tailscale/health/health.go

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

// Package health is a registry for other packages to report & check
// overall health status of the node.
package health

import (
	"errors"
	"fmt"
	"maps"
	"net/http"
	"os"
	"runtime"
	"sort"
	"sync"
	"sync/atomic"
	"time"

	"tailscale.com/envknob"
	"tailscale.com/tailcfg"
	"tailscale.com/types/opt"
	"tailscale.com/util/cibuild"
	"tailscale.com/util/mak"
	"tailscale.com/util/multierr"
	"tailscale.com/util/set"
	"tailscale.com/version"
)

var (
	mu           sync.Mutex
	debugHandler map[string]http.Handler
)

// ReceiveFunc is one of the three magicsock Receive funcs (IPv4, IPv6, or
// DERP).
type ReceiveFunc int

// ReceiveFunc indices for Tracker.MagicSockReceiveFuncs.
const (
	ReceiveIPv4 ReceiveFunc = 0
	ReceiveIPv6 ReceiveFunc = 1
	ReceiveDERP ReceiveFunc = 2
)

func (f ReceiveFunc) String() string {
	if f < 0 || int(f) >= len(receiveNames) {
		return fmt.Sprintf("ReceiveFunc(%d)", f)
	}
	return receiveNames[f]
}

var receiveNames = []string{
	ReceiveIPv4: "ReceiveIPv4",
	ReceiveIPv6: "ReceiveIPv6",
	ReceiveDERP: "ReceiveDERP",
}

// Tracker tracks the health of various Tailscale subsystems,
// comparing each subsystems' state with each other to make sure
// they're consistent based on the user's intended state.
type Tracker struct {
	// MagicSockReceiveFuncs tracks the state of the three
	// magicsock receive functions: IPv4, IPv6, and DERP.
	MagicSockReceiveFuncs [3]ReceiveFuncStats // indexed by ReceiveFunc values

	// mu guards everything that follows.
	mu sync.Mutex

	warnables   []*Warnable // keys ever set
	warnableVal map[*Warnable]*warningState

	// sysErr maps subsystems to their current error (or nil if the subsystem is healthy)
	// Deprecated: using Warnables should be preferred
	sysErr   map[Subsystem]error
	watchers set.HandleSet[func(*Warnable, *UnhealthyState)] // opt func to run if error state changes
	timer    *time.Timer

	latestVersion   *tailcfg.ClientVersion // or nil
	checkForUpdates bool

	inMapPoll               bool
	inMapPollSince          time.Time
	lastMapPollEndedAt      time.Time
	lastStreamedMapResponse time.Time
	derpHomeRegion          int
	derpHomeless            bool
	derpRegionConnected     map[int]bool
	derpRegionHealthProblem map[int]string
	derpRegionLastFrame     map[int]time.Time
	lastMapRequestHeard     time.Time // time we got a 200 from control for a MapRequest
	ipnState                string
	ipnWantRunning          bool
	anyInterfaceUp          opt.Bool // empty means unknown (assume true)
	udp4Unbound             bool
	controlHealth           []string
	lastLoginErr            error
	localLogConfigErr       error
	tlsConnectionErrors     map[string]error // map[ServerName]error
}

// Subsystem is the name of a subsystem whose health can be monitored.
//
// Deprecated: Registering a Warnable using Register() and updating its health state
// with  SetUnhealthy() and SetHealthy() should be preferred.
type Subsystem string

const (
	// SysRouter is the name of the wgengine/router subsystem.
	SysRouter = Subsystem("router")

	// SysDNS is the name of the net/dns subsystem.
	SysDNS = Subsystem("dns")

	// SysDNSOS is the name of the net/dns OSConfigurator subsystem.
	SysDNSOS = Subsystem("dns-os")

	// SysDNSManager is the name of the net/dns manager subsystem.
	SysDNSManager = Subsystem("dns-manager")

	// SysTKA is the name of the tailnet key authority subsystem.
	SysTKA = Subsystem("tailnet-lock")
)

var subsystemsWarnables = map[Subsystem]*Warnable{}

const legacyErrorArgKey = "LegacyError"

