portlist: document, clean up, fix an open fd spike, optimize a bit

I noticed portlist when looking at some profiles and hadn't looked at
the code much before. This is a first pass over it. It allocates a
fair bit. More love remains, but this does a bit:

name       old time/op    new time/op    delta
GetList-8    9.92ms ± 8%    9.64ms ±12%     ~     (p=0.247 n=10+10)

name       old alloc/op   new alloc/op   delta
GetList-8     931kB ± 0%     869kB ± 0%   -6.70%  (p=0.000 n=10+10)

name       old allocs/op  new allocs/op  delta
GetList-8     4.59k ± 0%     3.69k ± 1%  -19.71%  (p=0.000 n=10+10)

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

ipn/local.go

@@ -5,6 +5,7 @@
 package ipn
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"log"
@@ -27,6 +28,8 @@ import (
 // plane and the local network stack, wiring up NetworkMap updates
 // from the cloud to the local WireGuard engine.
 type LocalBackend struct {
+	ctx             context.Context    // valid until Close
+	ctxCancel       context.CancelFunc // closes ctx
 	logf            logger.Logf
 	e               wgengine.Engine
 	store           StateStore
@@ -66,12 +69,15 @@ func NewLocalBackend(logf logger.Logf, logid string, store StateStore, e wgengin
 	// Default filter blocks everything, until Start() is called.
 	e.SetFilter(filter.NewAllowNone())
 
+	ctx, cancel := context.WithCancel(context.Background())
 	portpoll, err := portlist.NewPoller()
 	if err != nil {
 		logf("skipping portlist: %s\n", err)
 	}
 
 	b := &LocalBackend{
+		ctx:          ctx,
+		ctxCancel:    cancel,
 		logf:         logf,
 		e:            e,
 		store:        store,
@@ -84,7 +90,7 @@ func NewLocalBackend(logf logger.Logf, logid string, store StateStore, e wgengin
 	e.SetNetInfoCallback(b.SetNetInfo)
 
 	if b.portpoll != nil {
-		go b.portpoll.Run()
+		go b.portpoll.Run(ctx)
 		go b.runPoller()
 	}
 
@@ -92,9 +98,7 @@ func NewLocalBackend(logf logger.Logf, logid string, store StateStore, e wgengin
 }
 
 func (b *LocalBackend) Shutdown() {
-	if b.portpoll != nil {
-		b.portpoll.Close()
-	}
+	b.ctxCancel()
 	b.c.Shutdown()
 	b.e.Close()
 	b.e.Wait()
@@ -313,9 +317,9 @@ func (b *LocalBackend) updateFilter(netMap *controlclient.NetworkMap) {
 
 func (b *LocalBackend) runPoller() {
 	for {
-		ports := <-b.portpoll.C
-		if ports == nil {
-			break
+		ports, ok := <-b.portpoll.C
+		if !ok {
+			return
 		}
 		sl := []tailcfg.Service{}
 		for _, p := range ports {

portlist/netstat_test.go

@@ -5,7 +5,6 @@
 package portlist
 
 import (
-	"fmt"
 	"testing"
 )
 
@@ -62,28 +61,26 @@ udp4       0      0  *.5553                 *.*
 `
 
 func TestParsePortsNetstat(t *testing.T) {
-	expect := List{
+	want := List{
 		Port{"tcp", 22, "", ""},
 		Port{"tcp", 23, "", ""},
 		Port{"tcp", 24, "", ""},
-		Port{"tcp", 32, "", "sshd"},
-		Port{"udp", 53, "", "chrome"},
-		Port{"udp", 53, "", "funball"},
-		Port{"udp", 5050, "", "CDPSvc"},
+		Port{"tcp", 32, "sshd", ""},
+		Port{"udp", 53, "chrome", ""},
+		Port{"udp", 53, "funball", ""},
+		Port{"udp", 5050, "CDPSvc", ""},
 		Port{"udp", 5353, "", ""},
 		Port{"udp", 5354, "", ""},
 		Port{"udp", 5453, "", ""},
 		Port{"udp", 5553, "", ""},
-		Port{"udp", 9353, "", "iTunes"},
+		Port{"udp", 9353, "iTunes", ""},
 	}
 
