cmd/tailscaled: unify the two Windows paths + separate IPN server path

tailscaled on Windows had two entirely separate start-up paths for running
as a service vs in the foreground. It's been causing problems for ages.
This unifies the two paths, making them be the same as the path used
for every other platform.

Also, it uses the new async LocalBackend support in ipnserver.Server
so the Server can start serving HTTP immediately, even if tun takes
awhile to come up.

Updates #6535

Change-Id: Icc8c4f96d4887b54a024d7ac15ad11096b5a58cf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/server.go

@@ -6,12 +6,14 @@ package ipnserver
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
 	"net"
 	"net/http"
 	"os/user"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -25,6 +27,7 @@ import (
 	"tailscale.com/ipn/localapi"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/mak"
+	"tailscale.com/util/set"
 	"tailscale.com/util/systemd"
 )
 
@@ -46,9 +49,10 @@ type Server struct {
 
 	// mu guards the fields that follow.
 	// lock order: mu, then LocalBackend.mu
-	mu         sync.Mutex
-	lastUserID ipn.WindowsUserID // tracks last userid; on change, Reset state for paranoia
-	activeReqs map[*http.Request]*ipnauth.ConnIdentity
+	mu            sync.Mutex
+	lastUserID    ipn.WindowsUserID // tracks last userid; on change, Reset state for paranoia
+	activeReqs    map[*http.Request]*ipnauth.ConnIdentity
+	backendWaiter set.HandleSet[context.CancelFunc] // values are wake-up funcs of lb waiters
 }
 
 func (s *Server) mustBackend() *ipnlocal.LocalBackend {
@@ -59,7 +63,64 @@ func (s *Server) mustBackend() *ipnlocal.LocalBackend {
 	return lb
 }
 
+func (s *Server) awaitBackend(ctx context.Context) (_ *ipnlocal.LocalBackend, ok bool) {
+	lb := s.lb.Load()
+	if lb != nil {
+		return lb, true
+	}
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	s.mu.Lock()
+	h := s.backendWaiter.Add(cancel)
+	s.mu.Unlock()
+	defer func() {
+		s.mu.Lock()
+		delete(s.backendWaiter, h)
+		s.mu.Unlock()
+	}()
+
+	// Try again, now that we've registered, in case there was a
+	// race.
+	lb = s.lb.Load()
+	if lb != nil {
+		return lb, true
+	}
+
+	<-ctx.Done()
+	lb = s.lb.Load()
+	return lb, lb != nil
+}
+
+// serveServerStatus serves the /server-status endpoint which reports whether
+// the LocalBackend is up yet.
+// This is primarily for the Windows GUI, because wintun can take awhile to
+// come up. See https://github.com/tailscale/tailscale/issues/6522.
+func (s *Server) serveServerStatus(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+
+	w.Header().Set("Content-Type", "application/json")
+	var res struct {
+		Error string `json:"error,omitempty"`
+	}
+
+	lb := s.lb.Load()
+	if lb == nil {
+		w.WriteHeader(http.StatusServiceUnavailable)
+		if wait, _ := strconv.ParseBool(r.FormValue("wait")); wait {
+			w.(http.Flusher).Flush()
+			lb, _ = s.awaitBackend(ctx)
+		}
+	}
+
+	if lb == nil {
+		res.Error = "backend not ready"
+	}
+	json.NewEncoder(w).Encode(res)
+}
+
 func (s *Server) serveHTTP(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
 	if r.Method == "CONNECT" {
 		if envknob.GOOS() == "windows" {
 			// For the GUI client when using an exit node. See docs on handleProxyConnectConn.
@@ -70,11 +131,17 @@ func (s *Server) serveHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// TODO(bradfitz): add a status HTTP handler that returns whether there's a
-	// LocalBackend yet, optionally blocking until there is one. See
-	// https://github.com/tailscale/tailscale/issues/6522
-	lb := s.lb.Load()
-	if lb == nil {
+	// Check for this method before the awaitBackend call, as it reports whether
+	// the backend is available.
+	if r.Method == "GET" && r.URL.Path == "/server-status" {
+		s.serveServerStatus(w, r)
+		return
+	}
+
+	lb, ok := s.awaitBackend(ctx)
+	if !ok {
+		// Almost certainly because the context was canceled so the response
+		// here doesn't really matter. The client is gone.
 		http.Error(w, "no backend", http.StatusServiceUnavailable)
 		return
 	}
@@ -304,6 +371,13 @@ func (s *Server) SetLocalBackend(lb *ipnlocal.LocalBackend) {
 		panic("already set")
 	}
 	s.startBackendIfNeeded()
+
+	s.mu.Lock()
+	for _, wake := range s.backendWaiter {
+		wake() // they'll remove themselves when woken
+	}
+	s.mu.Unlock()
+
 	// TODO(bradfitz): send status update to GUI long poller waiter. See
 	// https://github.com/tailscale/tailscale/issues/6522
 }