok := creds.UserID()
	if !ok {
		logf("connection from peer with unknown userid; read-only")
		return ro
	}
	if uid == "0" {
		logf("connection from userid %v; root has access", uid)
		return rw
	}
	if selfUID := os.Getuid(); selfUID != 0 && uid == strconv.Itoa(selfUID) {
		logf("connection from userid %v; connection from non-root user matching daemon has access", uid)
		return rw
	}
	if operatorUID != "" && uid == operatorUID {
		logf("connection from userid %v; is configured operator", uid)
		return rw
	}
	if yes, err := isLocalAdmin(uid); err != nil {
		logf("connection from userid %v; read-only; %v", uid, err)
		return ro
	} else if yes {
		logf("connection from userid %v; is local admin, has access", uid)
		return rw
	}
	logf("connection from userid %v; read-only", uid)
	return ro
}

func isLocalAdmin(uid string) (bool, error) {
	u, err := user.LookupId(uid)
	if err != nil {
		return false, err
	}
	var adminGroup string
	switch {
	case runtime.GOOS == "darwin":
		adminGroup = "admin"
	case distro.Get() == distro.QNAP:
		adminGroup = "administrators"
	default:
		return false, fmt.Errorf("no system admin group found")
	}
	return groupmember.IsMemberOfGroup(adminGroup, u.Username)
}

// inUseOtherUserError is the error type for when the server is in use
// by a different local user.
type inUseOtherUserError struct{ error }

func (e inUseOtherUserError) Unwrap() error { return e.error }

// checkConnIdentityLocked checks whether the provided identity is
// allowed to connect to the server.
//
// The returned error, when non-nil, will be of type inUseOtherUserError.
//
// s.mu must be held.
func (s *Server) checkConnIdentityLocked(ci connIdentity) error {
	// If clients are already connected, verify they're the same user.
	// This mostly matters on Windows at the moment.
	if len(s.allClients) > 0 {
		var active connIdentity
		for _, active = range s.allClients {
			break
		}
		if ci.UserID != active.UserID {
			return inUseOtherUserError{fmt.Errorf("Tailscale already in use by %s, pid %d", active.User.Username, active.Pid)}
		}
	}
	if su := s.serverModeUser; su != nil && ci.UserID != su.Uid {
		return inUseOtherUserError{fmt.Errorf("Tailscale already in use by %s", su.Username)}
	}
	return nil
}

// localAPIPermissions returns the permissions for the given identity accessing
// the Tailscale local daemon API.
//
// s.mu must not be held.
func (s *Server) localAPIPermissions(ci connIdentity) (read, write bool) {
	switch runtime.GOOS {
	case "windows":
		s.mu.Lock()
		defer s.mu.Unlock()
		if s.checkConnIdentityLocked(ci) == nil {
			return true, true
		}
		return false, false
	case "js":
		return true, true
	}
	if ci.IsUnixSock {
		return true, !isReadonlyConn(ci, s.b.OperatorUserID(), logger.Discard)
	}
	return false, false
}

// connCanFetchCerts reports whether ci is allowed to fetch HTTPS
// certs from this server when it wouldn't otherwise be able to.
//
// That is, this reports whether ci should grant additional
// capabilities over what the conn would otherwise be able to do.
//
// For now this only returns true on Unix machines when
// TS_PERMIT_CERT_UID is set the to the userid of the peer
// connection. It's intended to give your non-root webserver access
// (www-data, caddy, nginx, etc) to certs.
func (s *Server) connCanFetchCerts(ci connIdentity) bool {
	if ci.IsUnixSock && ci.Creds != nil {
		connUID, ok := ci.Creds.UserID()
		if ok && connUID == envknob.String("TS_PERMIT_CERT_UID") {
			return true
		}
	}
	return false
}

// registerDisconnectSub adds ch as a subscribe to connection disconnect
// events. If add is false, the subscriber is removed.
func (s *Server) registerDisconnectSub(ch chan<- struct{}, add bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if add {
		if s.disconnectSub == nil {
			s.disconnectSub = make(map[chan<- struct{}]struct{})
		}
		s.disconnectSub[ch] = struct{}{}
	} else {
		delete(s.disconnectSub, ch)
	}

