// Package tailcore is the Go shim that gomobile-binds the real Tailscale core (tailscale.com) into a
// native framework (iOS .xcframework / Android .aar) for OffTail. Everything the NE / VpnService needs
// from the core is exposed here as gomobile-friendly functions (simple params + return types).
//
// This is the WHOLE-DEVICE path: Start() takes the OS tun file descriptor handed over by the platform
// (iOS NEPacketTunnelProvider's utun, Android VpnService.establish()) and drives it through wgengine +
// LocalBackend, pointed at a self-hosted Headscale. Modeled directly on tsnet.Server.start() — the
// canonical in-repo node bring-up — but with a real fd-backed tun instead of netstack-only.
//
// STATUS: compiles + binds; the control-plane bring-up (register with Headscale via the pre-auth key)
// is wired. The last mile is the native ROUTER/DNS bridge — routes + DNS must be applied on the OS side
// (NEPacketTunnelNetworkSettings / VpnService.Builder), which needs on-device testing. Until that's in,
// a nil Go router means packets reach the tun but the OS isn't told which routes to send through it.
package tailcore

import (
	"context"
	"encoding/json"
	"fmt"
	"log"
	"os"
	"path/filepath"
	"sync"

	"tailscale.com/ipn"
	"tailscale.com/ipn/ipnlocal"
	"tailscale.com/ipn/store"
	"tailscale.com/tailcfg"
	"tailscale.com/net/tsdial"
	"tailscale.com/tsd"
	"tailscale.com/types/logid"
	"tailscale.com/types/opt"
	"tailscale.com/version"
	"tailscale.com/wgengine"
	"tailscale.com/wgengine/netstack"
)

var (
	mu      sync.Mutex
	backend *ipnlocal.LocalBackend
	cancel  context.CancelFunc
	// Kept alongside `cancel` so operations started AFTER Start — LoginTo's interactive login — share
	// the tunnel's lifetime and are cancelled with it, rather than running on a detached Background.
	runCtx  context.Context
)

// Version returns the embedded Tailscale core version — a no-network call proving the core is linked in.
func Version() string { return version.Long() }

// Ping is a trivial liveness check for the native bridge.
func Ping() string { return "tailcore-ok" }

// Start brings up a whole-device Tailscale node on the OS tun file descriptor, pointed at a Headscale
// control server, registering with a pre-auth key. gomobile-friendly signature (int32/string/error).
func Start(tunFD int32, controlURL, authKey, hostname, stateDir string) error {
	mu.Lock()
	defer mu.Unlock()
	if backend != nil {
		return nil // already running
	}

	// Give Tailscale's logpolicy a writable directory for its log-state/ID. On Android (GOOS=android)
	// logpolicy.LogsDir() matches none of its OS cases and its fallbacks all fail — DefaultTailscaledStateFile
	// is empty, UserCacheDir errors (no $HOME), cwd is "/", and MkdirTemp("") fails (no writable /tmp) — so it
	// PANICS ("no safe place found to store log state"). TS_LOGS_DIR is the first thing it honors; point it at
	// the per-app state dir the platform handed us (Android filesDir / iOS Application Support). Harmless on
	// iOS, where a cache dir already exists.
	_ = os.MkdirAll(stateDir, 0o700)
	_ = os.Setenv("TS_LOGS_DIR", stateDir)

	// Log to stderr → surfaces in the device's unified log (Console.app, process OffTailTunnel) so we can
	// watch the core register/connect. TODO: quiet or gate this once the tunnel is proven.
	logf := func(format string, args ...any) { log.Printf("[tailcore] "+format, args...) }