// Warnable() returns a Warnable representing a legacy Subsystem. This is used
// *temporarily* while we migrate the old health infrastructure based on
// Subsystems to the new Warnables architecture.
func (s Subsystem) Warnable() *Warnable {
	if w, ok := subsystemsWarnables[s]; ok {
		return w
	} else {
		w := Register(&Warnable{
			Code:     WarnableCode(s),
			Severity: SeverityMedium,
			Text: func(args Args) string {
				return args[legacyErrorArgKey]
			},
		})
		subsystemsWarnables[s] = w
		return w
	}
}

var registeredWarnables = map[WarnableCode]*Warnable{}

// Register registers a new Warnable with the health package and returns it.
// Register panics if the Warnable was already registered, because Warnables
// should be unique across the program.
func Register(w *Warnable) *Warnable {
	if registeredWarnables[w.Code] != nil {
		panic(fmt.Sprintf("health: a Warnable with code %q was already registered", w.Code))
	}
	mak.Set(&registeredWarnables, w.Code, w)
	return w
}

// unregister removes a Warnable from the health package. It should only be used
// for testing purposes.
func unregister(w *Warnable) {
	if registeredWarnables[w.Code] == nil {
		panic(fmt.Sprintf("health: attempting to unregister Warnable %q that was not registered", w.Code))
	}
	delete(registeredWarnables, w.Code)
}

// WarnableCode is a string that distinguishes each Warnable from others. It is globally unique within
// the program.
type WarnableCode string

// A Warnable is something that we might want to warn the user about, or not. A Warnable is either
// in an healthy or unhealth state. A Warnable is unhealthy if the Tracker knows about a WarningState
// affecting the Warnable.
// In most cases, Warnables are components of the backend (for instance, "DNS" or "Magicsock").
// Warnables are similar to the Subsystem type previously used in this package, but they provide
// a unique identifying code for each Warnable, along with more metadata that makes it easier for
// a GUI to display the Warnable in a user-friendly way.
type Warnable struct {
	// Code is a string that uniquely identifies this Warnable across the entire Tailscale backend,
	// and can be mapped to a user-displayable localized string.
	Code WarnableCode
	// Title is a string that the GUI uses as title for any message involving this Warnable. The title
	// should be short and fit in a single line.
	Title string
	// Text is a function that generates an extended string that the GUI will display to the user when
	// this Warnable is in an unhealthy state. The function can use the Args map to provide dynamic
	// information to the user.
	Text func(args Args) string
	// Severity is the severity of the Warnable, which the GUI can use to determine how to display it.
	// For instance, a Warnable with SeverityHigh could trigger a modal view, while a Warnable with
	// SeverityLow could be displayed in a less intrusive way.
	// TODO(angott): turn this into a SeverityFunc, which allows the Warnable to change its severity based on
	// the Args of the unhappy state, just like we do in the Text function.
	Severity Severity
	// DependsOn is a set of Warnables that this Warnable depends, on and need to be healthy
	// before this Warnable can also be healthy again. The GUI can use this information to ignore
	// this Warnable if one of its dependencies is unhealthy.
	DependsOn []*Warnable

	// MapDebugFlag is a MapRequest.DebugFlag that is sent to control when this Warnable is unhealthy
	//
	// Deprecated: this is only used in one case, and will be removed in a future PR
	MapDebugFlag string

	// If true, this warnable is related to configuration of networking stack
	// on the machine that impacts connectivity.
	ImpactsConnectivity bool
}

// StaticMessage returns a function that always returns the input string, to be used in
// simple Warnables that do not use the Args map to generate their Text.
func StaticMessage(s string) func(Args) string {
	return func(Args) string { return s }
}

// nil reports whether t is nil.
// It exists to accept nil *Tracker receivers on all methods
// to at least not crash. But because a nil receiver indicates
// some lost Tracker plumbing, we want to capture stack trace
// samples when it occurs.
func (t *Tracker) nil() bool {
	if t != nil {
		return false
	}

	if cibuild.On() {
		stack := make([]byte, 1<<10)
		stack = stack[:runtime.Stack(stack, false)]
		fmt.Fprintf(os.Stderr, "## WARNING: (non-fatal) nil health.Tracker (being strict in CI):\n%s\n", stack)
	}
	// TODO(bradfitz): open source our "unexpected" package
	// and use it here to capture samples of stacks where
	// t is nil.
	return true
}