}

// addConn adds c to the server's list of clients.
//
// If the returned error is of type inUseOtherUserError then the
// returned connIdentity is also valid.
func (s *Server) addConn(c net.Conn, isHTTP bool) (ci connIdentity, err error) {
	ci, err = s.getConnIdentity(c)
	if err != nil {
		return
	}

	// If the connected user changes, reset the backend server state to make
	// sure node keys don't leak between users.
	var doReset bool
	defer func() {
		if doReset {
			s.logf("identity changed; resetting server")
			s.b.ResetForClientDisconnect()
		}
	}()

	s.mu.Lock()
	defer s.mu.Unlock()

	if s.clients == nil {
		s.clients = map[net.Conn]bool{}
	}
	if s.allClients == nil {
		s.allClients = map[net.Conn]connIdentity{}
	}

	if err := s.checkConnIdentityLocked(ci); err != nil {
		return ci, err
	}

	if !isHTTP {
		s.clients[c] = true
	}
	s.allClients[c] = ci

	if s.lastUserID != ci.UserID {
		if s.lastUserID != "" {
			doReset = true
		}
		s.lastUserID = ci.UserID
	}
	return ci, nil
}

func (s *Server) removeAndCloseConn(c net.Conn) {
	s.mu.Lock()
	delete(s.clients, c)
	delete(s.allClients, c)
	remain := len(s.allClients)
	for sub := range s.disconnectSub {
		select {
		case sub <- struct{}{}:
		default:
		}
	}
	s.mu.Unlock()

	if remain == 0 && s.resetOnZero {
		if s.b.InServerMode() {
			s.logf("client disconnected; staying alive in server mode")
		} else {
			s.logf("client disconnected; stopping server")
			s.b.ResetForClientDisconnect()
		}
	}
	c.Close()
}

func (s *Server) stopAll() {
	s.mu.Lock()
	defer s.mu.Unlock()
	for c := range s.clients {
		safesocket.ConnCloseRead(c)
		safesocket.ConnCloseWrite(c)
	}
	s.clients = nil
}

// setServerModeUserLocked is called when we're in server mode but our s.serverModeUser is nil.
//
// s.mu must be held
func (s *Server) setServerModeUserLocked() {
	var ci connIdentity
	var ok bool
	for _, ci = range s.allClients {
		ok = true
		break
	}
	if !ok {
		s.logf("ipnserver: [unexpected] now in server mode, but no connected client")
		return
	}
	if ci.NotWindows {
		return
	}
	if ci.User != nil {
		s.logf("ipnserver: now in server mode; user=%v", ci.User.Username)
		s.serverModeUser = ci.User
	} else {
		s.logf("ipnserver: [unexpected] now in server mode, but nil User")
	}
}

var jsonEscapedZero = []byte(`\u0000`)

func (s *Server) writeToClients(n ipn.Notify) {
	inServerMode := s.b.InServerMode()

	s.mu.Lock()
	defer s.mu.Unlock()

	if inServerMode {
		if s.serverModeUser == nil {
			s.setServerModeUserLocked()
		}
	} else {
		if s.serverModeUser != nil {
			s.logf("ipnserver: no longer in server mode")
			s.serverModeUser = nil
		}
	}

	if len(s.clients) == 0 {
		// Common case (at least on busy servers): nobody
		// connected (no GUI, etc), so return before
		// serializing JSON.
		return
	}

	if b, ok := marshalNotify(n, s.logf); ok {
		for c := range s.clients {
			ipn.WriteMsg(c, b)
		}
	}
}

