tailscale/tsweb/tsweb.go

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

// Package tsweb contains code used in various Tailscale webservers.
package tsweb

import (
	"bufio"
	"bytes"
	"context"
	"errors"
	"expvar"
	"fmt"
	"io"
	"net"
	"net/http"
	_ "net/http/pprof"
	"net/netip"
	"os"
	"path/filepath"
	"reflect"
	"runtime"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"go4.org/mem"
	"tailscale.com/envknob"
	"tailscale.com/metrics"
	"tailscale.com/net/tsaddr"
	"tailscale.com/types/logger"
	"tailscale.com/util/vizerror"
	"tailscale.com/version"
)

func init() {
	expvar.Publish("process_start_unix_time", expvar.Func(func() any { return timeStart.Unix() }))
	expvar.Publish("version", expvar.Func(func() any { return version.Long }))
	expvar.Publish("go_version", expvar.Func(func() any { return runtime.Version() }))
	expvar.Publish("counter_uptime_sec", expvar.Func(func() any { return int64(Uptime().Seconds()) }))
	expvar.Publish("gauge_goroutines", expvar.Func(func() any { return runtime.NumGoroutine() }))
}

const (
	gaugePrefix    = "gauge_"
	counterPrefix  = "counter_"
	labelMapPrefix = "labelmap_"
)

// prefixesToTrim contains key prefixes to remove when exporting and sorting metrics.
var prefixesToTrim = []string{gaugePrefix, counterPrefix, labelMapPrefix}

// DevMode controls whether extra output in shown, for when the binary is being run in dev mode.
var DevMode bool

func DefaultCertDir(leafDir string) string {
	cacheDir, err := os.UserCacheDir()
	if err == nil {
		return filepath.Join(cacheDir, "tailscale", leafDir)
	}
	return ""
}

// IsProd443 reports whether addr is a Go listen address for port 443.
func IsProd443(addr string) bool {
	_, port, _ := net.SplitHostPort(addr)
	return port == "443" || port == "https"
}

// AllowDebugAccess reports whether r should be permitted to access
// various debug endpoints.
func AllowDebugAccess(r *http.Request) bool {
	if r.Header.Get("X-Forwarded-For") != "" {
		// TODO if/when needed. For now, conservative:
		return false
	}
	ipStr, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return false
	}
	ip, err := netip.ParseAddr(ipStr)
	if err != nil {
		return false
	}
	if tsaddr.IsTailscaleIP(ip) || ip.IsLoopback() || ipStr == envknob.String("TS_ALLOW_DEBUG_IP") {
		return true
	}
	if r.Method == "GET" {
		urlKey := r.FormValue("debugkey")
		keyPath := envknob.String("TS_DEBUG_KEY_PATH")
		if urlKey != "" && keyPath != "" {
			slurp, err := os.ReadFile(keyPath)
			if err == nil && string(bytes.TrimSpace(slurp)) == urlKey {
				return true
			}
		}
	}
	return false
}

// AcceptsEncoding reports whether r accepts the named encoding
// ("gzip", "br", etc).
func AcceptsEncoding(r *http.Request, enc string) bool {
	h := r.Header.Get("Accept-Encoding")
	if h == "" {
		return false
	}
	if !strings.Contains(h, enc) && !mem.ContainsFold(mem.S(h), mem.S(enc)) {
		return false
	}
	remain := h
	for len(remain) > 0 {
		var part string
		part, remain, _ = strings.Cut(remain, ",")
		part = strings.TrimSpace(part)
		part, _, _ = strings.Cut(part, ";")
		if part == enc {
			return true
		}
	}
	return false
}

