expvar.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package expvar provides a standardized interface to public variables, such
     6  // as operation counters in servers. It exposes these variables via HTTP at
     7  // /debug/vars in JSON format. As of Go 1.22, the /debug/vars request must
     8  // use GET.
     9  //
    10  // Operations to set or modify these public variables are atomic.
    11  //
    12  // In addition to adding the HTTP handler, this package registers the
    13  // following variables:
    14  //
    15  //	cmdline   os.Args
    16  //	memstats  runtime.Memstats
    17  //
    18  // The package is sometimes only imported for the side effect of
    19  // registering its HTTP handler and the above variables. To use it
    20  // this way, link this package into your program:
    21  //
    22  //	import _ "expvar"
    23  package expvar
    24  
    25  import (
    26  	"encoding/json"
    27  	"internal/godebug"
    28  	"log"
    29  	"math"
    30  	"net/http"
    31  	"os"
    32  	"runtime"
    33  	"slices"
    34  	"strconv"
    35  	"sync"
    36  	"sync/atomic"
    37  	"unicode/utf8"
    38  )
    39  
    40  // Var is an abstract type for all exported variables.
    41  type Var interface {
    42  	// String returns a valid JSON value for the variable.
    43  	// Types with String methods that do not return valid JSON
    44  	// (such as time.Time) must not be used as a Var.
    45  	String() string
    46  }
    47  
    48  type jsonVar interface {
    49  	// appendJSON appends the JSON representation of the receiver to b.
    50  	appendJSON(b []byte) []byte
    51  }
    52  
    53  // Int is a 64-bit integer variable that satisfies the [Var] interface.
    54  type Int struct {
    55  	i atomic.Int64
    56  }
    57  
    58  func (v *Int) Value() int64 {
    59  	return v.i.Load()
    60  }
    61  
    62  func (v *Int) String() string {
    63  	return string(v.appendJSON(nil))
    64  }
    65  
    66  func (v *Int) appendJSON(b []byte) []byte {
    67  	return strconv.AppendInt(b, v.i.Load(), 10)
    68  }
    69  
    70  func (v *Int) Add(delta int64) {
    71  	v.i.Add(delta)
    72  }
    73  
    74  func (v *Int) Set(value int64) {
    75  	v.i.Store(value)
    76  }
    77  
    78  // Float is a 64-bit float variable that satisfies the [Var] interface.
    79  type Float struct {
    80  	f atomic.Uint64
    81  }
    82  
    83  func (v *Float) Value() float64 {
    84  	return math.Float64frombits(v.f.Load())
    85  }
    86  
    87  func (v *Float) String() string {
    88  	return string(v.appendJSON(nil))
    89  }
    90  
    91  func (v *Float) appendJSON(b []byte) []byte {
    92  	return strconv.AppendFloat(b, math.Float64frombits(v.f.Load()), 'g', -1, 64)
    93  }
    94  
    95  // Add adds delta to v.
    96  func (v *Float) Add(delta float64) {
    97  	for {
    98  		cur := v.f.Load()
    99  		curVal := math.Float64frombits(cur)
   100  		nxtVal := curVal + delta
   101  		nxt := math.Float64bits(nxtVal)
   102  		if v.f.CompareAndSwap(cur, nxt) {
   103  			return
   104  		}
   105  	}
   106  }
   107  
   108  // Set sets v to value.
   109  func (v *Float) Set(value float64) {
   110  	v.f.Store(math.Float64bits(value))
   111  }
   112  
   113  // Map is a string-to-Var map variable that satisfies the [Var] interface.
   114  type Map struct {
   115  	m      sync.Map // map[string]Var
   116  	keysMu sync.RWMutex
   117  	keys   []string // sorted
   118  }
   119  
   120  // KeyValue represents a single entry in a [Map].
   121  type KeyValue struct {
   122  	Key   string
   123  	Value Var
   124  }
   125  
   126  func (v *Map) String() string {
   127  	return string(v.appendJSON(nil))
   128  }
   129  
   130  func (v *Map) appendJSON(b []byte) []byte {
   131  	return v.appendJSONMayExpand(b, false)
   132  }
   133  
   134  func (v *Map) appendJSONMayExpand(b []byte, expand bool) []byte {
   135  	afterCommaDelim := byte(' ')
   136  	mayAppendNewline := func(b []byte) []byte { return b }
   137  	if expand {
   138  		afterCommaDelim = '\n'
   139  		mayAppendNewline = func(b []byte) []byte { return append(b, '\n') }
   140  	}
   141  
   142  	b = append(b, '{')
   143  	b = mayAppendNewline(b)
   144  	first := true
   145  	v.Do(func(kv KeyValue) {
   146  		if !first {
   147  			b = append(b, ',', afterCommaDelim)
   148  		}
   149  		first = false
   150  		b = appendJSONQuote(b, kv.Key)
   151  		b = append(b, ':', ' ')
   152  		switch v := kv.Value.(type) {
   153  		case nil:
   154  			b = append(b, "null"...)