	// Wrap the OS-provided tun fd (NEPacketTunnelProvider's utun on iOS, VpnService.establish() on
	// Android). The wrapping differs per-OS — iOS must skip setMTU, Android must skip netlink — so it
	// lives behind build-tagged makeTUN (maketun_apple.go / maketun_droid.go).
	dev, err := makeTUN(tunFD)
	if err != nil {
		return fmt.Errorf("tun from fd: %w", err)
	}

	sys := tsd.NewSystem()
	dialer := &tsdial.Dialer{Logf: logf}
	dialer.SetBus(sys.Bus.Get())

	eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
		Tun:           dev,
		EventBus:      sys.Bus.Get(),
		Dialer:        dialer,
		SetSubsystem:  sys.Set,
		ControlKnobs:  sys.ControlKnobs(),
		HealthTracker: sys.HealthTracker.Get(),
		Metrics:       sys.UserMetricsRegistry(), // required by NewUserspaceEngine
		// Router stays nil: the OS applies routes itself (NE settings / VpnService.Builder).
		//
		// DNS is NOT nil, and that is the fix for MagicDNS. Left nil, the engine substitutes a fake
		// no-op OSConfigurator ("using fake (no-op) DNS configurator") and builds its dns.Manager
		// around it — so the resolver at 100.100.100.100 had no base config to forward non-tailnet
		// queries to. Ours returns the platform's real resolvers from GetBaseConfig; see dns.go.
		//
		// Passed here rather than constructed separately: the engine creates and registers the
		// Manager itself, and calling sys.Set on a second one panics with "*dns.Manager is already
		// set".
		DNS: mobileOSConfigurator{},
	})
	if err != nil {
		return fmt.Errorf("wgengine: %w", err)
	}
	sys.Set(eng)

	ns, err := netstack.Create(logf, sys.Tun.Get(), eng, sys.MagicSock.Get(), dialer, sys.DNSManager.Get(), sys.ProxyMapper())
	if err != nil {
		return fmt.Errorf("netstack: %w", err)
	}
	sys.Tun.Get().Start()
	sys.Set(ns)

	st, err := store.New(logf, filepath.Join(stateDir, "tailscaled.state"))
	if err != nil {
		return fmt.Errorf("store: %w", err)
	}
	sys.Set(st)

	lb, err := ipnlocal.NewLocalBackend(logf, logid.PublicID{}, sys, 0)
	if err != nil {
		return fmt.Errorf("localbackend: %w", err)
	}

	// Start netstack against the LocalBackend. netstack.Create() only INSTALLS its tstun hooks (incl. the
	// PreFilterPacketOutboundToWireGuardNetstackIntercept); ns.Start() is what marks netstack ready, wires
	// its LocalBackend, and launches the inject goroutine. WITHOUT this call the outbound intercept is live
	// but netstack isn't running, so every outbound packet is dropped and no peer is ever materialized in
	// wireguard-go (peers stay in the netmap + magicsock but never reach the engine — disco works, data
	// doesn't). tsnet and tailscaled both call ns.Start(lb); omitting it was the whole-device data-path bug.
	// Defaults (ProcessLocalIPs/ProcessSubnets = false) are correct here: this is a real-tun whole-device
	// node, so tailnet traffic must flow through the OS tun to peers, not be terminated in netstack.
	if err := ns.Start(lb); err != nil {
		return fmt.Errorf("netstack start: %w", err)
	}

	ctx, cancelFn := context.WithCancel(context.Background())
	cancel = cancelFn
	runCtx = ctx

	/*
	 NEVER rewrite an existing profile's control server.

	 The app keeps ONE stored control URL, but every profile has its own — and this Start runs on
	 whichever profile happens to be current, on every single tunnel start. So a restart while the app
	 held a different tailnet's URL silently rewrote the current profile to point at the wrong control
	 server: a Tailscale profile ended up carrying headscale.pastilhas.eu, mislabelled and unable to
	 reach its own tailnet.

	 A profile's control server is set once, when the profile is created (here for the first one,
	 LoginTo for the rest). After that the stored value wins and the caller's is ignored.
	*/
	effectiveControlURL := controlURL
	// Hostname is the SAME class of bug: one app-level value, applied to whichever profile is current,
	// on every start. Each profile advertises its own node name, and a restart would rename the node
	// on one tailnet to whatever was last typed for another.
	effectiveHostname := hostname
	if p := lb.Prefs(); p.Valid() {
		if p.ControlURL() != "" {
			effectiveControlURL = p.ControlURL()
		}
		if p.Hostname() != "" {
			effectiveHostname = p.Hostname()
		}
	}