// Protected wraps a provided debug handler, h, returning a Handler
// that enforces AllowDebugAccess and returns forbidden replies for
// unauthorized requests.
func Protected(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if !AllowDebugAccess(r) {
			msg := "debug access denied"
			if DevMode {
				ipStr, _, _ := net.SplitHostPort(r.RemoteAddr)
				msg += fmt.Sprintf("; to permit access, set TS_ALLOW_DEBUG_IP=%v", ipStr)
			}
			http.Error(w, msg, http.StatusForbidden)
			return
		}
		h.ServeHTTP(w, r)
	})
}

var timeStart = time.Now()

func Uptime() time.Duration { return time.Since(timeStart).Round(time.Second) }

// Port80Handler is the handler to be given to
// autocert.Manager.HTTPHandler.  The inner handler is the mux
// returned by NewMux containing registered /debug handlers.
type Port80Handler struct {
	Main http.Handler
	// FQDN is used to redirect incoming requests to https://<FQDN>.
	// If it is not set, the hostname is calculated from the incoming
	// request.
	FQDN string
}

func (h Port80Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	path := r.RequestURI
	if path == "/debug" || strings.HasPrefix(path, "/debug") {
		h.Main.ServeHTTP(w, r)
		return
	}
	if r.Method != "GET" && r.Method != "HEAD" {
		http.Error(w, "Use HTTPS", http.StatusBadRequest)
		return
	}
	if path == "/" && AllowDebugAccess(r) {
		// Redirect authorized user to the debug handler.
		path = "/debug/"
	}
	host := h.FQDN
	if host == "" {
		host = r.Host
	}
	target := "https://" + host + path
	http.Redirect(w, r, target, http.StatusFound)
}

// ReturnHandler is like net/http.Handler, but the handler can return an
// error instead of writing to its ResponseWriter.
type ReturnHandler interface {
	// ServeHTTPReturn is like http.Handler.ServeHTTP, except that
	// it can choose to return an error instead of writing to its
	// http.ResponseWriter.
	//
	// If ServeHTTPReturn returns an error, it caller should handle
	// an error by serving an HTTP 500 response to the user. The
	// error details should not be sent to the client, as they may
	// contain sensitive information. If the error is an
	// HTTPError, though, callers should use the HTTP response
	// code and message as the response to the client.
	ServeHTTPReturn(http.ResponseWriter, *http.Request) error
}

type HandlerOptions struct {
	QuietLoggingIfSuccessful bool // if set, do not log successfully handled HTTP requests (200 and 304 status codes)
	Logf                     logger.Logf
	Now                      func() time.Time // if nil, defaults to time.Now

	// If non-nil, StatusCodeCounters maintains counters
	// of status codes for handled responses.
	// The keys are "1xx", "2xx", "3xx", "4xx", and "5xx".
	StatusCodeCounters *expvar.Map
	// If non-nil, StatusCodeCountersFull maintains counters of status
	// codes for handled responses.
	// The keys are HTTP numeric response codes e.g. 200, 404, ...
	StatusCodeCountersFull *expvar.Map

	// OnError is called if the handler returned a HTTPError. This
	// is intended to be used to present pretty error pages if
	// the user agent is determined to be a browser.
	OnError ErrorHandlerFunc
}

// ErrorHandlerFunc is called to present a error response.
type ErrorHandlerFunc func(http.ResponseWriter, *http.Request, HTTPError)

// ReturnHandlerFunc is an adapter to allow the use of ordinary
// functions as ReturnHandlers. If f is a function with the
// appropriate signature, ReturnHandlerFunc(f) is a ReturnHandler that
// calls f.
type ReturnHandlerFunc func(http.ResponseWriter, *http.Request) error

// ServeHTTPReturn calls f(w, r).
func (f ReturnHandlerFunc) ServeHTTPReturn(w http.ResponseWriter, r *http.Request) error {
	return f(w, r)
}

// StdHandler converts a ReturnHandler into a standard http.Handler.
// Handled requests are logged using opts.Logf, as are any errors.
// Errors are handled as specified by the Handler interface.
func StdHandler(h ReturnHandler, opts HandlerOptions) http.Handler {
	if opts.Now == nil {
		opts.Now = time.Now
	}
	if opts.Logf == nil {
		opts.Logf = logger.Discard
	}
	return retHandler{h, opts}
}

