net: sort records returned by LookupSRV

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4518061

diff --git a/src/pkg/net/dnsclient.go b/src/pkg/net/dnsclient.go
index 3466003fab8eae531237df4c91a96ec8e16809cc..ae9ca8430522a2e2a6b5dc1745992537b1932e80 100644 (file)
--- a/src/pkg/net/dnsclient.go
+++ b/src/pkg/net/dnsclient.go
@@ -398,10 +398,49 @@ type SRV struct {
        Weight   uint16
 }
 
+// byPriorityWeight sorts SRV records by ascending priority and weight.
+type byPriorityWeight []*SRV
+
+func (s byPriorityWeight) Len() int { return len(s) }
+
+func (s byPriorityWeight) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
+
+func (s byPriorityWeight) Less(i, j int) bool {
+       return s[i].Priority < s[j].Priority ||
+               (s[i].Priority == s[j].Priority && s[i].Weight < s[j].Weight)
+}
+
+// shuffleSRVByWeight shuffles SRV records by weight using the algorithm
+// described in RFC 2782.  
+func shuffleSRVByWeight(addrs []*SRV) {
+       sum := 0
+       for _, addr := range addrs {
+               sum += int(addr.Weight)
+       }
+       for sum > 0 && len(addrs) > 1 {
+               s := 0
+               n := rand.Intn(sum + 1)
+               for i := range addrs {
+                       s += int(addrs[i].Weight)
+                       if s >= n {
+                               if i > 0 {
+                                       t := addrs[i]
+                                       copy(addrs[1:i+1], addrs[0:i])
+                                       addrs[0] = t
+                               }
+                               break
+                       }
+               }
+               sum -= int(addrs[0].Weight)
+               addrs = addrs[1:]
+       }
+}
+
 // LookupSRV tries to resolve an SRV query of the given service,
 // protocol, and domain name, as specified in RFC 2782. In most cases
 // the proto argument can be the same as the corresponding
-// Addr.Network().
+// Addr.Network(). The returned records are sorted by priority 
+// and randomized by weight within a priority.
 func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err os.Error) {
        target := "_" + service + "._" + proto + "." + name
        var records []dnsRR
@@ -414,6 +453,15 @@ func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err os.
                r := rr.(*dnsRR_SRV)
                addrs[i] = &SRV{r.Target, r.Port, r.Priority, r.Weight}
        }
+       sort.Sort(byPriorityWeight(addrs))
+       i := 0
+       for j := 1; j < len(addrs); j++ {
+               if addrs[i].Priority != addrs[j].Priority {
+                       shuffleSRVByWeight(addrs[i:j])
+                       i = j
+               }
+       }
+       shuffleSRVByWeight(addrs[i:len(addrs)])
        return
 }