	if err := lb.Start(ipn.Options{
		UpdatePrefs: &ipn.Prefs{
			ControlURL:  effectiveControlURL,
			WantRunning: true,
			Hostname:    effectiveHostname,
			// Do stateful packet filtering (double negative: NoStatefulFiltering=false ⇒ ON) so replies to
			// THIS node's outbound connections are allowed back through the netmap's inbound filter. Without
			// it, phone→service traffic goes out but every reply is dropped (the connection just hangs) unless
			// the ACL happens to also grant the reverse direction. tailscaled's NewPrefs defaults this OFF
			// (NoStatefulFiltering=true); a minimal bring-up like ours must opt in explicitly.
			NoStatefulFiltering: opt.NewBool(false),
			// Accept the DNS config the control plane sends — MagicDNS. Same trap as the line above:
			// tailscaled's NewPrefs() defaults this ON, but we build Prefs as a struct literal so it
			// gets Go's zero value, false. The node then tells Headscale it does not want DNS, and the
			// netmap arrives with {DefaultResolvers:[] Routes:{} SearchDomains:[] Hosts:0} — which is
			// why `alpha` never resolved no matter what the server was configured to send. Headscale's
			// own docs call this out: MagicDNS only helps a node with accept-dns on (CorpDNS true).
			CorpDNS: true,
		},
		AuthKey: authKey,
	}); err != nil {
		cancelFn()
		return fmt.Errorf("localbackend start: %w", err)
	}
	if lb.State() == ipn.NeedsLogin {
		if err := lb.StartLoginInteractive(ctx); err != nil {
			cancelFn()
			return fmt.Errorf("login: %w", err)
		}
	}

	backend = lb
	return nil
}

// ── Exit nodes ─────────────────────────────────────────────────────────────────────────────────
//
// Routing ALL traffic through another node on the tailnet. The single most-used control in the real
// Tailscale app after the on/off switch, and the reason a prefs-editing verb had to exist at all:
// everything else here is set once at Start, but an exit node is chosen and changed while running.

// SetExitNode routes all traffic through the peer with this stable node id. An empty id turns it off.
//
// EditPrefs rather than Start: this changes a preference on a RUNNING node, and restarting to apply
// it would drop the tunnel for something the core can do live. `ExitNodeIP` is deliberately not used
// — an IP can be reassigned, while the stable id survives the node changing address.
func SetExitNode(stableID string) error {
	mu.Lock()
	defer mu.Unlock()
	lb := backend
	if lb == nil {
		return fmt.Errorf("not started")
	}
	mp := &ipn.MaskedPrefs{ExitNodeIDSet: true}
	mp.ExitNodeID = tailcfg.StableNodeID(stableID)
	// Allowing LAN access alongside an exit node is what people expect: routing everything through a
	// remote node should not cut off the printer in the same room.
	mp.ExitNodeAllowLANAccessSet = true
	mp.ExitNodeAllowLANAccess = true
	if _, err := lb.EditPrefs(mp); err != nil {
		return fmt.Errorf("set exit node: %w", err)
	}
	return nil
}

// SetAcceptRoutes turns on using subnet routes other nodes advertise.
//
// Off by default in Tailscale, and the reason a node can see its peers but not the network behind
// one of them — which looks like a broken route rather than a preference.
func SetAcceptRoutes(on bool) error {
	mu.Lock()
	defer mu.Unlock()
	lb := backend
	if lb == nil {
		return fmt.Errorf("not started")
	}
	mp := &ipn.MaskedPrefs{RouteAllSet: true}
	mp.RouteAll = on
	if _, err := lb.EditPrefs(mp); err != nil {
		return fmt.Errorf("set accept routes: %w", err)
	}
	return nil
}