// Severity represents how serious an error is. Each GUI interprets this severity value in different ways,
// to surface the error in a more or less visible way. For instance, the macOS GUI could change its menubar
// icon to display an exclamation mark and present a modal notification for SeverityHigh warnings, but not
// for SeverityLow messages, which would only appear in the Settings window.
type Severity string

const (
	SeverityHigh   Severity = "high"
	SeverityMedium Severity = "medium"
	SeverityLow    Severity = "low"
)

// Args is a map of Args to string values that can be used to provide parameters regarding
// the unhealthy state of a Warnable.
// For instance, if you have a Warnable to track the health of DNS lookups, here you can include
// the hostname that failed to resolve, or the IP address of the DNS server that has been failing
// to respond. You can then use these parameters in the Text function of the Warnable to provide a detailed
// error message to the user.
type Args map[Arg]string

// A warningState is a condition affecting a Warnable. For each Warnable known to the Tracker, a Warnable
// is in an unhappy state if there is a warningState associated with the Warnable.
type warningState struct {
	BrokenSince time.Time // when the Warnable became unhealthy
	Args        Args      // args can be used to provide parameters to the function that generates the Text in the Warnable
}

func (ws *warningState) Equal(other *warningState) bool {
	if ws == nil && other == nil {
		return true
	}
	if ws == nil || other == nil {
		return false
	}
	return ws.BrokenSince.Equal(other.BrokenSince) && maps.Equal(ws.Args, other.Args)
}

// SetUnhealthy sets a warningState for the given Warnable with the provided Args, and should be
// called when a Warnable becomes unhealthy, or its unhealthy status needs to be updated.
// SetUnhealthy takes ownership of args. The args can be nil if no additional information is
// needed for the unhealthy state.
func (t *Tracker) SetUnhealthy(w *Warnable, args Args) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.setUnhealthyLocked(w, args)
}

func (t *Tracker) setUnhealthyLocked(w *Warnable, args Args) {
	if w == nil {
		return
	}

	// If we already have a warningState for this Warnable with an earlier BrokenSince time, keep that
	// BrokenSince time.
	brokenSince := time.Now()
	if existingWS := t.warnableVal[w]; existingWS != nil {
		brokenSince = existingWS.BrokenSince
	}

	if t.warnableVal[w] == nil {
		t.warnables = append(t.warnables, w)
	}
	ws := &warningState{
		BrokenSince: brokenSince,
		Args:        args,
	}
	prevWs := t.warnableVal[w]
	mak.Set(&t.warnableVal, w, ws)
	if !ws.Equal(prevWs) {
		for _, cb := range t.watchers {
			go cb(w, w.unhealthyState(ws))
		}
	}
}

// SetHealthy removes any warningState for the given Warnable.
func (t *Tracker) SetHealthy(w *Warnable) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.setHealthyLocked(w)
}

func (t *Tracker) setHealthyLocked(w *Warnable) {
	if t.warnableVal[w] == nil {
		// Nothing to remove
		return
	}

	delete(t.warnableVal, w)
	for _, cb := range t.watchers {
		go cb(w, nil)
	}
}

// AppendWarnableDebugFlags appends to base any health items that are currently in failed
// state and were created with MapDebugFlag.
func (t *Tracker) AppendWarnableDebugFlags(base []string) []string {
	if t.nil() {
		return base
	}

	ret := base

	t.mu.Lock()
	defer t.mu.Unlock()
	for w, err := range t.warnableVal {
		if w.MapDebugFlag == "" {
			continue
		}
		if err != nil {
			ret = append(ret, w.MapDebugFlag)
		}
	}
	sort.Strings(ret[len(base):]) // sort the new ones
	return ret
}