 	pl := parsePortsNetstat(netstat_output)
-	fmt.Printf("--- expect:\n%v\n", expect)
-	fmt.Printf("--- got:\n%v\n", pl)
 	for i := range pl {
-		if expect[i] != pl[i] {
-			t.Fatalf("row#%d\n expect=%v\n    got=%v\n",
-				i, expect[i], pl[i])
+		if pl[i] != want[i] {
+			t.Errorf("row#%d\n got: %#v\n\nwant: %#v\n",
+				i, pl[i], want[i])
 		}
 	}
 }

portlist/poller.go

@@ -5,40 +5,67 @@
 package portlist
 
 import (
+	"context"
 	"time"
 )
 
+// Poller scans the systems for listening ports periodically and sends
+// the results to C.
 type Poller struct {
-	C      chan List     // new data when it arrives; closed when done
+	// C received the list of ports periodically. It's closed when
+	// Run completes, after which Err can be checked.
+	C <-chan List
+
+	c chan List
+
+	// Err is the error from the final GetList call. It is only
+	// valid to read once C has been closed. Err is nil if Close
+	// is called or the context is canceled.
+	Err error
+
 	quitCh chan struct{} // close this to force exit
-	Err    error         // last returned error code, if any
 	prev   List          // most recent data
 }
 
+// NewPoller returns a new portlist Poller. It returns an error
+// if the portlist couldn't be obtained. Subsequent
 func NewPoller() (*Poller, error) {
 	p := &Poller{
-		C:      make(chan List),
+		c:      make(chan List),
 		quitCh: make(chan struct{}),
 	}
-	// Do one initial poll synchronously, so the caller can react
-	// to any obvious errors.
-	p.prev, p.Err = GetList(nil)
-	return p, p.Err
+	p.C = p.c
+
+	// Do one initial poll synchronously so we can return an error
+	// early.
+	var err error
+	p.prev, err = GetList(nil)
+	if err != nil {
+		return nil, err
+	}
+	return p, nil
 }
 
-func (p *Poller) Close() {
+func (p *Poller) Close() error {
+	select {
+	case <-p.quitCh:
+		return nil
+	default:
+	}
 	close(p.quitCh)
 	<-p.C
+	return nil
 }
 
-// Poll periodically. Run this in a goroutine if you want.
-func (p *Poller) Run() error {
-	defer close(p.C)
-	tick := time.NewTicker(POLL_SECONDS * time.Second)
+// Run runs the Poller periodically until either the context
+// is done, or the Close is called.
+func (p *Poller) Run(ctx context.Context) error {
+	defer close(p.c)
+	tick := time.NewTicker(pollInterval)
 	defer tick.Stop()
 
 	// Send out the pre-generated initial value
-	p.C <- p.prev
+	p.c <- p.prev
 
 	for {
 		select {
@@ -46,12 +73,21 @@ func (p *Poller) Run() error {
 			pl, err := GetList(p.prev)
 			if err != nil {
 				p.Err = err
-				return p.Err
+				return err
+			}
+			if pl.SameInodes(p.prev) {
+				continue
 			}
-			if !pl.SameInodes(p.prev) {
-				p.prev = pl
-				p.C <- pl
+			p.prev = pl
+			select {
+			case p.c <- pl:
+			case <-ctx.Done():
+				return ctx.Err()
+			case <-p.quitCh:
+				return nil
 			}
+		case <-ctx.Done():
+			return ctx.Err()
 		case <-p.quitCh:
 			return nil
 		}

portlist/portlist.go

@@ -9,13 +9,16 @@ import (
 	"strings"
 )
 
+// Port is a listening port on the machine.
 type Port struct {
-	Proto   string
-	Port    uint16
-	inode   string
-	Process string
+	Proto   string // "tcp" or "udp"
+	Port    uint16 // port number
+	Process string // optional process name, if found
+
+	inode string // OS-specific; "socket:[165614651]" on Linux
 }
 