// tryWindowsAppDataMigration attempts to copy the Windows state file
// from its old location to the new location. (Issue 2856)
//
// Tailscale 1.14 and before stored state under %LocalAppData%
// (usually "C:\WINDOWS\system32\config\systemprofile\AppData\Local"
// when tailscaled.exe is running as a non-user system service).
// However it is frequently cleared for almost any reason: Windows
// updates, System Restore, even various System Cleaner utilities.
//
// Returns a string of the path to use for the state file.
// This will be a fallback %LocalAppData% path if migration fails,
// a %ProgramData% path otherwise.
func tryWindowsAppDataMigration(logf logger.Logf, path string) string {
	if path != paths.DefaultTailscaledStateFile() {
		// If they're specifying a non-default path, just trust that they know
		// what they are doing.
		return path
	}
	oldFile := paths.LegacyStateFilePath()
	return paths.TryConfigFileMigration(logf, oldFile, path)
}

// StateStore returns a StateStore from path.
//
// The path should be an absolute path to a file.
//
// Special cases:
//
//   * empty string means to use an in-memory store
//   * if the string begins with "kube:", the suffix
//     is a Kubernetes secret name
//   * if the string begins with "arn:", the value is
//     an AWS ARN for an SSM.
func StateStore(path string, logf logger.Logf) (ipn.StateStore, error) {
	if path == "" {
		return &ipn.MemoryStore{}, nil
	}
	const kubePrefix = "kube:"
	const arnPrefix = "arn:"
	switch {
	case strings.HasPrefix(path, kubePrefix):
		secretName := strings.TrimPrefix(path, kubePrefix)
		store, err := ipn.NewKubeStore(secretName)
		if err != nil {
			return nil, fmt.Errorf("ipn.NewKubeStore(%q): %v", secretName, err)
		}
		return store, nil
	case strings.HasPrefix(path, arnPrefix):
		store, err := aws.NewStore(path)
		if err != nil {
			return nil, fmt.Errorf("aws.NewStore(%q): %v", path, err)
		}
		return store, nil
	}
	if runtime.GOOS == "windows" {
		path = tryWindowsAppDataMigration(logf, path)
	}
	store, err := ipn.NewFileStore(path)
	if err != nil {
		return nil, fmt.Errorf("ipn.NewFileStore(%q): %v", path, err)
	}
	return store, nil
}

// Run runs a Tailscale backend service.
// The getEngine func is called repeatedly, once per connection, until it returns an engine successfully.
//
// Deprecated: use New and Server.Run instead.
func Run(ctx context.Context, logf logger.Logf, ln net.Listener, store ipn.StateStore, linkMon *monitor.Mon, dialer *tsdial.Dialer, logid string, getEngine func() (wgengine.Engine, error), opts Options) error {
	getEngine = getEngineUntilItWorksWrapper(getEngine)
	runDone := make(chan struct{})
	defer close(runDone)

	var serverMu sync.Mutex
	var serverOrNil *Server

	// When the context is closed or when we return, whichever is first, close our listener
	// and all open connections.
	go func() {
		select {
		case <-ctx.Done():
		case <-runDone:
		}
		serverMu.Lock()
		if s := serverOrNil; s != nil {
			s.stopAll()
		}
		serverMu.Unlock()
		ln.Close()
	}()
	logf("Listening on %v", ln.Addr())

	var serverModeUser *user.User
	if opts.AutostartStateKey == "" {
		autoStartKey, err := store.ReadState(ipn.ServerModeStartKey)
		if err != nil && err != ipn.ErrStateNotExist {
			return fmt.Errorf("calling ReadState on state store: %w", err)
		}
		key := string(autoStartKey)
		if strings.HasPrefix(key, "user-") {
			uid := strings.TrimPrefix(key, "user-")
			u, err := lookupUserFromID(logf, uid)
			if err != nil {
				logf("ipnserver: found server mode auto-start key %q; failed to load user: %v", key, err)
			} else {
				logf("ipnserver: found server mode auto-start key %q (user %s)", key, u.Username)
				serverModeUser = u
			}
			opts.AutostartStateKey = ipn.StateKey(key)
		}
	}

	bo := backoff.NewBackoff("ipnserver", logf, 30*time.Second)
	var unservedConn net.Conn // if non-nil, accepted, but hasn't served yet

