portlist: de-dup services on same (proto, port) on both IPv4/IPv6

Fixes #1703

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

portlist/portlist.go

@@ -6,6 +6,7 @@
 
 import (
 	"fmt"
+	"sort"
 	"strings"
 )
 
@@ -76,6 +77,7 @@ func GetList(prev List) (List, error) {
 	if err != nil {
 		return nil, fmt.Errorf("listPorts: %s", err)
 	}
+	pl = sortAndDedup(pl)
 	if pl.sameInodes(prev) {
 		// Nothing changed, skip inode lookup
 		return prev, nil
@@ -86,3 +88,22 @@ func GetList(prev List) (List, error) {
 	}
 	return pl, nil
 }
+
+// sortAndDedup sorts ps in place (by Port.lessThan) and then returns
+// a subset of it with duplicate (Proto, Port) removed.
+func sortAndDedup(ps List) List {
+	sort.Slice(ps, func(i, j int) bool {
+		return (&ps[i]).lessThan(&ps[j])
+	})
+	out := ps[:0]
+	var last Port
+	for _, p := range ps {
+		protoPort := Port{Proto: p.Proto, Port: p.Port}
+		if last == protoPort {
+			continue
+		}
+		out = append(out, p)
+		last = protoPort
+	}
+	return out
+}

portlist/portlist_linux.go

@@ -12,7 +12,6 @@
 	"os"
 	"path/filepath"
 	"runtime"
-	"sort"
 	"strconv"
 	"strings"
 	"syscall"
@@ -71,11 +70,6 @@ func listPorts() (List, error) {
 
 		l = append(l, ports...)
 	}
-
-	sort.Slice(l, func(i, j int) bool {
-		return (&l[i]).lessThan(&l[j])
-	})
-
 	return l, nil
 }