   155  		case jsonVar:
   156  			b = v.appendJSON(b)
   157  		default:
   158  			b = append(b, v.String()...)
   159  		}
   160  	})
   161  	b = mayAppendNewline(b)
   162  	b = append(b, '}')
   163  	b = mayAppendNewline(b)
   164  	return b
   165  }
   166  
   167  // Init removes all keys from the map.
   168  func (v *Map) Init() *Map {
   169  	v.keysMu.Lock()
   170  	defer v.keysMu.Unlock()
   171  	v.keys = v.keys[:0]
   172  	v.m.Range(func(k, _ any) bool {
   173  		v.m.Delete(k)
   174  		return true
   175  	})
   176  	return v
   177  }
   178  
   179  // addKey updates the sorted list of keys in v.keys.
   180  func (v *Map) addKey(key string) {
   181  	v.keysMu.Lock()
   182  	defer v.keysMu.Unlock()
   183  	// Using insertion sort to place key into the already-sorted v.keys.
   184  	i, found := slices.BinarySearch(v.keys, key)
   185  	if found {
   186  		return
   187  	}
   188  	v.keys = slices.Insert(v.keys, i, key)
   189  }
   190  
   191  func (v *Map) Get(key string) Var {
   192  	i, _ := v.m.Load(key)
   193  	av, _ := i.(Var)
   194  	return av
   195  }
   196  
   197  func (v *Map) Set(key string, av Var) {
   198  	// Before we store the value, check to see whether the key is new. Try a Load
   199  	// before LoadOrStore: LoadOrStore causes the key interface to escape even on
   200  	// the Load path.
   201  	if _, ok := v.m.Load(key); !ok {
   202  		if _, dup := v.m.LoadOrStore(key, av); !dup {
   203  			v.addKey(key)
   204  			return
   205  		}
   206  	}
   207  
   208  	v.m.Store(key, av)
   209  }
   210  
   211  // Add adds delta to the *[Int] value stored under the given map key.
   212  func (v *Map) Add(key string, delta int64) {
   213  	i, ok := v.m.Load(key)
   214  	if !ok {
   215  		var dup bool
   216  		i, dup = v.m.LoadOrStore(key, new(Int))
   217  		if !dup {
   218  			v.addKey(key)
   219  		}
   220  	}
   221  
   222  	// Add to Int; ignore otherwise.
   223  	if iv, ok := i.(*Int); ok {
   224  		iv.Add(delta)
   225  	}
   226  }
   227  
   228  // AddFloat adds delta to the *[Float] value stored under the given map key.
   229  func (v *Map) AddFloat(key string, delta float64) {
   230  	i, ok := v.m.Load(key)
   231  	if !ok {
   232  		var dup bool
   233  		i, dup = v.m.LoadOrStore(key, new(Float))
   234  		if !dup {
   235  			v.addKey(key)
   236  		}
   237  	}
   238  
   239  	// Add to Float; ignore otherwise.
   240  	if iv, ok := i.(*Float); ok {
   241  		iv.Add(delta)
   242  	}
   243  }
   244  
   245  // Delete deletes the given key from the map.
   246  func (v *Map) Delete(key string) {
   247  	v.keysMu.Lock()
   248  	defer v.keysMu.Unlock()
   249  	i, found := slices.BinarySearch(v.keys, key)
   250  	if found {
   251  		v.keys = slices.Delete(v.keys, i, i+1)
   252  		v.m.Delete(key)
   253  	}
   254  }
   255  
   256  // Do calls f for each entry in the map.
   257  // The map is locked during the iteration,
   258  // but existing entries may be concurrently updated.
   259  func (v *Map) Do(f func(KeyValue)) {
   260  	v.keysMu.RLock()
   261  	defer v.keysMu.RUnlock()
   262  	for _, k := range v.keys {
   263  		i, _ := v.m.Load(k)
   264  		val, _ := i.(Var)
   265  		f(KeyValue{k, val})
   266  	}
   267  }
   268  
   269  // String is a string variable, and satisfies the [Var] interface.
   270  type String struct {
   271  	s atomic.Value // string
   272  }
   273  
   274  func (v *String) Value() string {
   275  	p, _ := v.s.Load().(string)
   276  	return p
   277  }
   278  
   279  // String implements the [Var] interface. To get the unquoted string
   280  // use [String.Value].