// retHandler is an http.Handler that wraps a Handler and handles errors.
type retHandler struct {
	rh   ReturnHandler
	opts HandlerOptions
}

// ServeHTTP implements the http.Handler interface.
func (h retHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	msg := AccessLogRecord{
		When:       h.opts.Now(),
		RemoteAddr: r.RemoteAddr,
		Proto:      r.Proto,
		TLS:        r.TLS != nil,
		Host:       r.Host,
		Method:     r.Method,
		RequestURI: r.URL.RequestURI(),
		UserAgent:  r.UserAgent(),
		Referer:    r.Referer(),
	}

	lw := &loggingResponseWriter{ResponseWriter: w, logf: h.opts.Logf}
	err := h.rh.ServeHTTPReturn(lw, r)

	var hErr HTTPError
	var hErrOK bool
	if errors.As(err, &hErr) {
		hErrOK = true
	} else if vizErr, ok := vizerror.As(err); ok {
		hErrOK = true
		hErr = HTTPError{Msg: vizErr.Error()}
	}

	if lw.code == 0 && err == nil && !lw.hijacked {
		// If the handler didn't write and didn't send a header, that still means 200.
		// (See https://play.golang.org/p/4P7nx_Tap7p)
		lw.code = 200
	}

	msg.Seconds = h.opts.Now().Sub(msg.When).Seconds()
	msg.Code = lw.code
	msg.Bytes = lw.bytes

	switch {
	case lw.hijacked:
		// Connection no longer belongs to us, just log that we
		// switched protocols away from HTTP.
		if msg.Code == 0 {
			msg.Code = http.StatusSwitchingProtocols
		}
	case err != nil && r.Context().Err() == context.Canceled:
		msg.Code = 499 // nginx convention: Client Closed Request
		msg.Err = context.Canceled.Error()
	case hErrOK:
		// Handler asked us to send an error. Do so, if we haven't
		// already sent a response.
		msg.Err = hErr.Msg
		if hErr.Err != nil {
			if msg.Err == "" {
				msg.Err = hErr.Err.Error()
			} else {
				msg.Err = msg.Err + ": " + hErr.Err.Error()
			}
		}
		if lw.code != 0 {
			h.opts.Logf("[unexpected] handler returned HTTPError %v, but already sent a response with code %d", hErr, lw.code)
			break
		}
		msg.Code = hErr.Code
		if msg.Code == 0 {
			h.opts.Logf("[unexpected] HTTPError %v did not contain an HTTP status code, sending internal server error", hErr)
			msg.Code = http.StatusInternalServerError
		}
		if h.opts.OnError != nil {
			h.opts.OnError(lw, r, hErr)
		} else {
			// Default headers set by http.Error.
			lw.Header().Set("Content-Type", "text/plain; charset=utf-8")
			lw.Header().Set("X-Content-Type-Options", "nosniff")
			for k, vs := range hErr.Header {
				lw.Header()[k] = vs
			}
			lw.WriteHeader(msg.Code)
			fmt.Fprintln(lw, hErr.Msg)
		}
	case err != nil:
		// Handler returned a generic error. Serve an internal server
		// error, if necessary.
		msg.Err = err.Error()
		if lw.code == 0 {
			msg.Code = http.StatusInternalServerError
			http.Error(lw, "internal server error", msg.Code)
		}
	}

	if !h.opts.QuietLoggingIfSuccessful || (msg.Code != http.StatusOK && msg.Code != http.StatusNotModified) {
		h.opts.Logf("%s", msg)
	}

	if h.opts.StatusCodeCounters != nil {
		h.opts.StatusCodeCounters.Add(responseCodeString(msg.Code/100), 1)
	}

	if h.opts.StatusCodeCountersFull != nil {
		h.opts.StatusCodeCountersFull.Add(responseCodeString(msg.Code), 1)
	}
}

func responseCodeString(code int) string {
	if v, ok := responseCodeCache.Load(code); ok {
		return v.(string)
	}

	var ret string
	if code < 10 {
		ret = fmt.Sprintf("%dxx", code)
	} else {
		ret = strconv.Itoa(code)
	}
	responseCodeCache.Store(code, ret)
	return ret
}