// RegisterWatcher adds a function that will be called whenever the health state of any Warnable changes.
// If a Warnable becomes unhealthy or its unhealthy state is updated, the callback will be called with its
// current Representation.
// If a Warnable becomes healthy, the callback will be called with ws set to nil.
// The provided callback function will be executed in its own goroutine. The returned function can be used
// to unregister the callback.
func (t *Tracker) RegisterWatcher(cb func(w *Warnable, r *UnhealthyState)) (unregister func()) {
	if t.nil() {
		return func() {}
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.watchers == nil {
		t.watchers = set.HandleSet[func(*Warnable, *UnhealthyState)]{}
	}
	handle := t.watchers.Add(cb)
	if t.timer == nil {
		t.timer = time.AfterFunc(time.Minute, t.timerSelfCheck)
	}
	return func() {
		t.mu.Lock()
		defer t.mu.Unlock()
		delete(t.watchers, handle)
		if len(t.watchers) == 0 && t.timer != nil {
			t.timer.Stop()
			t.timer = nil
		}
	}
}

// SetRouterHealth sets the state of the wgengine/router.Router.
//
// Deprecated: Warnables should be preferred over Subsystem errors.
func (t *Tracker) SetRouterHealth(err error) { t.setErr(SysRouter, err) }

// RouterHealth returns the wgengine/router.Router error state.
//
// Deprecated: Warnables should be preferred over Subsystem errors.
func (t *Tracker) RouterHealth() error { return t.get(SysRouter) }

// SetDNSHealth sets the state of the net/dns.Manager
//
// Deprecated: Warnables should be preferred over Subsystem errors.
func (t *Tracker) SetDNSHealth(err error) { t.setErr(SysDNS, err) }

// DNSHealth returns the net/dns.Manager error state.
//
// Deprecated: Warnables should be preferred over Subsystem errors.
func (t *Tracker) DNSHealth() error { return t.get(SysDNS) }

// SetDNSOSHealth sets the state of the net/dns.OSConfigurator
//
// Deprecated: Warnables should be preferred over Subsystem errors.
func (t *Tracker) SetDNSOSHealth(err error) { t.setErr(SysDNSOS, err) }

// SetDNSManagerHealth sets the state of the Linux net/dns manager's
// discovery of the /etc/resolv.conf situation.
//
// Deprecated: Warnables should be preferred over Subsystem errors.
func (t *Tracker) SetDNSManagerHealth(err error) { t.setErr(SysDNSManager, err) }

// DNSOSHealth returns the net/dns.OSConfigurator error state.
//
// Deprecated: Warnables should be preferred over Subsystem errors.
func (t *Tracker) DNSOSHealth() error { return t.get(SysDNSOS) }

// SetTKAHealth sets the health of the tailnet key authority.
//
// Deprecated: Warnables should be preferred over Subsystem errors.
func (t *Tracker) SetTKAHealth(err error) { t.setErr(SysTKA, err) }

// TKAHealth returns the tailnet key authority error state.
//
// Deprecated: Warnables should be preferred over Subsystem errors.
func (t *Tracker) TKAHealth() error { return t.get(SysTKA) }

// SetLocalLogConfigHealth sets the error state of this client's local log configuration.
func (t *Tracker) SetLocalLogConfigHealth(err error) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.localLogConfigErr = err
}

// SetTLSConnectionError sets the error state for connections to a specific
// host. Setting the error to nil will clear any previously-set error.
func (t *Tracker) SetTLSConnectionError(host string, err error) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	if err == nil {
		delete(t.tlsConnectionErrors, host)
	} else {
		mak.Set(&t.tlsConnectionErrors, host, err)
	}
}

func RegisterDebugHandler(typ string, h http.Handler) {
	mu.Lock()
	defer mu.Unlock()
	mak.Set(&debugHandler, typ, h)
}

func DebugHandler(typ string) http.Handler {
	mu.Lock()
	defer mu.Unlock()
	return debugHandler[typ]
}

func (t *Tracker) get(key Subsystem) error {
	if t.nil() {
		return nil
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	return t.sysErr[key]
}

func (t *Tracker) setErr(key Subsystem, err error) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.setLocked(key, err)
}