   281  func (v *String) String() string {
   282  	return string(v.appendJSON(nil))
   283  }
   284  
   285  func (v *String) appendJSON(b []byte) []byte {
   286  	return appendJSONQuote(b, v.Value())
   287  }
   288  
   289  func (v *String) Set(value string) {
   290  	v.s.Store(value)
   291  }
   292  
   293  // Func implements [Var] by calling the function
   294  // and formatting the returned value using JSON.
   295  type Func func() any
   296  
   297  func (f Func) Value() any {
   298  	return f()
   299  }
   300  
   301  func (f Func) String() string {
   302  	v, _ := json.Marshal(f())
   303  	return string(v)
   304  }
   305  
   306  // All published variables.
   307  var vars Map
   308  
   309  // Publish declares a named exported variable. This should be called from a
   310  // package's init function when it creates its Vars. If the name is already
   311  // registered then this will log.Panic.
   312  func Publish(name string, v Var) {
   313  	if _, dup := vars.m.LoadOrStore(name, v); dup {
   314  		log.Panicln("Reuse of exported var name:", name)
   315  	}
   316  	vars.keysMu.Lock()
   317  	defer vars.keysMu.Unlock()
   318  	vars.keys = append(vars.keys, name)
   319  	slices.Sort(vars.keys)
   320  }
   321  
   322  // Get retrieves a named exported variable. It returns nil if the name has
   323  // not been registered.
   324  func Get(name string) Var {
   325  	return vars.Get(name)
   326  }
   327  
   328  // Convenience functions for creating new exported variables.
   329  
   330  func NewInt(name string) *Int {
   331  	v := new(Int)
   332  	Publish(name, v)
   333  	return v
   334  }
   335  
   336  func NewFloat(name string) *Float {
   337  	v := new(Float)
   338  	Publish(name, v)
   339  	return v
   340  }
   341  
   342  func NewMap(name string) *Map {
   343  	v := new(Map).Init()
   344  	Publish(name, v)
   345  	return v
   346  }
   347  
   348  func NewString(name string) *String {
   349  	v := new(String)
   350  	Publish(name, v)
   351  	return v
   352  }
   353  
   354  // Do calls f for each exported variable.
   355  // The global variable map is locked during the iteration,
   356  // but existing entries may be concurrently updated.
   357  func Do(f func(KeyValue)) {
   358  	vars.Do(f)
   359  }
   360  
   361  func expvarHandler(w http.ResponseWriter, r *http.Request) {
   362  	w.Header().Set("Content-Type", "application/json; charset=utf-8")
   363  	w.Write(vars.appendJSONMayExpand(nil, true))
   364  }
   365  
   366  // Handler returns the expvar HTTP Handler.
   367  //
   368  // This is only needed to install the handler in a non-standard location.
   369  func Handler() http.Handler {
   370  	return http.HandlerFunc(expvarHandler)
   371  }
   372  
   373  func cmdline() any {
   374  	return os.Args
   375  }
   376  
   377  func memstats() any {
   378  	stats := new(runtime.MemStats)
   379  	runtime.ReadMemStats(stats)
   380  	return *stats
   381  }
   382  
   383  func init() {
   384  	if godebug.New("httpmuxgo121").Value() == "1" {
   385  		http.HandleFunc("/debug/vars", expvarHandler)
   386  	} else {
   387  		http.HandleFunc("GET /debug/vars", expvarHandler)
   388  	}
   389  	Publish("cmdline", Func(cmdline))
   390  	Publish("memstats", Func(memstats))
   391  }
   392  
   393  // TODO: Use json.appendString instead.
   394  func appendJSONQuote(b []byte, s string) []byte {
   395  	const hex = "0123456789abcdef"
   396  	b = append(b, '"')
   397  	for _, r := range s {
   398  		switch {
   399  		case r < ' ' || r == '\\' || r == '"' || r == '<' || r == '>' || r == '&' || r == '\u2028' || r == '\u2029':
   400  			switch r {
   401  			case '\\', '"':
   402  				b = append(b, '\\', byte(r))
   403  			case '\n':
   404  				b = append(b, '\\', 'n')
   405  			case '\r':
   406  				b = append(b, '\\', 'r')
   407  			case '\t':
   408  				b = append(b, '\\', 't')
   409  			default:
   410  				b = append(b, '\\', 'u', hex[(r>>12)&0xf], hex[(r>>8)&0xf], hex[(r>>4)&0xf], hex[(r>>0)&0xf])
   411  			}
   412  		case r < utf8.RuneSelf:
   413  			b = append(b, byte(r))
   414  		default:
   415  			b = utf8.AppendRune(b, r)
   416  		}
   417  	}
   418  	b = append(b, '"')
   419  	return b
   420  }
   421
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