// responseCodeCache memoizes the string form of HTTP response codes,
// so that the hot request-handling codepath doesn't have to allocate
// in strconv/fmt for every request.
//
// Keys are either full HTTP response code ints (200, 404) or "family"
// ints representing entire families (e.g. 2 for 2xx codes). Values
// are the string form of that code/family.
var responseCodeCache sync.Map

// loggingResponseWriter wraps a ResponseWriter and record the HTTP
// response code that gets sent, if any.
type loggingResponseWriter struct {
	http.ResponseWriter
	code     int
	bytes    int
	hijacked bool
	logf     logger.Logf
}

// WriteHeader implements http.Handler.
func (l *loggingResponseWriter) WriteHeader(statusCode int) {
	if l.code != 0 {
		l.logf("[unexpected] HTTP handler set statusCode twice (%d and %d)", l.code, statusCode)
		return
	}
	l.code = statusCode
	l.ResponseWriter.WriteHeader(statusCode)
}

// Write implements http.Handler.
func (l *loggingResponseWriter) Write(bs []byte) (int, error) {
	if l.code == 0 {
		l.code = 200
	}
	n, err := l.ResponseWriter.Write(bs)
	l.bytes += n
	return n, err
}

// Hijack implements http.Hijacker. Note that hijacking can still fail
// because the wrapped ResponseWriter is not required to implement
// Hijacker, as this breaks HTTP/2.
func (l *loggingResponseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	h, ok := l.ResponseWriter.(http.Hijacker)
	if !ok {
		return nil, nil, errors.New("ResponseWriter is not a Hijacker")
	}
	conn, buf, err := h.Hijack()
	if err == nil {
		l.hijacked = true
	}
	return conn, buf, err
}

func (l loggingResponseWriter) Flush() {
	f, _ := l.ResponseWriter.(http.Flusher)
	if f == nil {
		l.logf("[unexpected] tried to Flush a ResponseWriter that can't flush")
		return
	}
	f.Flush()
}

// HTTPError is an error with embedded HTTP response information.
//
// It is the error type to be (optionally) used by Handler.ServeHTTPReturn.
type HTTPError struct {
	Code   int         // HTTP response code to send to client; 0 means 500
	Msg    string      // Response body to send to client
	Err    error       // Detailed error to log on the server
	Header http.Header // Optional set of HTTP headers to set in the response
}

// Error implements the error interface.
func (e HTTPError) Error() string { return fmt.Sprintf("httperror{%d, %q, %v}", e.Code, e.Msg, e.Err) }

func (e HTTPError) Unwrap() error { return e.Err }

// Error returns an HTTPError containing the given information.
func Error(code int, msg string, err error) HTTPError {
	return HTTPError{Code: code, Msg: msg, Err: err}
}

// PrometheusVar is a value that knows how to format itself into
// Prometheus metric syntax.
type PrometheusVar interface {
	// WritePrometheus writes the value of the var to w, in Prometheus
	// metric syntax. All variables names written out must start with
	// prefix (or write out a single variable named exactly prefix)
	WritePrometheus(w io.Writer, prefix string)
}

