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expvar.go
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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package expvar provides a standardized interface to public variables, such
// as operation counters in servers. It exposes these variables via HTTP at
// /debug/vars in JSON format.
//
// Operations to set or modify these public variables are atomic.
//
// In addition to adding the HTTP handler, this package registers the
// following variables:
//
// cmdline os.Args
// memstats runtime.Memstats
//
// The package is sometimes only imported for the side effect of
// registering its HTTP handler and the above variables. To use it
// this way, link this package into your program:
//
// import _ "expvar"
package expvar
import (
"encoding/json"
"fmt"
"log"
"math"
"net/http"
"os"
"runtime"
"sort"
"strconv"
"strings"
"sync"
"sync/atomic"
)
// Var is an abstract type for all exported variables.
type Var interface {
// String returns a valid JSON value for the variable.
// Types with String methods that do not return valid JSON
// (such as time.Time) must not be used as a Var.
String() string
}
// Int is a 64-bit integer variable that satisfies the Var interface.
type Int struct {
i int64
}
func (v *Int) Value() int64 {
return atomic.LoadInt64(&v.i)
}
func (v *Int) String() string {
return strconv.FormatInt(atomic.LoadInt64(&v.i), 10)
}
func (v *Int) Add(delta int64) {
atomic.AddInt64(&v.i, delta)
}
func (v *Int) Set(value int64) {
atomic.StoreInt64(&v.i, value)
}
// Float is a 64-bit float variable that satisfies the Var interface.
type Float struct {
f uint64
}
func (v *Float) Value() float64 {
return math.Float64frombits(atomic.LoadUint64(&v.f))
}
func (v *Float) String() string {
return strconv.FormatFloat(
math.Float64frombits(atomic.LoadUint64(&v.f)), 'g', -1, 64)
}
// Add adds delta to v.
func (v *Float) Add(delta float64) {
for {
cur := atomic.LoadUint64(&v.f)
curVal := math.Float64frombits(cur)
nxtVal := curVal + delta
nxt := math.Float64bits(nxtVal)
if atomic.CompareAndSwapUint64(&v.f, cur, nxt) {
return
}
}
}
// Set sets v to value.
func (v *Float) Set(value float64) {
atomic.StoreUint64(&v.f, math.Float64bits(value))
}
// Map is a string-to-Var map variable that satisfies the Var interface.
type Map struct {
m sync.Map // map[string]Var
keysMu sync.RWMutex
keys []string // sorted
}
// KeyValue represents a single entry in a Map.
type KeyValue struct {
Key string
Value Var
}
func (v *Map) String() string {
var b strings.Builder
fmt.Fprintf(&b, "{")
first := true
v.Do(func(kv KeyValue) {
if !first {
fmt.Fprintf(&b, ", ")
}
fmt.Fprintf(&b, "%q: %v", kv.Key, kv.Value)
first = false
})
fmt.Fprintf(&b, "}")
return b.String()
}
// Init removes all keys from the map.
func (v *Map) Init() *Map {
v.keysMu.Lock()
defer v.keysMu.Unlock()
v.keys = v.keys[:0]
v.m.Range(func(k, _ any) bool {
v.m.Delete(k)
return true
})
return v
}
// addKey updates the sorted list of keys in v.keys.
func (v *Map) addKey(key string) {
v.keysMu.Lock()
defer v.keysMu.Unlock()
// Using insertion sort to place key into the already-sorted v.keys.
if i := sort.SearchStrings(v.keys, key); i >= len(v.keys) {
v.keys = append(v.keys, key)
} else if v.keys[i] != key {
v.keys = append(v.keys, "")
copy(v.keys[i+1:], v.keys[i:])
v.keys[i] = key
}
}
func (v *Map) Get(key string) Var {
i, _ := v.m.Load(key)
av, _ := i.(Var)
return av
}
func (v *Map) Set(key string, av Var) {
// Before we store the value, check to see whether the key is new. Try a Load
// before LoadOrStore: LoadOrStore causes the key interface to escape even on
// the Load path.
if _, ok := v.m.Load(key); !ok {
if _, dup := v.m.LoadOrStore(key, av); !dup {
v.addKey(key)
return
}
}
v.m.Store(key, av)
}
// Add adds delta to the *Int value stored under the given map key.
