expvar: sort maps, fix race

It's pretty distracting to use expvar with the output of both
the top-level map and map values jumping around randomly.

Also fixes a potential race where multiple clients trying to
increment a map int or float key at the same time could lose
updates.

R=golang-codereviews, couchmoney
CC=golang-codereviews
https://golang.org/cl/54320043

src/pkg/expvar/expvar.go

@@ -29,6 +29,7 @@ import (
 	"net/http"
 	"os"
 	"runtime"
+	"sort"
 	"strconv"
 	"sync"
 )
@@ -40,8 +41,8 @@ type Var interface {
 
 // Int is a 64-bit integer variable that satisfies the Var interface.
 type Int struct {
-	i  int64
 	mu sync.RWMutex
+	i  int64
 }
 
 func (v *Int) String() string {
@@ -64,8 +65,8 @@ func (v *Int) Set(value int64) {
 
 // Float is a 64-bit float variable that satisfies the Var interface.
 type Float struct {
-	f  float64
 	mu sync.RWMutex
+	f  float64
 }
 
 func (v *Float) String() string {
@@ -90,8 +91,9 @@ func (v *Float) Set(value float64) {
 
 // Map is a string-to-Var map variable that satisfies the Var interface.
 type Map struct {
-	m  map[string]Var
-	mu sync.RWMutex
+	mu   sync.RWMutex
+	m    map[string]Var
+	keys []string // sorted
 }
 
 // KeyValue represents a single entry in a Map.
@@ -106,13 +108,13 @@ func (v *Map) String() string {
 	var b bytes.Buffer
 	fmt.Fprintf(&b, "{")
 	first := true
-	for key, val := range v.m {
+	v.Do(func(kv KeyValue) {
 		if !first {
 			fmt.Fprintf(&b, ", ")
 		}
-		fmt.Fprintf(&b, "\"%s\": %v", key, val)
+		fmt.Fprintf(&b, "\"%s\": %v", kv.Key, kv.Value)
 		first = false
-	}
+	})
 	fmt.Fprintf(&b, "}")
 	return b.String()
 }
@@ -122,6 +124,20 @@ func (v *Map) Init() *Map {
 	return v
 }
 
+// updateKeys updates the sorted list of keys in v.keys.
+// must be called with v.mu held.
+func (v *Map) updateKeys() {
+	if len(v.m) == len(v.keys) {
+		// No new key.
+		return
+	}
+	v.keys = v.keys[:0]
+	for k := range v.m {
+		v.keys = append(v.keys, k)
+	}
+	sort.Strings(v.keys)
+}
+
 func (v *Map) Get(key string) Var {
 	v.mu.RLock()
 	defer v.mu.RUnlock()
@@ -132,6 +148,7 @@ func (v *Map) Set(key string, av Var) {
 	v.mu.Lock()
 	defer v.mu.Unlock()
 	v.m[key] = av
+	v.updateKeys()
 }
 
 func (v *Map) Add(key string, delta int64) {
@@ -141,9 +158,11 @@ func (v *Map) Add(key string, delta int64) {
 	if !ok {
 		// check again under the write lock
 		v.mu.Lock()
-		if _, ok = v.m[key]; !ok {
+		av, ok = v.m[key]
+		if !ok {
 			av = new(Int)
 			v.m[key] = av
+			v.updateKeys()
 		}
 		v.mu.Unlock()
 	}
@@ -162,9 +181,11 @@ func (v *Map) AddFloat(key string, delta float64) {
 	if !ok {
 		// check again under the write lock
 		v.mu.Lock()
-		if _, ok = v.m[key]; !ok {
+		av, ok = v.m[key]
+		if !ok {
 			av = new(Float)
 			v.m[key] = av
+			v.updateKeys()
 		}
 		v.mu.Unlock()
 	}
@@ -181,15 +202,15 @@ func (v *Map) AddFloat(key string, delta float64) {
 func (v *Map) Do(f func(KeyValue)) {
 	v.mu.RLock()
 	defer v.mu.RUnlock()
-	for k, v := range v.m {
-		f(KeyValue{k, v})
+	for _, k := range v.keys {
+		f(KeyValue{k, v.m[k]})
 	}
 }
 
 // String is a string variable, and satisfies the Var interface.
 type String struct {
-	s  string
 	mu sync.RWMutex
+	s  string
 }
 
 func (v *String) String() string {
@@ -215,8 +236,9 @@ func (f Func) String() string {
 
 // All published variables.
 var (
-	mutex sync.RWMutex
-	vars  map[string]Var = make(map[string]Var)
+	mutex   sync.RWMutex
+	vars    = make(map[string]Var)
+	varKeys []string // sorted
 )
 
 // Publish declares a named exported variable. This should be called from a
@@ -229,6 +251,8 @@ func Publish(name string, v Var) {
 		log.Panicln("Reuse of exported var name:", name)
 	}
 	vars[name] = v
+	varKeys = append(varKeys, name)
+	sort.Strings(varKeys)
 }
 
 // Get retrieves a named exported variable.
@@ -270,8 +294,8 @@ func NewString(name string) *String {
 func Do(f func(KeyValue)) {
 	mutex.RLock()
 	defer mutex.RUnlock()
-	for k, v := range vars {
-		f(KeyValue{k, v})
+	for _, k := range varKeys {
+		f(KeyValue{k, vars[k]})
 	}
 }
 

src/pkg/expvar/expvar_test.go

@@ -5,7 +5,10 @@
 package expvar
 
 import (
+	"bytes"
 	"encoding/json"
+	"net/http/httptest"
+	"strconv"
 	"testing"
 )
 
@@ -15,6 +18,7 @@ func RemoveAll() {
 	mutex.Lock()
 	defer mutex.Unlock()
 	vars = make(map[string]Var)
+	varKeys = nil
 }
 
 func TestInt(t *testing.T) {
@@ -139,3 +143,25 @@ func TestFunc(t *testing.T) {
 		t.Errorf(`f.String() = %q, want %q`, s, exp)
 	}
 }
+
+func TestHandler(t *testing.T) {
+	RemoveAll()
+	m := NewMap("map1")
+	m.Add("a", 1)
+	m.Add("z", 2)
+	m2 := NewMap("map2")
+	for i := 0; i < 9; i++ {
+		m2.Add(strconv.Itoa(i), int64(i))
+	}
+	rr := httptest.NewRecorder()
+	rr.Body = new(bytes.Buffer)
+	expvarHandler(rr, nil)
+	want := `{
+"map1": {"a": 1, "z": 2},
+"map2": {"0": 0, "1": 1, "2": 2, "3": 3, "4": 4, "5": 5, "6": 6, "7": 7, "8": 8}
+}
+`
+	if got := rr.Body.String(); got != want {
+		t.Errorf("HTTP handler wrote:\n%s\nWant:\n%s", got, want)
+	}
+}