func (t *Tracker) setLocked(key Subsystem, err error) {
	if t.sysErr == nil {
		t.sysErr = map[Subsystem]error{}
	}
	old, ok := t.sysErr[key]
	if !ok && err == nil {
		// Initial happy path.
		t.sysErr[key] = nil
		t.selfCheckLocked()
		return
	}
	if ok && (old == nil) == (err == nil) {
		// No change in overall error status (nil-vs-not), so
		// don't run callbacks, but exact error might've
		// changed, so note it.
		if err != nil {
			t.sysErr[key] = err
		}
		return
	}
	t.sysErr[key] = err
	t.selfCheckLocked()
}

// updateLegacyErrorWarnableLocked takes a legacy Subsystem and an optional error, and
// updates the WarningState for that legacy Subsystem, setting it to healthy or unhealthy.
// It is used temporarily while we migrate from Subsystems to Warnables.
//
// Deprecated: this function will be removed after migrating all subsystem errors to use
// Warnables instead.
func (t *Tracker) updateLegacyErrorWarnableLocked(key Subsystem, err error) {
	w := key.Warnable()
	if err != nil {
		t.setUnhealthyLocked(key.Warnable(), Args{legacyErrorArgKey: err.Error()})
	} else {
		t.setHealthyLocked(w)
	}
}

func (t *Tracker) SetControlHealth(problems []string) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.controlHealth = problems
	t.selfCheckLocked()
}

// GotStreamedMapResponse notes that we got a tailcfg.MapResponse
// message in streaming mode, even if it's just a keep-alive message.
//
// This also notes that a map poll is in progress. To unset that, call
// SetOutOfPollNetMap().
func (t *Tracker) GotStreamedMapResponse() {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.lastStreamedMapResponse = time.Now()
	if !t.inMapPoll {
		t.inMapPoll = true
		t.inMapPollSince = time.Now()
	}
	t.selfCheckLocked()
}

// SetOutOfPollNetMap records that the client is no longer in
// an HTTP map request long poll to the control plane.
func (t *Tracker) SetOutOfPollNetMap() {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	if !t.inMapPoll {
		return
	}
	t.inMapPoll = false
	t.lastMapPollEndedAt = time.Now()
	t.selfCheckLocked()
}

// GetInPollNetMap reports whether the client has an open
// HTTP long poll open to the control plane.
func (t *Tracker) GetInPollNetMap() bool {
	if t.nil() {
		return false
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	return t.inMapPoll
}

// SetMagicSockDERPHome notes what magicsock's view of its home DERP is.
//
// The homeless parameter is whether magicsock is running in DERP-disconnected
// mode, without discovering and maintaining a connection to its home DERP.
func (t *Tracker) SetMagicSockDERPHome(region int, homeless bool) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.derpHomeRegion = region
	t.derpHomeless = homeless
	t.selfCheckLocked()
}

// NoteMapRequestHeard notes whenever we successfully sent a map request
// to control for which we received a 200 response.
func (t *Tracker) NoteMapRequestHeard(mr *tailcfg.MapRequest) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	// TODO: extract mr.HostInfo.NetInfo.PreferredDERP, compare
	// against SetMagicSockDERPHome and
	// SetDERPRegionConnectedState

	t.lastMapRequestHeard = time.Now()
	t.selfCheckLocked()
}

func (t *Tracker) SetDERPRegionConnectedState(region int, connected bool) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	mak.Set(&t.derpRegionConnected, region, connected)
	t.selfCheckLocked()
}

// SetDERPRegionHealth sets or clears any problem associated with the
// provided DERP region.
func (t *Tracker) SetDERPRegionHealth(region int, problem string) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	if problem == "" {
		delete(t.derpRegionHealthProblem, region)
	} else {
		mak.Set(&t.derpRegionHealthProblem, region, problem)
	}
	t.selfCheckLocked()
}

// NoteDERPRegionReceivedFrame is called to note that a frame was received from
// the given DERP region at the current time.
func (t *Tracker) NoteDERPRegionReceivedFrame(region int) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	mak.Set(&t.derpRegionLastFrame, region, time.Now())
	t.selfCheckLocked()
}