// WritePrometheusExpvar writes kv to w in Prometheus metrics format.
//
// See VarzHandler for conventions. This is exported primarily for
// people to test their varz.
func WritePrometheusExpvar(w io.Writer, kv expvar.KeyValue) {
	writePromExpVar(w, "", kv)
}

type prometheusMetricDetails struct {
	Name  string
	Type  string
	Label string
}

var prometheusMetricCache sync.Map // string => *prometheusMetricDetails

func prometheusMetric(prefix string, key string) (string, string, string) {
	cachekey := prefix + key
	if v, ok := prometheusMetricCache.Load(cachekey); ok {
		d := v.(*prometheusMetricDetails)
		return d.Name, d.Type, d.Label
	}
	var typ string
	var label string
	switch {
	case strings.HasPrefix(key, gaugePrefix):
		typ = "gauge"
		key = strings.TrimPrefix(key, gaugePrefix)

	case strings.HasPrefix(key, counterPrefix):
		typ = "counter"
		key = strings.TrimPrefix(key, counterPrefix)
	}
	if strings.HasPrefix(key, labelMapPrefix) {
		key = strings.TrimPrefix(key, labelMapPrefix)
		if a, b, ok := strings.Cut(key, "_"); ok {
			label, key = a, b
		}
	}
	d := &prometheusMetricDetails{
		Name:  strings.ReplaceAll(prefix+key, "-", "_"),
		Type:  typ,
		Label: label,
	}
	prometheusMetricCache.Store(cachekey, d)
	return d.Name, d.Type, d.Label
}

func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
	key := kv.Key
	name, typ, label := prometheusMetric(prefix, key)

	switch v := kv.Value.(type) {
	case PrometheusVar:
		v.WritePrometheus(w, name)
		return
	case *expvar.Int:
		if typ == "" {
			typ = "counter"
		}
		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, typ, name, v.Value())
		return
	case *expvar.Float:
		if typ == "" {
			typ = "gauge"
		}
		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, typ, name, v.Value())
		return
	case *metrics.Set:
		v.Do(func(kv expvar.KeyValue) {
			writePromExpVar(w, name+"_", kv)
		})
		return
	case PrometheusMetricsReflectRooter:
		root := v.PrometheusMetricsReflectRoot()
		rv := reflect.ValueOf(root)
		if rv.Type().Kind() == reflect.Ptr {
			if rv.IsNil() {
				return
			}
			rv = rv.Elem()
		}
		if rv.Type().Kind() != reflect.Struct {
			fmt.Fprintf(w, "# skipping expvar %q; unknown root type\n", name)
			return
		}
		foreachExportedStructField(rv, func(fieldOrJSONName, metricType string, rv reflect.Value) {
			mname := name + "_" + fieldOrJSONName
			switch rv.Kind() {
			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
				fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", mname, metricType, mname, rv.Int())
			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
				fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", mname, metricType, mname, rv.Uint())
			case reflect.Float32, reflect.Float64:
				fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", mname, metricType, mname, rv.Float())
			case reflect.Struct:
				if rv.CanAddr() {
					// Slight optimization, not copying big structs if they're addressable:
					writePromExpVar(w, name+"_", expvar.KeyValue{Key: fieldOrJSONName, Value: expVarPromStructRoot{rv.Addr().Interface()}})
				} else {
					writePromExpVar(w, name+"_", expvar.KeyValue{Key: fieldOrJSONName, Value: expVarPromStructRoot{rv.Interface()}})
				}
			}
			return
		})
		return
	}

	if typ == "" {
		var funcRet string
		if f, ok := kv.Value.(expvar.Func); ok {
			v := f()
			if ms, ok := v.(runtime.MemStats); ok && name == "memstats" {
				writeMemstats(w, &ms)
				return
			}
			if vs, ok := v.(string); ok && strings.HasSuffix(name, "version") {
				fmt.Fprintf(w, "%s{version=%q} 1\n", name, vs)
				return
			}
			switch v := v.(type) {
			case int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, uintptr, float32, float64:
				fmt.Fprintf(w, "%s %v\n", name, v)
				return
			}
			funcRet = fmt.Sprintf(" returning %T", v)
		}
		switch kv.Value.(type) {
		default:
			fmt.Fprintf(w, "# skipping expvar %q (Go type %T%s) with undeclared Prometheus type\n", name, kv.Value, funcRet)
			return
		case *metrics.LabelMap, *expvar.Map:
			// Permit typeless LabelMap and expvar.Map for
			// compatibility with old expvar-registered
			// metrics.LabelMap.
		}
	}