	eng, err := getEngine()
	if err != nil {
		logf("ipnserver: initial getEngine call: %v", err)
		for i := 1; ctx.Err() == nil; i++ {
			c, err := ln.Accept()
			if err != nil {
				logf("%d: Accept: %v", i, err)
				bo.BackOff(ctx, err)
				continue
			}
			logf("ipnserver: try%d: trying getEngine again...", i)
			eng, err = getEngine()
			if err == nil {
				logf("%d: GetEngine worked; exiting failure loop", i)
				unservedConn = c
				break
			}
			logf("ipnserver%d: getEngine failed again: %v", i, err)
			errMsg := err.Error()
			go func() {
				defer c.Close()
				bs := ipn.NewBackendServer(logf, nil, jsonNotifier(c, logf))
				bs.SendErrorMessage(errMsg)
				time.Sleep(time.Second)
			}()
		}
		if err := ctx.Err(); err != nil {
			return err
		}
	}
	if unservedConn != nil {
		ln = &listenerWithReadyConn{
			Listener: ln,
			c:        unservedConn,
		}
	}

	server, err := New(logf, logid, store, eng, dialer, serverModeUser, opts)
	if err != nil {
		return err
	}
	serverMu.Lock()
	serverOrNil = server
	serverMu.Unlock()
	return server.Run(ctx, ln)
}

// New returns a new Server.
//
// To start it, use the Server.Run method.
func New(logf logger.Logf, logid string, store ipn.StateStore, eng wgengine.Engine, dialer *tsdial.Dialer, serverModeUser *user.User, opts Options) (*Server, error) {
	b, err := ipnlocal.NewLocalBackend(logf, logid, store, dialer, eng)
	if err != nil {
		return nil, fmt.Errorf("NewLocalBackend: %v", err)
	}
	b.SetVarRoot(opts.VarRoot)
	b.SetDecompressor(func() (controlclient.Decompressor, error) {
		return smallzstd.NewDecoder(nil)
	})

	dg := distro.Get()
	switch dg {
	case distro.Synology, distro.TrueNAS:
		// See if they have a "Taildrop" share.
		// See https://github.com/tailscale/tailscale/issues/2179#issuecomment-982821319
		path, err := findTaildropDir(dg)
		if err != nil {
			logf("%s Taildrop support: %v", dg, err)
		} else {
			logf("%s Taildrop: using %v", dg, path)
			b.SetDirectFileRoot(path)
			b.SetDirectFileDoFinalRename(true)
		}

	}

	if opts.AutostartStateKey == "" {
		autoStartKey, err := store.ReadState(ipn.ServerModeStartKey)
		if err != nil && err != ipn.ErrStateNotExist {
			return nil, fmt.Errorf("calling ReadState on store: %w", err)
		}
		key := string(autoStartKey)
		if strings.HasPrefix(key, "user-") {
			uid := strings.TrimPrefix(key, "user-")
			u, err := lookupUserFromID(logf, uid)
			if err != nil {
				logf("ipnserver: found server mode auto-start key %q; failed to load user: %v", key, err)
			} else {
				logf("ipnserver: found server mode auto-start key %q (user %s)", key, u.Username)
				serverModeUser = u
			}
			opts.AutostartStateKey = ipn.StateKey(key)
		}
	}

	server := &Server{
		b:                 b,
		backendLogID:      logid,
		logf:              logf,
		resetOnZero:       !opts.SurviveDisconnects,
		serverModeUser:    serverModeUser,
		autostartStateKey: opts.AutostartStateKey,
	}
	server.bs = ipn.NewBackendServer(logf, b, server.writeToClients)
	return server, nil
}

// Run runs the server, accepting connections from ln forever.
//
// If the context is done, the listener is closed.
func (s *Server) Run(ctx context.Context, ln net.Listener) error {
	defer s.b.Shutdown()

	runDone := make(chan struct{})
	defer close(runDone)

