portlist: add Poller.IncludeLocalhost option

This PR parameterizes receiving loopback updates from the portlist package.
Callers can now include services bound to localhost if they want.
Note that this option is off by default still.

Fixes #8171

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

portlist/portlist_linux.go

@@ -35,25 +35,28 @@ type linuxImpl struct {
 	procNetFiles    []*os.File // seeked to start & reused between calls
 	readlinkPathBuf []byte
 
-	known map[string]*portMeta // inode string => metadata
-	br    *bufio.Reader
+	known            map[string]*portMeta // inode string => metadata
+	br               *bufio.Reader
+	includeLocalhost bool
 }
 
 type portMeta struct {
 	port          Port
+	pid           int
 	keep          bool
 	needsProcName bool
 }
 
-func newLinuxImplBase() *linuxImpl {
+func newLinuxImplBase(includeLocalhost bool) *linuxImpl {
 	return &linuxImpl{
-		br:    bufio.NewReader(eofReader),
-		known: map[string]*portMeta{},
+		br:               bufio.NewReader(eofReader),
+		known:            map[string]*portMeta{},
+		includeLocalhost: includeLocalhost,
 	}
 }
 
-func newLinuxImpl() osImpl {
-	li := newLinuxImplBase()
+func newLinuxImpl(includeLocalhost bool) osImpl {
+	li := newLinuxImplBase(includeLocalhost)
 	for _, name := range []string{
 		"/proc/net/tcp",
 		"/proc/net/tcp6",
@@ -220,7 +223,7 @@ func (li *linuxImpl) parseProcNetFile(r *bufio.Reader, fileBase string) error {
 		// If a port is bound to localhost, ignore it.
 		// TODO: localhost is bigger than 1 IP, we need to ignore
 		// more things.
-		if mem.HasPrefix(local, mem.S(v4Localhost)) || mem.HasPrefix(local, mem.S(v6Localhost)) {
+		if !li.includeLocalhost && (mem.HasPrefix(local, mem.S(v4Localhost)) || mem.HasPrefix(local, mem.S(v6Localhost))) {
 			continue
 		}
 
@@ -315,6 +318,9 @@ func (li *linuxImpl) findProcessNames(need map[string]*portMeta) error {
 			}
 
 			argv := strings.Split(strings.TrimSuffix(string(bs), "\x00"), "\x00")
+			if p, err := mem.ParseInt(pid, 10, 0); err == nil {
+				pe.pid = int(p)
+			}
 			pe.port.Process = argvSubject(argv...)
 			pe.needsProcName = false
 			delete(need, string(targetBuf[:n]))