	switch v := kv.Value.(type) {
	case expvar.Func:
		val := v()
		switch val.(type) {
		case float64, int64, int:
			fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, typ, name, val)
		default:
			fmt.Fprintf(w, "# skipping expvar func %q returning unknown type %T\n", name, val)
		}

	case *metrics.LabelMap:
		if typ != "" {
			fmt.Fprintf(w, "# TYPE %s %s\n", name, typ)
		}
		// IntMap uses expvar.Map on the inside, which presorts
		// keys. The output ordering is deterministic.
		v.Do(func(kv expvar.KeyValue) {
			fmt.Fprintf(w, "%s{%s=%q} %v\n", name, v.Label, kv.Key, kv.Value)
		})
	case *expvar.Map:
		if label != "" && typ != "" {
			fmt.Fprintf(w, "# TYPE %s %s\n", name, typ)
			v.Do(func(kv expvar.KeyValue) {
				fmt.Fprintf(w, "%s{%s=%q} %v\n", name, label, kv.Key, kv.Value)
			})
		} else {
			v.Do(func(kv expvar.KeyValue) {
				fmt.Fprintf(w, "%s_%s %v\n", name, kv.Key, kv.Value)
			})
		}
	}
}

var sortedKVsPool = &sync.Pool{New: func() any { return new(sortedKVs) }}

// sortedKV is a KeyValue with a sort key.
type sortedKV struct {
	expvar.KeyValue
	sortKey string // KeyValue.Key with type prefix removed
}

type sortedKVs struct {
	kvs []sortedKV
}

// VarzHandler is an HTTP handler to write expvar values into the
// prometheus export format:
//
//	https://github.com/prometheus/docs/blob/master/content/docs/instrumenting/exposition_formats.md
//
// It makes the following assumptions:
//
//   - *expvar.Int are counters (unless marked as a gauge_; see below)
//   - a *tailscale/metrics.Set is descended into, joining keys with
//     underscores. So use underscores as your metric names.
//   - an expvar named starting with "gauge_" or "counter_" is of that
//     Prometheus type, and has that prefix stripped.
//   - anything else is untyped and thus not exported.
//   - expvar.Func can return an int or int64 (for now) and anything else
//     is not exported.
//
// This will evolve over time, or perhaps be replaced.
func VarzHandler(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/plain; version=0.0.4")

	s := sortedKVsPool.Get().(*sortedKVs)
	defer sortedKVsPool.Put(s)
	s.kvs = s.kvs[:0]
	expvarDo(func(kv expvar.KeyValue) {
		s.kvs = append(s.kvs, sortedKV{kv, removeTypePrefixes(kv.Key)})
	})
	sort.Slice(s.kvs, func(i, j int) bool {
		return s.kvs[i].sortKey < s.kvs[j].sortKey
	})
	for _, e := range s.kvs {
		writePromExpVar(w, "", e.KeyValue)
	}
}

// PrometheusMetricsReflectRooter is an optional interface that expvar.Var implementations
// can implement to indicate that they should be walked recursively with reflect to find
// sets of fields to export.
type PrometheusMetricsReflectRooter interface {
	expvar.Var

