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.
     8  //
     9  // Operations to set or modify these public variables are atomic.
    10  //
    11  // In addition to adding the HTTP handler, this package registers the
    12  // following variables:
    13  //
    14  //	cmdline   os.Args
    15  //	memstats  runtime.Memstats
    16  //
    17  // The package is sometimes only imported for the side effect of
    18  // registering its HTTP handler and the above variables. To use it
    19  // this way, link this package into your program:
    20  //	import _ "expvar"
    21  //
    22  package expvar
    23  
    24  import (
    25  	"encoding/json"
    26  	"fmt"
    27  	"log"
    28  	"math"
    29  	"net/http"
    30  	"os"
    31  	"runtime"
    32  	"sort"
    33  	"strconv"
    34  	"strings"
    35  	"sync"
    36  	"sync/atomic"
    37  )
    38  
    39  // Var is an abstract type for all exported variables.
    40  type Var interface {
    41  	// String returns a valid JSON value for the variable.
    42  	// Types with String methods that do not return valid JSON
    43  	// (such as time.Time) must not be used as a Var.
    44  	String() string
    45  }
    46  
    47  // Int is a 64-bit integer variable that satisfies the Var interface.
    48  type Int struct {
    49  	i int64
    50  }
    51  
    52  func (v *Int) Value() int64 {
    53  	return atomic.LoadInt64(&v.i)
    54  }
    55  
    56  func (v *Int) String() string {
    57  	return strconv.FormatInt(atomic.LoadInt64(&v.i), 10)
    58  }
    59  
    60  func (v *Int) Add(delta int64) {
    61  	atomic.AddInt64(&v.i, delta)
    62  }
    63  
    64  func (v *Int) Set(value int64) {
    65  	atomic.StoreInt64(&v.i, value)
    66  }
    67  
    68  // Float is a 64-bit float variable that satisfies the Var interface.
    69  type Float struct {
    70  	f uint64
    71  }
    72  
    73  func (v *Float) Value() float64 {
    74  	return math.Float64frombits(atomic.LoadUint64(&v.f))
    75  }
    76  
    77  func (v *Float) String() string {
    78  	return strconv.FormatFloat(
    79  		math.Float64frombits(atomic.LoadUint64(&v.f)), 'g', -1, 64)
    80  }
    81  
    82  // Add adds delta to v.
    83  func (v *Float) Add(delta float64) {
    84  	for {
    85  		cur := atomic.LoadUint64(&v.f)
    86  		curVal := math.Float64frombits(cur)
    87  		nxtVal := curVal + delta
    88  		nxt := math.Float64bits(nxtVal)
    89  		if atomic.CompareAndSwapUint64(&v.f, cur, nxt) {
    90  			return
    91  		}
    92  	}
    93  }
    94  
    95  // Set sets v to value.
    96  func (v *Float) Set(value float64) {
    97  	atomic.StoreUint64(&v.f, math.Float64bits(value))
    98  }
    99  
   100  // Map is a string-to-Var map variable that satisfies the Var interface.
   101  type Map struct {
   102  	m      sync.Map // map[string]Var
   103  	keysMu sync.RWMutex
   104  	keys   []string // sorted
   105  }
   106  
   107  // KeyValue represents a single entry in a Map.
   108  type KeyValue struct {
   109  	Key   string
   110  	Value Var
   111  }
   112  
   113  func (v *Map) String() string {
   114  	var b strings.Builder
   115  	fmt.Fprintf(&b, "{")
   116  	first := true
   117  	v.Do(func(kv KeyValue) {
   118  		if !first {
   119  			fmt.Fprintf(&b, ", ")
   120  		}
   121  		fmt.Fprintf(&b, "%q: %v", kv.Key, kv.Value)
   122  		first = false
   123  	})
   124  	fmt.Fprintf(&b, "}")
   125  	return b.String()
   126  }
   127  
   128  // Init removes all keys from the map.