	// When the context is closed or when we return, whichever is first, close our listener
	// and all open connections.
	go func() {
		select {
		case <-ctx.Done():
		case <-runDone:
		}
		s.stopAll()
		ln.Close()
	}()

	if s.autostartStateKey != "" {
		s.bs.GotCommand(ctx, &ipn.Command{
			Version: version.Long,
			Start: &ipn.StartArgs{
				Opts: ipn.Options{StateKey: s.autostartStateKey},
			},
		})
	}

	systemd.Ready()
	bo := backoff.NewBackoff("ipnserver", s.logf, 30*time.Second)
	var connNum int
	for {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		c, err := ln.Accept()
		if err != nil {
			if ctx.Err() != nil {
				return ctx.Err()
			}
			s.logf("ipnserver: Accept: %v", err)
			bo.BackOff(ctx, err)
			continue
		}
		connNum++
		go s.serveConn(ctx, c, logger.WithPrefix(s.logf, fmt.Sprintf("ipnserver: conn%d: ", connNum)))
	}
}

// BabysitProc runs the current executable as a child process with the
// provided args, capturing its output, writing it to files, and
// restarting the process on any crashes.
//
// It's only currently (2020-10-29) used on Windows.
func BabysitProc(ctx context.Context, args []string, logf logger.Logf) {

	executable, err := os.Executable()
	if err != nil {
		panic("cannot determine executable: " + err.Error())
	}

	var proc struct {
		mu sync.Mutex
		p  *os.Process
	}

	done := make(chan struct{})
	go func() {
		interrupt := make(chan os.Signal, 1)
		signal.Notify(interrupt, syscall.SIGINT, syscall.SIGTERM)
		var sig os.Signal
		select {
		case sig = <-interrupt:
			logf("BabysitProc: got signal: %v", sig)
			close(done)
		case <-ctx.Done():
			logf("BabysitProc: context done")
			sig = os.Kill
			close(done)
		}

		proc.mu.Lock()
		proc.p.Signal(sig)
		proc.mu.Unlock()
	}()

	bo := backoff.NewBackoff("BabysitProc", logf, 30*time.Second)

	for {
		startTime := time.Now()
		log.Printf("exec: %#v %v", executable, args)
		cmd := exec.Command(executable, args...)
		if runtime.GOOS == "windows" {
			extraEnv, err := loadExtraEnv()
			if err != nil {
				logf("errors loading extra env file; ignoring: %v", err)
			} else {
				cmd.Env = append(os.Environ(), extraEnv...)
			}
		}

		// Create a pipe object to use as the subproc's stdin.
		// When the writer goes away, the reader gets EOF.
		// A subproc can watch its stdin and exit when it gets EOF;
		// this is a very reliable way to have a subproc die when
		// its parent (us) disappears.
		// We never need to actually write to wStdin.
		rStdin, wStdin, err := os.Pipe()
		if err != nil {
			log.Printf("os.Pipe 1: %v", err)
			return
		}

		// Create a pipe object to use as the subproc's stdout/stderr.
		// We'll read from this pipe and send it to logf, line by line.
		// We can't use os.exec's io.Writer for this because it
		// doesn't care about lines, and thus ends up merging multiple
		// log lines into one or splitting one line into multiple
		// logf() calls. bufio is more appropriate.
		rStdout, wStdout, err := os.Pipe()
		if err != nil {
			log.Printf("os.Pipe 2: %v", err)
		}
		go func(r *os.File) {
			defer r.Close()
			rb := bufio.NewReader(r)
			for {
				s, err := rb.ReadString('\n')
				if s != "" {
					logf("%s", s)
				}
				if err != nil {
					break
				}
			}
		}(rStdout)

		cmd.Stdin = rStdin
		cmd.Stdout = wStdout
		cmd.Stderr = wStdout
		err = cmd.Start()

