// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause

// Package routecheck performs status checks for routes from the current host.
package routecheck

import (
	"context"
	"errors"
	"fmt"
	"net/netip"
	"sync/atomic"
	"time"

	"tailscale.com/envknob"
	"tailscale.com/ipn/ipnstate"
	"tailscale.com/tailcfg"
	"tailscale.com/types/logger"
	"tailscale.com/types/netmap"
	"tailscale.com/util/clientmetric"
)

var (
	metricRefresh = clientmetric.NewCounter("routecheck_refresh")
)

// DebugForceClientSideReachabilityRoutecheck reports whether routecheck should be forced on or off.
// If the TS_DEBUG_FORCE_CLIENT_SIDE_REACHABILITY_ROUTECHECK environment variable is true,
// then routecheck is forced on. If it is false, then routecheck is forced off.
// If unset, then the client respects the client-side-reachability and
// client-side-reachability-routecheck node attributes.
var DebugForceClientSideReachabilityRoutecheck = envknob.RegisterOptBool("TS_DEBUG_FORCE_CLIENT_SIDE_REACHABILITY_ROUTECHECK")

// IsEnabled reports whether routecheck probing has been enabled for this client.
func IsEnabled(self tailcfg.NodeView) bool {
	if v, ok := DebugForceClientSideReachabilityRoutecheck().Get(); ok {
		return v // forced
	}
	if !self.Valid() {
		return false
	}
	// TODO(sfllaw): We intend to eventually enable this behaviour by default.
	return self.HasCap(tailcfg.NodeAttrClientSideReachability) &&
		self.HasCap(tailcfg.NodeAttrClientSideReachabilityRouteCheck)
}

// Client generates Reports describing the result of both passive and active
// reachability probing.
type Client struct {
	// Verbose enables verbose logging.
	Verbose bool

	// Logf optionally specifies where to log to.
	// If nil, log.Printf is used.
	Logf logger.Logf

	// These elements are read-only after initialization.
	nb     NodeBackender
	nm     NetMapper
	pinger Pinger

	// HasNetMap is a channel that can be closed to wake up goroutines
	// waiting for the netmap received after connecting to the control plane.
	// This channel gets swapped out for a new one whenever it is closed,
	// to handle disconnecting and reconnecting to the control plane.
	hasNetMap atomic.Pointer[chan struct{}]
}

// NetMapper is the interface that returns the current [netmap.NetworkMap].
type NetMapper interface {
	// NetMapNoPeers returns the latest cached network map received from
	// controlclient WITHOUT a freshly-built Peers slice.
	//
	// On a tailnet with frequent peer churn the cached netmap's Peers slice
	// can be stale relative to the live per-node-backend peers map; non-Peers
	// fields (SelfNode, DNS, PacketFilter, capabilities, ...) are always
	// current. Use this for any caller that does not need to iterate Peers,
	// since it's O(1) regardless of tailnet size.
	//
	// Returns nil if no network map has been received yet.
	NetMapNoPeers() *netmap.NetworkMap

	// NetMapWithPeers returns the latest network map with the Peers slice
	// populated.
	//
	// Currently this is the same as [LocalBackend.NetMapNoPeers]: the cached
	// netmap's Peers slice may be stale relative to the live per-node-backend
	// peers map. A follow-up change will switch this method to return a
	// freshly-built netmap with up-to-date Peers, at O(N) cost per call.
	// Callers that genuinely need the up-to-date peer set should use this
	// method (and document why) so the upcoming change reaches them.
	//
	// Returns nil if no network map has been received yet.
	NetMapWithPeers() *netmap.NetworkMap
}

// NodeBackender is the interface that returns the current [NodeBackend].
type NodeBackender interface {
	NodeBackend() NodeBackend
}

// NodeBackend is an interface to query the current node and its peers.
//
// It is not a snapshot in time but is locked to a particular node.
type NodeBackend interface {
	// Self returns the current node.
	Self() tailcfg.NodeView

	// Peers returns all the current peers.
	Peers() []tailcfg.NodeView
}

// Pinger is the interface that wraps the [tailscale.com/ipn/ipnlocal.LocalBackend.Ping] method.
type Pinger interface {
	Ping(ip netip.Addr, pingType tailcfg.PingType, size int, cb func(*ipnstate.PingResult))
}

// NewClient returns a client that probes its peers using this LocalBackend.
func NewClient(logf logger.Logf, nb NodeBackender, nm NetMapper, pinger Pinger) (*Client, error) {
	if nb == nil {
		return nil, errors.New("NodeBackender must be set")
	}
	if nm == nil {
		return nil, errors.New("NetMapper must be set")
	}
	if pinger == nil {
		return nil, errors.New("Pinger must be set")
	}
	c := &Client{
		Logf:   logf,
		nb:     nb,
		nm:     nm,
		pinger: pinger,
	}
	c.hasNetMap.Store(new(make(chan struct{})))
	return c, nil
}

// NotifyNetMapAvailable wakes up goroutines that have been waiting for the
// non-nil network map that the control plane sends after reconnecting.
func (c *Client) NotifyNetMapAvailable(nm *netmap.NetworkMap) {
	if nm == nil {
		return // client disconnected
	}
	var nextCh *chan struct{}
	for {
		ch := c.hasNetMap.Load()
		if ch == nil || *ch == nil {
			return // Client has been Closed
		}

		if nextCh == nil {
			nextCh = new(make(chan struct{})) // prepare for next non-nil netmap
		}
		if c.hasNetMap.CompareAndSwap(ch, nextCh) {
			close(*ch)
			return
		}
	}
}

func (c *Client) waitForNetMap(ctx context.Context) (*netmap.NetworkMap, error) {
	for {
		ch := c.hasNetMap.Load()
		if ch == nil || *ch == nil {
			return nil, errors.New("routecheck client closed")
		}

		if nm := c.nm.NetMapNoPeers(); nm != nil {
			return nm, nil
		}

		select {
		case <-ctx.Done():
			return nil, ctx.Err()
		case <-*ch: // woken up by NotifyNetMapAvailable
		}
	}
}

// Refresh generates a new reachability report and returns it.
// A peer is considered unreachable if it doesn’t respond within the timeout.
func (c *Client) Refresh(ctx context.Context, timeout time.Duration) (*Report, error) {
	metricRefresh.Add(1)
	r, err := c.ProbeAllHARouters(ctx, 5, timeout)
	if err != nil {
		return nil, fmt.Errorf("error probing routers: %w", err)
	}
	return r, nil
}

// Close immediately stops all active probes.
func (c *Client) Close() error {
	if c == nil {
		return nil
	}

	ch := c.hasNetMap.Swap(nil) // clear before waking anything up
	if ch != nil && *ch != nil {
		close(*ch)
	}

	return nil
}