   129  func (v *Map) Init() *Map {
   130  	v.keysMu.Lock()
   131  	defer v.keysMu.Unlock()
   132  	v.keys = v.keys[:0]
   133  	v.m.Range(func(k, _ any) bool {
   134  		v.m.Delete(k)
   135  		return true
   136  	})
   137  	return v
   138  }
   139  
   140  // addKey updates the sorted list of keys in v.keys.
   141  func (v *Map) addKey(key string) {
   142  	v.keysMu.Lock()
   143  	defer v.keysMu.Unlock()
   144  	// Using insertion sort to place key into the already-sorted v.keys.
   145  	if i := sort.SearchStrings(v.keys, key); i >= len(v.keys) {
   146  		v.keys = append(v.keys, key)
   147  	} else if v.keys[i] != key {
   148  		v.keys = append(v.keys, "")
   149  		copy(v.keys[i+1:], v.keys[i:])
   150  		v.keys[i] = key
   151  	}
   152  }
   153  
   154  func (v *Map) Get(key string) Var {
   155  	i, _ := v.m.Load(key)
   156  	av, _ := i.(Var)
   157  	return av
   158  }
   159  
   160  func (v *Map) Set(key string, av Var) {
   161  	// Before we store the value, check to see whether the key is new. Try a Load
   162  	// before LoadOrStore: LoadOrStore causes the key interface to escape even on
   163  	// the Load path.
   164  	if _, ok := v.m.Load(key); !ok {
   165  		if _, dup := v.m.LoadOrStore(key, av); !dup {
   166  			v.addKey(key)
   167  			return
   168  		}
   169  	}
   170  
   171  	v.m.Store(key, av)
   172  }
   173  
   174  // Add adds delta to the *Int value stored under the given map key.
   175  func (v *Map) Add(key string, delta int64) {
   176  	i, ok := v.m.Load(key)
   177  	if !ok {
   178  		var dup bool
   179  		i, dup = v.m.LoadOrStore(key, new(Int))
   180  		if !dup {
   181  			v.addKey(key)
   182  		}
   183  	}
   184  
   185  	// Add to Int; ignore otherwise.
   186  	if iv, ok := i.(*Int); ok {
   187  		iv.Add(delta)
   188  	}
   189  }
   190  
   191  // AddFloat adds delta to the *Float value stored under the given map key.
   192  func (v *Map) AddFloat(key string, delta float64) {
   193  	i, ok := v.m.Load(key)
   194  	if !ok {
   195  		var dup bool
   196  		i, dup = v.m.LoadOrStore(key, new(Float))
   197  		if !dup {
   198  			v.addKey(key)
   199  		}
   200  	}
   201  
   202  	// Add to Float; ignore otherwise.
   203  	if iv, ok := i.(*Float); ok {
   204  		iv.Add(delta)
   205  	}
   206  }
   207  
   208  // Delete deletes the given key from the map.
   209  func (v *Map) Delete(key string) {
   210  	v.keysMu.Lock()
   211  	defer v.keysMu.Unlock()
   212  	i := sort.SearchStrings(v.keys, key)
   213  	if i < len(v.keys) && key == v.keys[i] {
   214  		v.keys = append(v.keys[:i], v.keys[i+1:]...)
   215  		v.m.Delete(key)
   216  	}
   217  }
   218  
   219  // Do calls f for each entry in the map.
   220  // The map is locked during the iteration,
   221  // but existing entries may be concurrently updated.
   222  func (v *Map) Do(f func(KeyValue)) {
   223  	v.keysMu.RLock()
   224  	defer v.keysMu.RUnlock()
   225  	for _, k := range v.keys {
   226  		i, _ := v.m.Load(k)
   227  		f(KeyValue{k, i.(Var)})
   228  	}
   229  }
   230  
   231  // String is a string variable, and satisfies the Var interface.
   232  type String struct {
   233  	s atomic.Value // string
   234  }
   235  
   236  func (v *String) Value() string {
   237  	p, _ := v.s.Load().(string)
   238  	return p
   239  }
   240  
   241  // String implements the Var interface. To get the unquoted string
   242  // use Value.