	// PrometheusMetricsReflectRoot returns the struct or struct pointer to walk.
	PrometheusMetricsReflectRoot() any
}

var expvarDo = expvar.Do // pulled out for tests

func writeMemstats(w io.Writer, ms *runtime.MemStats) {
	out := func(name, typ string, v uint64, help string) {
		if help != "" {
			fmt.Fprintf(w, "# HELP memstats_%s %s\n", name, help)
		}
		fmt.Fprintf(w, "# TYPE memstats_%s %s\nmemstats_%s %v\n", name, typ, name, v)
	}
	g := func(name string, v uint64, help string) { out(name, "gauge", v, help) }
	c := func(name string, v uint64, help string) { out(name, "counter", v, help) }
	g("heap_alloc", ms.HeapAlloc, "current bytes of allocated heap objects (up/down smoothly)")
	c("total_alloc", ms.TotalAlloc, "cumulative bytes allocated for heap objects")
	g("sys", ms.Sys, "total bytes of memory obtained from the OS")
	c("mallocs", ms.Mallocs, "cumulative count of heap objects allocated")
	c("frees", ms.Frees, "cumulative count of heap objects freed")
	c("num_gc", uint64(ms.NumGC), "number of completed GC cycles")
}

// sortedStructField is metadata about a struct field used both for sorting once
// (by structTypeSortedFields) and at serving time (by
// foreachExportedStructField).
type sortedStructField struct {
	Index           int    // index of struct field in struct
	Name            string // struct field name, or "json" name
	SortName        string // Name with "foo_" type prefixes removed
	MetricType      string // the "metrictype" struct tag
	StructFieldType *reflect.StructField
}

var structSortedFieldsCache sync.Map // reflect.Type => []sortedStructField

// structTypeSortedFields returns the sorted fields of t, caching as needed.
func structTypeSortedFields(t reflect.Type) []sortedStructField {
	if v, ok := structSortedFieldsCache.Load(t); ok {
		return v.([]sortedStructField)
	}
	fields := make([]sortedStructField, 0, t.NumField())
	for i, n := 0, t.NumField(); i < n; i++ {
		sf := t.Field(i)
		name := sf.Name
		if v := sf.Tag.Get("json"); v != "" {
			v, _, _ = strings.Cut(v, ",")
			if v == "-" {
				// Skip it, regardless of its metrictype.
				continue
			}
			if v != "" {
				name = v
			}
		}
		fields = append(fields, sortedStructField{
			Index:           i,
			Name:            name,
			SortName:        removeTypePrefixes(name),
			MetricType:      sf.Tag.Get("metrictype"),
			StructFieldType: &sf,
		})
	}
	sort.Slice(fields, func(i, j int) bool {
		return fields[i].SortName < fields[j].SortName
	})
	structSortedFieldsCache.Store(t, fields)
	return fields
}

// removeTypePrefixes returns s with the first "foo_" prefix in prefixesToTrim
// removed.
func removeTypePrefixes(s string) string {
	for _, prefix := range prefixesToTrim {
		if trimmed, ok := strings.CutPrefix(s, prefix); ok {
			return trimmed
		}
	}
	return s
}

// foreachExportedStructField iterates over the fields in sorted order of
// their name, after removing metric prefixes. This is not necessarily the
// order they were declared in the struct
func foreachExportedStructField(rv reflect.Value, f func(fieldOrJSONName, metricType string, rv reflect.Value)) {
	t := rv.Type()
	for _, ssf := range structTypeSortedFields(t) {
		sf := ssf.StructFieldType
		if ssf.MetricType != "" || sf.Type.Kind() == reflect.Struct {
			f(ssf.Name, ssf.MetricType, rv.Field(ssf.Index))
		} else if sf.Type.Kind() == reflect.Ptr && sf.Type.Elem().Kind() == reflect.Struct {
			fv := rv.Field(ssf.Index)
			if !fv.IsNil() {
				f(ssf.Name, ssf.MetricType, fv.Elem())
			}
		}
	}
}

type expVarPromStructRoot struct{ v any }

func (r expVarPromStructRoot) PrometheusMetricsReflectRoot() any { return r.v }
func (r expVarPromStructRoot) String() string                    { panic("unused") }

var (
	_ PrometheusMetricsReflectRooter = expVarPromStructRoot{}
	_ expvar.Var                     = expVarPromStructRoot{}
)