// SubnetRoutes returns the subnet routes this node should carry, as a JSON array of CIDR strings.
//
// Empty unless RouteAll is on — accepting routes is opt-in, and a tun that claimed them regardless
// would capture traffic the user never asked us to carry.
//
// These come from peers' PrimaryRoutes: the prefixes each node is currently the primary router for.
// The tailnet's own ranges are filtered out because the tun already claims those, and re-adding them
// as subnet routes would be redundant at best.
//
// It exists because accepting a route in the CORE is only half of it. The OS decides what enters the
// tunnel, so unless the tun claims these prefixes the traffic never reaches us — the same gap that
// made choosing an exit node do nothing observable.
func SubnetRoutes() string {
	mu.Lock()
	defer mu.Unlock()
	if backend == nil {
		return "[]"
	}
	p := backend.Prefs()
	if !p.Valid() || !p.RouteAll() {
		return "[]"
	}
	st := backend.Status()
	seen := map[string]bool{}
	out := []string{}
	for _, ps := range st.Peer {
		if ps == nil || ps.PrimaryRoutes == nil {
			continue
		}
		for _, r := range ps.PrimaryRoutes.All() {
			// Skip the tailnet's own space and default routes: the first is already claimed, and the
			// second is an exit node advertising itself, which is a separate choice the user makes
			// explicitly rather than something to inherit by turning on subnet routes.
			if r.Bits() == 0 {
				continue
			}
			a := r.Addr()
			if a.Is4() && a.As4()[0] == 100 && a.As4()[1] >= 64 && a.As4()[1] <= 127 {
				continue
			}
			cidr := r.String()
			if !seen[cidr] {
				seen[cidr] = true
				out = append(out, cidr)
			}
		}
	}
	b, err := json.Marshal(out)
	if err != nil {
		return "[]"
	}
	return string(b)
}

// Prefs returns the live preferences as JSON, so the app can show what is actually set rather than
// what it last asked for. gomobile binds simple types only, hence a string.
func Prefs() string {
	mu.Lock()
	defer mu.Unlock()
	lb := backend
	if lb == nil {
		return "{}"
	}
	p := lb.Prefs()
	if !p.Valid() {
		return "{}"
	}
	out := map[string]any{
		"exitNodeID":   string(p.ExitNodeID()),
		"acceptRoutes": p.RouteAll(),
		"corpDNS":      p.CorpDNS(),
	}
	b, err := json.Marshal(out)
	if err != nil {
		return "{}"
	}
	return string(b)
}

// NOTE ON LOCKING
//
// These accessors used to take the mutex only to COPY the backend pointer, then release it and call
// methods on that copy. That is a use-after-shutdown waiting to happen: Stop() calls Shutdown() and
// nils the pointer under the same mutex, so a caller that had already copied it goes on to use a
// backend that is being torn down. Android polls Status() every two seconds and a profile switch
// calls Stop() — the two race, and the result is a native crash with no Java stack, intermittently.
//
// iOS never showed it because the extension is a separate process torn down whole.
//
// So the short operations below hold the lock for the WHOLE call. They are microseconds of work;
// serialising them against Stop costs nothing. LoginTo is deliberately excluded — it blocks on
// StartLoginInteractive, and holding the mutex across a network round trip would block Stop instead.

// ── Profiles ───────────────────────────────────────────────────────────────────────────────────
//
// Several tailnets on one device, switched between: a self-hosted Headscale and a login.tailscale.com
// account, say. The core has supported this the whole time — LocalBackend keeps a profile per login,
// each with its own control URL, node key and prefs — it was simply never exposed.
//
// gomobile only binds simple types, so these take and return strings; the list is JSON.

// ProfileInfo is the gomobile-safe shape of a login profile. `ipn.LoginProfileView` cannot cross the
// binding, and most of it is uninteresting to a picker.
type profileInfo struct {
	ID      string `json:"id"`
	Name    string `json:"name"`    // the login name, e.g. andre@example.com
	Tailnet string `json:"tailnet"` // NetworkProfile.DomainName — which tailnet this is
	Current bool   `json:"current"`
	// ControlURL is which control plane this profile talks to, and it is the ONLY authoritative way to
	// say whether a stored login is Tailscale or a self-hosted Headscale. Inferring it from the tailnet
	// name does not work: a personal Tailscale tailnet is named after the login email, and a Headscale
	// namespace can be called anything at all.
	ControlURL string `json:"controlUrl"`
}