// GetDERPRegionReceivedTime returns the last time that a frame was received
// from the given DERP region, or the zero time if no communication with that
// region has occurred.
func (t *Tracker) GetDERPRegionReceivedTime(region int) time.Time {
	if t.nil() {
		return time.Time{}
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	return t.derpRegionLastFrame[region]
}

// state is an ipn.State.String() value: "Running", "Stopped", "NeedsLogin", etc.
func (t *Tracker) SetIPNState(state string, wantRunning bool) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.ipnState = state
	t.ipnWantRunning = wantRunning
	t.selfCheckLocked()
}

// SetAnyInterfaceUp sets whether any network interface is up.
func (t *Tracker) SetAnyInterfaceUp(up bool) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.anyInterfaceUp.Set(up)
	t.selfCheckLocked()
}

// SetUDP4Unbound sets whether the udp4 bind failed completely.
func (t *Tracker) SetUDP4Unbound(unbound bool) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.udp4Unbound = unbound
	t.selfCheckLocked()
}

// SetAuthRoutineInError records the latest error encountered as a result of a
// login attempt. Providing a nil error indicates successful login, or that
// being logged in w/coordination is not currently desired.
func (t *Tracker) SetAuthRoutineInError(err error) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	if err == nil && t.lastLoginErr == nil {
		return
	}
	t.lastLoginErr = err
	t.selfCheckLocked()
}

// SetLatestVersion records the latest version of the Tailscale client.
// v can be nil if unknown.
func (t *Tracker) SetLatestVersion(v *tailcfg.ClientVersion) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.latestVersion = v
	t.selfCheckLocked()
}

// SetCheckForUpdates sets whether the client wants to check for updates.
func (t *Tracker) SetCheckForUpdates(v bool) {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.checkForUpdates == v {
		return
	}
	t.checkForUpdates = v
	t.selfCheckLocked()
}

func (t *Tracker) timerSelfCheck() {
	if t.nil() {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.checkReceiveFuncsLocked()
	t.selfCheckLocked()
	if t.timer != nil {
		t.timer.Reset(time.Minute)
	}
}

func (t *Tracker) selfCheckLocked() {
	if t.ipnState == "" {
		// Don't check yet.
		return
	}
	t.updateBuiltinWarnablesLocked()
}

// OverallError returns a summary of the health state.
//
// If there are multiple problems, the error will be of type
// multierr.Error.
func (t *Tracker) OverallError() error {
	if t.nil() {
		return nil
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.updateBuiltinWarnablesLocked()
	return t.multiErrLocked()
}

// Strings() returns a string array containing the Text of all Warnings
// currently known to the Tracker. These strings can be presented to the
// user, although ideally you would use the Code property on each Warning
// to show a localized version of them instead.
// This function is here for legacy compatibility purposes and is deprecated.
func (t *Tracker) Strings() []string {
	if t.nil() {
		return nil
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	return t.stringsLocked()
}

func (t *Tracker) stringsLocked() []string {
	result := []string{}
	for w, ws := range t.warnableVal {
		if ws.Args == nil {
			result = append(result, w.Text(Args{}))
		} else {
			result = append(result, w.Text(ws.Args))
		}
	}
	return result
}

// errorsLocked returns an array of errors where each error is the Text
// of a Warning known to the Tracker.
// This function is here for legacy compatibility purposes and is deprecated.
func (t *Tracker) errorsLocked() []error {
	strs := t.stringsLocked()
	errs := []error{}
	for _, str := range strs {
		errs = append(errs, errors.New(str))
	}
	return errs
}

// multiErrLocked returns an error listing all errors known to the Tracker.
// This function is here for legacy compatibility purposes and is deprecated.
func (t *Tracker) multiErrLocked() error {
	errs := t.errorsLocked()
	return multierr.New(errs...)
}

var fakeErrForTesting = envknob.RegisterString("TS_DEBUG_FAKE_HEALTH_ERROR")

