net: sort records returned by LookupMX

R=rog, adg, rsc
CC=golang-dev
https://golang.org/cl/4388048

diff --git a/src/pkg/net/dnsclient.go b/src/pkg/net/dnsclient.go
index 32cea6125eb05276a366f74ed6959a4bc03aa6e5..c3e727bcef46c15b7619437ae9c500674ffba0d9 100644 (file)
--- a/src/pkg/net/dnsclient.go
+++ b/src/pkg/net/dnsclient.go
@@ -21,6 +21,7 @@ import (
        "rand"
        "sync"
        "time"
+       "sort"
 )
 
 // DNSError represents a DNS lookup error.
@@ -410,18 +411,32 @@ type MX struct {
        Pref uint16
 }
 
-// LookupMX returns the DNS MX records associated with name.
-func LookupMX(name string) (entries []*MX, err os.Error) {
-       var records []dnsRR
-       _, records, err = lookup(name, dnsTypeMX)
+// byPref implements sort.Interface to sort MX records by preference
+type byPref []*MX
+
+func (s byPref) Len() int { return len(s) }
+
+func (s byPref) Less(i, j int) bool { return s[i].Pref < s[j].Pref }
+
+func (s byPref) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
+
+// LookupMX returns the DNS MX records for the given domain name sorted by preference.
+func LookupMX(name string) (mx []*MX, err os.Error) {
+       _, rr, err := lookup(name, dnsTypeMX)
        if err != nil {
                return
        }
-       entries = make([]*MX, len(records))
-       for i := range records {
-               r := records[i].(*dnsRR_MX)
-               entries[i] = &MX{r.Mx, r.Pref}
+       mx = make([]*MX, len(rr))
+       for i := range rr {
+               r := rr[i].(*dnsRR_MX)
+               mx[i] = &MX{r.Mx, r.Pref}
+       }
+       // Shuffle the records to match RFC 5321 when sorted
+       for i := range mx {
+               j := rand.Intn(i + 1)
+               mx[i], mx[j] = mx[j], mx[i]
        }
+       sort.Sort(byPref(mx))
        return
 }