func (v *Map) Add(key string, delta int64) {
i, ok := v.m.Load(key)
if !ok {
var dup bool
i, dup = v.m.LoadOrStore(key, new(Int))
if !dup {
v.addKey(key)
}
}
// Add to Int; ignore otherwise.
if iv, ok := i.(*Int); ok {
iv.Add(delta)
}
}
// AddFloat adds delta to the *Float value stored under the given map key.
func (v *Map) AddFloat(key string, delta float64) {
i, ok := v.m.Load(key)
if !ok {
var dup bool
i, dup = v.m.LoadOrStore(key, new(Float))
if !dup {
v.addKey(key)
}
}
// Add to Float; ignore otherwise.
if iv, ok := i.(*Float); ok {
iv.Add(delta)
}
}
// Delete deletes the given key from the map.
func (v *Map) Delete(key string) {
v.keysMu.Lock()
defer v.keysMu.Unlock()
i := sort.SearchStrings(v.keys, key)
if i < len(v.keys) && key == v.keys[i] {
v.keys = append(v.keys[:i], v.keys[i+1:]...)
v.m.Delete(key)
}
}
// Do calls f for each entry in the map.
// The map is locked during the iteration,
// but existing entries may be concurrently updated.
func (v *Map) Do(f func(KeyValue)) {
v.keysMu.RLock()
defer v.keysMu.RUnlock()
for _, k := range v.keys {
i, _ := v.m.Load(k)
f(KeyValue{k, i.(Var)})
}
}
// String is a string variable, and satisfies the Var interface.
type String struct {
s atomic.Value // string
}
func (v *String) Value() string {
p, _ := v.s.Load().(string)
return p
}
// String implements the Var interface. To get the unquoted string
// use Value.
func (v *String) String() string {
s := v.Value()
b, _ := json.Marshal(s)
return string(b)
}
func (v *String) Set(value string) {
v.s.Store(value)
}
// Func implements Var by calling the function
// and formatting the returned value using JSON.
type Func func() any
func (f Func) Value() any {
return f()
}
func (f Func) String() string {
v, _ := json.Marshal(f())
return string(v)
}
// All published variables.
var (
vars sync.Map // map[string]Var
varKeysMu sync.RWMutex
varKeys []string // sorted
)
// Publish declares a named exported variable. This should be called from a
// package's init function when it creates its Vars. If the name is already
// registered then this will log.Panic.
func Publish(name string, v Var) {
if _, dup := vars.LoadOrStore(name, v); dup {
log.Panicln("Reuse of exported var name:", name)
}
varKeysMu.Lock()
defer varKeysMu.Unlock()
varKeys = append(varKeys, name)
sort.Strings(varKeys)
}
// Get retrieves a named exported variable. It returns nil if the name has
// not been registered.
func Get(name string) Var {
i, _ := vars.Load(name)
v, _ := i.(Var)
return v
}
// Convenience functions for creating new exported variables.
func NewInt(name string) *Int {
v := new(Int)
Publish(name, v)
return v
}
func NewFloat(name string) *Float {
v := new(Float)
Publish(name, v)
return v
}
func NewMap(name string) *Map {
v := new(Map).Init()
Publish(name, v)
return v
}
func NewString(name string) *String {
v := new(String)
Publish(name, v)
return v
}
// Do calls f for each exported variable.
// The global variable map is locked during the iteration,
// but existing entries may be concurrently updated.
func Do(f func(KeyValue)) {
varKeysMu.RLock()
defer varKeysMu.RUnlock()
for _, k := range varKeys {
val, _ := vars.Load(k)
f(KeyValue{k, val.(Var)})
}
}
func expvarHandler(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "application/json; charset=utf-8")
fmt.Fprintf(w, "{\n")
first := true
Do(func(kv KeyValue) {
if !first {
fmt.Fprintf(w, ",\n")
}
first = false
fmt.Fprintf(w, "%q: %s", kv.Key, kv.Value)
})
fmt.Fprintf(w, "\n}\n")
}
// Handler returns the expvar HTTP Handler.
//
// This is only needed to install the handler in a non-standard location.
func Handler() http.Handler {
return http.HandlerFunc(expvarHandler)
}
func cmdline() any {
return os.Args
}
func memstats() any {
stats := new(runtime.MemStats)
runtime.ReadMemStats(stats)
return *stats
}
func init() {
http.HandleFunc("/debug/vars", expvarHandler)
Publish("cmdline", Func(cmdline))
Publish("memstats", Func(memstats))
}