// updateBuiltinWarnablesLocked performs a number of checks on the state of the backend,
// and adds/removes Warnings from the Tracker as needed.
func (t *Tracker) updateBuiltinWarnablesLocked() {
	if t.checkForUpdates {
		if cv := t.latestVersion; cv != nil && !cv.RunningLatest && cv.LatestVersion != "" {
			if cv.UrgentSecurityUpdate {
				t.setUnhealthyLocked(securityUpdateAvailableWarnable, Args{
					ArgCurrentVersion:   version.Short(),
					ArgAvailableVersion: cv.LatestVersion,
				})
			} else {
				t.setUnhealthyLocked(updateAvailableWarnable, Args{
					ArgCurrentVersion:   version.Short(),
					ArgAvailableVersion: cv.LatestVersion,
				})
			}
		}
	}

	if version.IsUnstableBuild() {
		t.setUnhealthyLocked(unstableWarnable, Args{
			ArgCurrentVersion: version.Short(),
		})
	}

	if v, ok := t.anyInterfaceUp.Get(); ok && !v {
		t.setUnhealthyLocked(NetworkStatusWarnable, nil)
		return
	} else {
		t.setHealthyLocked(NetworkStatusWarnable)
	}

	if t.localLogConfigErr != nil {
		t.setUnhealthyLocked(localLogWarnable, Args{
			ArgError: t.localLogConfigErr.Error(),
		})
		return
	} else {
		t.setHealthyLocked(localLogWarnable)
	}

	if !t.ipnWantRunning {
		t.setUnhealthyLocked(IPNStateWarnable, Args{
			"State": t.ipnState,
		})
		return
	} else {
		t.setHealthyLocked(IPNStateWarnable)
	}

	if t.lastLoginErr != nil {
		t.setUnhealthyLocked(LoginStateWarnable, Args{
			ArgError: t.lastLoginErr.Error(),
		})
		return
	} else {
		t.setHealthyLocked(LoginStateWarnable)
	}

	now := time.Now()
	if !t.inMapPoll && (t.lastMapPollEndedAt.IsZero() || now.Sub(t.lastMapPollEndedAt) > 10*time.Second) {
		t.setUnhealthyLocked(notInMapPollWarnable, nil)
		return
	} else {
		t.setHealthyLocked(notInMapPollWarnable)
	}

	const tooIdle = 2*time.Minute + 5*time.Second
	if d := now.Sub(t.lastStreamedMapResponse).Round(time.Second); d > tooIdle {
		t.setUnhealthyLocked(mapResponseTimeoutWarnable, Args{
			ArgDuration: d.String(),
		})
		return
	} else {
		t.setHealthyLocked(mapResponseTimeoutWarnable)
	}

	if !t.derpHomeless {
		rid := t.derpHomeRegion
		if rid == 0 {
			t.setUnhealthyLocked(noDERPHomeWarnable, nil)
			return
		} else if !t.derpRegionConnected[rid] {
			t.setUnhealthyLocked(noDERPConnectionWarnable, Args{
				ArgRegionID: fmt.Sprint(rid),
			})
			return
		} else if d := now.Sub(t.derpRegionLastFrame[rid]).Round(time.Second); d > tooIdle {
			t.setUnhealthyLocked(derpTimeoutWarnable, Args{
				ArgRegionID: fmt.Sprint(rid),
				ArgDuration: d.String(),
			})
			return
		}
	}
	t.setHealthyLocked(noDERPHomeWarnable)
	t.setHealthyLocked(noDERPConnectionWarnable)
	t.setHealthyLocked(derpTimeoutWarnable)

	if t.udp4Unbound {
		t.setUnhealthyLocked(noUDP4BindWarnable, nil)
		return
	} else {
		t.setHealthyLocked(noUDP4BindWarnable)
	}

	// TODO: use
	_ = t.inMapPollSince
	_ = t.lastMapPollEndedAt
	_ = t.lastStreamedMapResponse
	_ = t.lastMapRequestHeard

	shouldClearMagicsockWarnings := false
	for i := range t.MagicSockReceiveFuncs {
		f := &t.MagicSockReceiveFuncs[i]
		if f.missing {
			t.setUnhealthyLocked(magicsockReceiveFuncWarnable, Args{
				ArgMagicsockFunctionName: f.name,
			})
			shouldClearMagicsockWarnings = false
		}
	}
	if shouldClearMagicsockWarnings {
		t.setHealthyLocked(magicsockReceiveFuncWarnable)
	}