// ListProfiles returns every stored login as JSON, with the active one flagged.
func ListProfiles() string {
	mu.Lock()
	defer mu.Unlock()
	lb := backend
	if lb == nil {
		return "[]"
	}
	currentID := lb.CurrentProfile().ID()
	out := []profileInfo{}
	for _, p := range lb.ListProfiles() {
		out = append(out, profileInfo{
			ID:         string(p.ID()),
			Name:       p.Name(),
			Tailnet:    p.NetworkProfile().DomainName,
			Current:    p.ID() == currentID,
			ControlURL: p.ControlURL(),
		})
	}
	b, err := json.Marshal(out)
	if err != nil {
		return "[]"
	}
	return string(b)
}

// SwitchProfile makes an existing login the active one. The node re-registers under that identity;
// the caller is responsible for tearing the tun down and back up around it.
func SwitchProfile(id string) error {
	mu.Lock()
	defer mu.Unlock()
	lb := backend
	if lb == nil {
		return fmt.Errorf("not started")
	}
	return lb.SwitchProfile(ipn.ProfileID(id))
}

// NewProfile begins an additional login, leaving the existing ones intact. The node lands in
// NeedsLogin, which is the point: Status().AuthURL then carries where to send the user.
/*
LoginTo joins ANOTHER tailnet without restarting the tunnel.

NewProfile alone is not enough. It creates a blank profile on the running backend, but the control
URL and the pre-auth key only reach the core through Start — and on iOS Start only happens when the
Network Extension launches. Recycling the tunnel to trigger it *loses the blank profile*: an
unregistered profile is not persisted, so on reload the core falls back to the previous one and
Start then overwrites ITS control URL. The result is a second tailnet silently replacing the first,
same profile id and all.

Doing both here, against the live backend, is the only ordering where neither step undoes the other:
branch to a new profile, then start it with its own control URL and key. An empty authKey lands in
NeedsLogin and begins interactive login, exactly as the initial Start does.
*/
func LoginTo(controlURL, authKey, hostname string) error {
	mu.Lock()
	lb := backend
	ctxLocal := runCtx
	mu.Unlock()
	if lb == nil {
		return fmt.Errorf("not started")
	}
	if err := lb.NewProfile(); err != nil {
		return fmt.Errorf("new profile: %w", err)
	}
	// Same prefs as the initial Start — see the comments there for why CorpDNS and stateful
	// filtering must be set explicitly rather than left at Go's zero values.
	if err := lb.Start(ipn.Options{
		UpdatePrefs: &ipn.Prefs{
			ControlURL:          controlURL,
			WantRunning:         true,
			Hostname:            hostname,
			NoStatefulFiltering: opt.NewBool(false),
			CorpDNS:             true,
		},
		AuthKey: authKey,
	}); err != nil {
		return fmt.Errorf("start profile: %w", err)
	}
	if lb.State() == ipn.NeedsLogin {
		if ctxLocal == nil {
			ctxLocal = context.Background()
		}
		if err := lb.StartLoginInteractive(ctxLocal); err != nil {
			return fmt.Errorf("login: %w", err)
		}
	}
	return nil
}

func NewProfile() error {
	mu.Lock()
	defer mu.Unlock()
	lb := backend
	if lb == nil {
		return fmt.Errorf("not started")
	}
	return lb.NewProfile()
}

// DeleteProfile forgets a login. Deleting the ACTIVE profile is allowed by the core and leaves the
// node logged out, so callers should switch away first if that is not what they meant.
func DeleteProfile(id string) error {
	mu.Lock()
	defer mu.Unlock()
	lb := backend
	if lb == nil {
		return fmt.Errorf("not started")
	}
	return lb.DeleteProfile(ipn.ProfileID(id))
}

// Stop tears the node down.
func Stop() {
	mu.Lock()
	defer mu.Unlock()
	if cancel != nil {
		cancel()
		cancel = nil
	}
	if backend != nil {
		backend.Shutdown()
		backend = nil
	}
}

// Status returns the node status as JSON (self IPs, peers, backend state) — drives the RN UI.
func Status() string {
	mu.Lock()
	defer mu.Unlock()
	if backend == nil {
		return `{"BackendState":"Stopped"}`
	}
	b, err := json.Marshal(backend.Status())
	if err != nil {
		return `{"BackendState":"Error"}`
	}
	return string(b)
}