		// Now that the subproc is started, get rid of our copy of the
		// pipe reader. Bad things happen on Windows if more than one
		// process owns the read side of a pipe.
		rStdin.Close()
		wStdout.Close()

		if err != nil {
			log.Printf("starting subprocess failed: %v", err)
		} else {
			proc.mu.Lock()
			proc.p = cmd.Process
			proc.mu.Unlock()

			err = cmd.Wait()
			log.Printf("subprocess exited: %v", err)
		}

		// If the process finishes, clean up the write side of the
		// pipe. We'll make a new one when we restart the subproc.
		wStdin.Close()

		if os.Getenv("TS_DEBUG_RESTART_CRASHED") == "0" {
			log.Fatalf("Process ended.")
		}

		if time.Since(startTime) < 60*time.Second {
			bo.BackOff(ctx, fmt.Errorf("subproc early exit: %v", err))
		} else {
			// Reset the timeout, since the process ran for a while.
			bo.BackOff(ctx, nil)
		}

		select {
		case <-done:
			return
		default:
		}
	}
}

// FixedEngine returns a func that returns eng and a nil error.
func FixedEngine(eng wgengine.Engine) func() (wgengine.Engine, error) {
	return func() (wgengine.Engine, error) { return eng, nil }
}

// getEngineUntilItWorksWrapper returns a getEngine wrapper that does
// not call getEngine concurrently and stops calling getEngine once
// it's returned a working engine.
func getEngineUntilItWorksWrapper(getEngine func() (wgengine.Engine, error)) func() (wgengine.Engine, error) {
	var mu sync.Mutex
	var engGood wgengine.Engine
	return func() (wgengine.Engine, error) {
		mu.Lock()
		defer mu.Unlock()
		if engGood != nil {
			return engGood, nil
		}
		e, err := getEngine()
		if err != nil {
			return nil, err
		}
		engGood = e
		return e, nil
	}
}

type dummyAddr string
type oneConnListener struct {
	conn net.Conn
}

func (l *oneConnListener) Accept() (c net.Conn, err error) {
	c = l.conn
	if c == nil {
		err = io.EOF
		return
	}
	err = nil
	l.conn = nil
	return
}

func (l *oneConnListener) Close() error { return nil }

func (l *oneConnListener) Addr() net.Addr { return dummyAddr("unused-address") }

func (a dummyAddr) Network() string { return string(a) }
func (a dummyAddr) String() string  { return string(a) }

// protoSwitchConn is a net.Conn that's we want to speak HTTP to but
// it's already had a few bytes read from it to determine that it's
// HTTP. So we Read from its bufio.Reader. On Close, we we tell the
// server it's closed, so the server can account the who's connected.
type protoSwitchConn struct {
	s *Server
	net.Conn
	br        *bufio.Reader
	closeOnce sync.Once
}

func (psc *protoSwitchConn) Read(p []byte) (int, error) { return psc.br.Read(p) }
func (psc *protoSwitchConn) Close() error {
	psc.closeOnce.Do(func() { psc.s.removeAndCloseConn(psc.Conn) })
	return nil
}

func (s *Server) localhostHandler(ci connIdentity) http.Handler {
	lah := localapi.NewHandler(s.b, s.logf, s.backendLogID)
	lah.PermitRead, lah.PermitWrite = s.localAPIPermissions(ci)
	lah.PermitCert = s.connCanFetchCerts(ci)

	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if strings.HasPrefix(r.URL.Path, "/localapi/") {
			lah.ServeHTTP(w, r)
			return
		}
		if ci.NotWindows {
			io.WriteString(w, "<html><title>Tailscale</title><body><h1>Tailscale</h1>This is the local Tailscale daemon.")
			return
		}
		s.ServeHTMLStatus(w, r)
	})
}

func (s *Server) ServeHTMLStatus(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	st := s.b.Status()
	// TODO(bradfitz): add LogID and opts to st?
	st.WriteHTML(w)
}

func peerPid(entries []netstat.Entry, la, ra netaddr.IPPort) int {
	for _, e := range entries {
		if e.Local == ra && e.Remote == la {
			return e.Pid
		}
	}
	return 0
}

