flag: visit the flags in sorted order, for better messages.

Fixes #1601.

R=rsc
CC=golang-dev
https://golang.org/cl/4249070

src/pkg/flag/flag.go

@@ -68,6 +68,7 @@ package flag
 import (
 	"fmt"
 	"os"
+	"sort"
 	"strconv"
 )
 
@@ -205,16 +206,34 @@ type allFlags struct {
 
 var flags *allFlags
 
-// VisitAll visits the flags, calling fn for each. It visits all flags, even those not set.
+// sortFlags returns the flags as a slice in lexicographical sorted order.
+func sortFlags(flags map[string]*Flag) []*Flag {
+	list := make(sort.StringArray, len(flags))
+	i := 0
+	for _, f := range flags {
+		list[i] = f.Name
+		i++
+	}
+	list.Sort()
+	result := make([]*Flag, len(list))
+	for i, name := range list {
+		result[i] = flags[name]
+	}
+	return result
+}
+
+// VisitAll visits the flags in lexicographical order, calling fn for each.
+// It visits all flags, even those not set.
 func VisitAll(fn func(*Flag)) {
-	for _, f := range flags.formal {
+	for _, f := range sortFlags(flags.formal) {
 		fn(f)
 	}
 }
 
-// Visit visits the flags, calling fn for each. It visits only those flags that have been set.
+// Visit visits the flags in lexicographical order, calling fn for each.
+// It visits only those flags that have been set.
 func Visit(fn func(*Flag)) {
-	for _, f := range flags.actual {
+	for _, f := range sortFlags(flags.actual) {
 		fn(f)
 	}
 }

src/pkg/flag/flag_test.go

@@ -8,6 +8,7 @@ import (
 	. "flag"
 	"fmt"
 	"os"
+	"sort"
 	"testing"
 )
 
@@ -77,6 +78,12 @@ func TestEverything(t *testing.T) {
 			t.Log(k, *v)
 		}
 	}
+	// Now test they're visited in sort order.
+	var flagNames []string
+	Visit(func(f *Flag) { flagNames = append(flagNames, f.Name) })
+	if !sort.StringsAreSorted(flagNames) {
+		t.Errorf("flag names not sorted: %v", flagNames)
+	}
 }
 
 func TestUsage(t *testing.T) {