	// Iterates over the legacy subsystems and their error, and turns them into structured errors
	for sys, err := range t.sysErr {
		t.updateLegacyErrorWarnableLocked(sys, err)
	}

	if len(t.derpRegionHealthProblem) > 0 {
		for regionID, problem := range t.derpRegionHealthProblem {
			t.setUnhealthyLocked(derpRegionErrorWarnable, Args{
				ArgRegionID: fmt.Sprint(regionID),
				ArgError:    problem,
			})
		}
	} else {
		t.setHealthyLocked(derpRegionErrorWarnable)
	}

	if len(t.controlHealth) > 0 {
		for _, s := range t.controlHealth {
			t.setUnhealthyLocked(controlHealthWarnable, Args{
				ArgError: s,
			})
		}
	} else {
		t.setHealthyLocked(controlHealthWarnable)
	}

	if err := envknob.ApplyDiskConfigError(); err != nil {
		t.setUnhealthyLocked(applyDiskConfigWarnable, Args{
			ArgError: err.Error(),
		})
	} else {
		t.setHealthyLocked(applyDiskConfigWarnable)
	}

	if len(t.tlsConnectionErrors) > 0 {
		for serverName, err := range t.tlsConnectionErrors {
			t.setUnhealthyLocked(tlsConnectionFailedWarnable, Args{
				ArgServerName: serverName,
				ArgError:      err.Error(),
			})
		}
	} else {
		t.setHealthyLocked(tlsConnectionFailedWarnable)
	}

	if e := fakeErrForTesting(); len(t.warnables) == 0 && e != "" {
		t.setUnhealthyLocked(testWarnable, Args{
			ArgError: e,
		})
	} else {
		t.setHealthyLocked(testWarnable)
	}
}

// ReceiveFuncStats tracks the calls made to a wireguard-go receive func.
type ReceiveFuncStats struct {
	// name is the name of the receive func.
	// It's lazily populated.
	name string
	// numCalls is the number of times the receive func has ever been called.
	// It is required because it is possible for a receive func's wireguard-go goroutine
	// to be active even though the receive func isn't.
	// The wireguard-go goroutine alternates between calling the receive func and
	// processing what the func returned.
	numCalls atomic.Uint64
	// prevNumCalls is the value of numCalls last time the health check examined it.
	prevNumCalls uint64
	// inCall indicates whether the receive func is currently running.
	inCall atomic.Bool
	// missing indicates whether the receive func is not running.
	missing bool
}

func (s *ReceiveFuncStats) Enter() {
	s.numCalls.Add(1)
	s.inCall.Store(true)
}

func (s *ReceiveFuncStats) Exit() {
	s.inCall.Store(false)
}

// ReceiveFuncStats returns the ReceiveFuncStats tracker for the given func
// type.
//
// If t is nil, it returns nil.
func (t *Tracker) ReceiveFuncStats(which ReceiveFunc) *ReceiveFuncStats {
	if t == nil {
		return nil
	}
	return &t.MagicSockReceiveFuncs[which]
}

func (t *Tracker) checkReceiveFuncsLocked() {
	for i := range t.MagicSockReceiveFuncs {
		f := &t.MagicSockReceiveFuncs[i]
		if f.name == "" {
			f.name = (ReceiveFunc(i)).String()
		}
		if runtime.GOOS == "js" && i < 2 {
			// Skip IPv4 and IPv6 on js.
			continue
		}
		f.missing = false
		prev := f.prevNumCalls
		numCalls := f.numCalls.Load()
		f.prevNumCalls = numCalls
		if numCalls > prev {
			// OK: the function has gotten called since last we checked
			continue
		}
		if f.inCall.Load() {
			// OK: the function is active, probably blocked due to inactivity
			continue
		}
		// Not OK: The function is not active, and not accumulating new calls.
		// It is probably MIA.
		f.missing = true
	}
}