// jsonNotifier returns a notify-writer func that writes ipn.Notify
// messages to w.
func jsonNotifier(w io.Writer, logf logger.Logf) func(ipn.Notify) {
	return func(n ipn.Notify) {
		if b, ok := marshalNotify(n, logf); ok {
			ipn.WriteMsg(w, b)
		}
	}
}

func marshalNotify(n ipn.Notify, logf logger.Logf) (b []byte, ok bool) {
	b, err := json.Marshal(n)
	if err != nil {
		logf("ipnserver: [unexpected] error serializing JSON: %v", err)
		return nil, false
	}
	if bytes.Contains(b, jsonEscapedZero) {
		logf("[unexpected] zero byte in BackendServer.send notify message: %q", b)
	}
	return b, true
}

// listenerWithReadyConn is a net.Listener wrapper that has
// one net.Conn ready to be accepted already.
type listenerWithReadyConn struct {
	net.Listener

	mu sync.Mutex
	c  net.Conn // if non-nil, ready to be Accepted
}

func (ln *listenerWithReadyConn) Accept() (net.Conn, error) {
	ln.mu.Lock()
	c := ln.c
	ln.c = nil
	ln.mu.Unlock()
	if c != nil {
		return c, nil
	}
	return ln.Listener.Accept()
}

func findTaildropDir(dg distro.Distro) (string, error) {
	const name = "Taildrop"
	switch dg {
	case distro.Synology:
		return findSynologyTaildropDir(name)
	case distro.TrueNAS:
		return findTrueNASTaildropDir(name)
	}
	return "", fmt.Errorf("%s is an unsupported distro for Taildrop dir", dg)
}

// findSynologyTaildropDir looks for the first volume containing a
// "Taildrop" directory.  We'd run "synoshare --get Taildrop" command
// but on DSM7 at least, we lack permissions to run that.
func findSynologyTaildropDir(name string) (dir string, err error) {
	for i := 1; i <= 16; i++ {
		dir = fmt.Sprintf("/volume%v/%s", i, name)
		if fi, err := os.Stat(dir); err == nil && fi.IsDir() {
			return dir, nil
		}
	}
	return "", fmt.Errorf("shared folder %q not found", name)
}

// findTrueNASTaildropDir returns the first matching directory of
// /mnt/{name} or /mnt/*/{name}
func findTrueNASTaildropDir(name string) (dir string, err error) {
	// If we're running in a jail, a mount point could just be added at /mnt/Taildrop
	dir = fmt.Sprintf("/mnt/%s", name)
	if fi, err := os.Stat(dir); err == nil && fi.IsDir() {
		return dir, nil
	}

	// but if running on the host, it may be something like /mnt/Primary/Taildrop
	fis, err := ioutil.ReadDir("/mnt")
	if err != nil {
		return "", fmt.Errorf("error reading /mnt: %w", err)
	}
	for _, fi := range fis {
		dir = fmt.Sprintf("/mnt/%s/%s", fi.Name(), name)
		if fi, err := os.Stat(dir); err == nil && fi.IsDir() {
			return dir, nil
		}
	}
	return "", fmt.Errorf("shared folder %q not found", name)
}

func loadExtraEnv() (env []string, err error) {
	if runtime.GOOS != "windows" {
		return nil, nil
	}
	name := filepath.Join(os.Getenv("ProgramData"), "Tailscale", "tailscaled-env.txt")
	contents, err := os.ReadFile(name)
	if os.IsNotExist(err) {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	for _, line := range strings.Split(string(contents), "\n") {
		line = strings.TrimSpace(line)
		if line == "" || line[0] == '#' {
			continue
		}
		k, v, ok := stringsCut(line, "=")
		if !ok || k == "" {
			continue
		}
		if strings.HasPrefix(v, `"`) {
			var err error
			v, err = strconv.Unquote(v)
			if err != nil {
				return nil, fmt.Errorf("invalid value in line %q: %v", line, err)
			}
			env = append(env, k+"="+v)
		} else {
			env = append(env, line)
		}
	}
	return env, nil
}

// stringsCut is Go 1.18's strings.Cut.
// TODO(bradfitz): delete this when we depend on Go 1.18.
func stringsCut(s, sep string) (before, after string, found bool) {
	if i := strings.Index(s, sep); i >= 0 {
		return s[:i], s[i+len(sep):], true
	}
	return s, "", false
}