   243  func (v *String) String() string {
   244  	s := v.Value()
   245  	b, _ := json.Marshal(s)
   246  	return string(b)
   247  }
   248  
   249  func (v *String) Set(value string) {
   250  	v.s.Store(value)
   251  }
   252  
   253  // Func implements Var by calling the function
   254  // and formatting the returned value using JSON.
   255  type Func func() any
   256  
   257  func (f Func) Value() any {
   258  	return f()
   259  }
   260  
   261  func (f Func) String() string {
   262  	v, _ := json.Marshal(f())
   263  	return string(v)
   264  }
   265  
   266  // All published variables.
   267  var (
   268  	vars      sync.Map // map[string]Var
   269  	varKeysMu sync.RWMutex
   270  	varKeys   []string // sorted
   271  )
   272  
   273  // Publish declares a named exported variable. This should be called from a
   274  // package's init function when it creates its Vars. If the name is already
   275  // registered then this will log.Panic.
   276  func Publish(name string, v Var) {
   277  	if _, dup := vars.LoadOrStore(name, v); dup {
   278  		log.Panicln("Reuse of exported var name:", name)
   279  	}
   280  	varKeysMu.Lock()
   281  	defer varKeysMu.Unlock()
   282  	varKeys = append(varKeys, name)
   283  	sort.Strings(varKeys)
   284  }
   285  
   286  // Get retrieves a named exported variable. It returns nil if the name has
   287  // not been registered.
   288  func Get(name string) Var {
   289  	i, _ := vars.Load(name)
   290  	v, _ := i.(Var)
   291  	return v
   292  }
   293  
   294  // Convenience functions for creating new exported variables.
   295  
   296  func NewInt(name string) *Int {
   297  	v := new(Int)
   298  	Publish(name, v)
   299  	return v
   300  }
   301  
   302  func NewFloat(name string) *Float {
   303  	v := new(Float)
   304  	Publish(name, v)
   305  	return v
   306  }
   307  
   308  func NewMap(name string) *Map {
   309  	v := new(Map).Init()
   310  	Publish(name, v)
   311  	return v
   312  }
   313  
   314  func NewString(name string) *String {
   315  	v := new(String)
   316  	Publish(name, v)
   317  	return v
   318  }
   319  
   320  // Do calls f for each exported variable.
   321  // The global variable map is locked during the iteration,
   322  // but existing entries may be concurrently updated.
   323  func Do(f func(KeyValue)) {
   324  	varKeysMu.RLock()
   325  	defer varKeysMu.RUnlock()
   326  	for _, k := range varKeys {
   327  		val, _ := vars.Load(k)
   328  		f(KeyValue{k, val.(Var)})
   329  	}
   330  }
   331  
   332  func expvarHandler(w http.ResponseWriter, r *http.Request) {
   333  	w.Header().Set("Content-Type", "application/json; charset=utf-8")
   334  	fmt.Fprintf(w, "{\n")
   335  	first := true
   336  	Do(func(kv KeyValue) {
   337  		if !first {
   338  			fmt.Fprintf(w, ",\n")
   339  		}
   340  		first = false
   341  		fmt.Fprintf(w, "%q: %s", kv.Key, kv.Value)
   342  	})
   343  	fmt.Fprintf(w, "\n}\n")
   344  }
   345  
   346  // Handler returns the expvar HTTP Handler.
   347  //
   348  // This is only needed to install the handler in a non-standard location.
   349  func Handler() http.Handler {
   350  	return http.HandlerFunc(expvarHandler)
   351  }
   352  
   353  func cmdline() any {
   354  	return os.Args
   355  }
   356  
   357  func memstats() any {
   358  	stats := new(runtime.MemStats)
   359  	runtime.ReadMemStats(stats)
   360  	return *stats
   361  }
   362  
   363  func init() {
   364  	http.HandleFunc("/debug/vars", expvarHandler)
   365  	Publish("cmdline", Func(cmdline))
   366  	Publish("memstats", Func(memstats))
   367  }
   368
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