+// List is a list of Ports.
 type List []Port
 
 var protos = []string{"tcp", "udp"}
@@ -62,12 +65,12 @@ func (a List) SameInodes(b List) bool {
 }
 
 func (pl List) String() string {
-	out := []string{}
+	var sb strings.Builder
 	for _, v := range pl {
-		out = append(out, fmt.Sprintf("%-3s %5d %-17s %#v",
-			v.Proto, v.Port, v.inode, v.Process))
+		fmt.Fprintf(&sb, "%-3s %5d %-17s %#v\n",
+			v.Proto, v.Port, v.inode, v.Process)
 	}
-	return strings.Join(out, "\n")
+	return strings.TrimRight(sb.String(), "\n")
 }
 
 func GetList(prev List) (List, error) {

portlist/portlist_darwin.go

@@ -13,12 +13,13 @@ import (
 	"log"
 	"os"
 	"strings"
+	"time"
 
 	exec "tailscale.com/tempfork/osexec"
 )
 
 // We have to run netstat, which is a bit expensive, so don't do it too often.
-const POLL_SECONDS = 5
+const pollInterval = 5 * time.Second
 
 func listPorts() (List, error) {
 	return listPortsNetstat("-na")

portlist/portlist_linux.go

@@ -13,10 +13,13 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"time"
+
+	"golang.org/x/sys/unix"
 )
 
 // Reading the sockfiles on Linux is very fast, so we can do it often.
-const POLL_SECONDS = 1
+const pollInterval = 1 * time.Second
 
 // TODO(apenwarr): Include IPv6 ports eventually.
 // Right now we don't route IPv6 anyway so it's better to exclude them.
@@ -82,24 +85,73 @@ func listPorts() (List, error) {
 }
 
 func addProcesses(pl []Port) ([]Port, error) {
-	pm := map[string]*Port{}
-	for k := range pl {
-		pm[pl[k].inode] = &pl[k]
+	pm := map[string]*Port{} // by Port.inode
+	for i := range pl {
+		pm[pl[i].inode] = &pl[i]
+	}
+
+	err := foreachPID(func(pid string) error {
+		fdDir, err := os.Open(fmt.Sprintf("/proc/%s/fd", pid))
+		if err != nil {
+			// Can't open fd list for this pid. Maybe
+			// don't have access. Ignore it.
+			return nil
+		}
+		defer fdDir.Close()
+
+		targetBuf := make([]byte, 64) // plenty big for "socket:[165614651]"
+		for {
+			fds, err := fdDir.Readdirnames(100)
+			if err == io.EOF {
+				return nil
+			}
+			if err != nil {
+				return fmt.Errorf("readdir: %w", err)
+			}
+			for _, fd := range fds {
+				n, err := unix.Readlink(fmt.Sprintf("/proc/%s/fd/%s", pid, fd), targetBuf)
+				if err != nil {
+					// Not a symlink or no permission.
+					// Skip it.
+					continue
+				}
+
+				// TODO(apenwarr): use /proc/*/cmdline instead of /comm?
+				// Unsure right now whether users will want the extra detail
+				// or not.
+				pe := pm[string(targetBuf[:n])] // m[string([]byte)] avoids alloc
+				if pe != nil {
+					comm, err := ioutil.ReadFile(fmt.Sprintf("/proc/%s/comm", pid))
+					if err != nil {
+						// Usually shouldn't happen. One possibility is
+						// the process has gone away, so let's skip it.
+						continue
+					}
+					pe.Process = strings.TrimSpace(string(comm))
+				}
+			}
+		}
+	})
+	if err != nil {
+		return nil, err
 	}
+	return pl, nil
+}
 
+func foreachPID(fn func(pidStr string) error) error {
 	pdir, err := os.Open("/proc")
 	if err != nil {
-		return nil, fmt.Errorf("/proc: %s", err)
+		return err
 	}
 	defer pdir.Close()
 
 	for {
 		pids, err := pdir.Readdirnames(100)
 		if err == io.EOF {
-			break
+			return nil
 		}
 		if err != nil {
-			return nil, fmt.Errorf("/proc: %s", err)
+			return err
 		}
 
 		for _, pid := range pids {
@@ -109,47 +161,9 @@ func addProcesses(pl []Port) ([]Port, error) {
 				// /proc has lots of non-pid stuff in it.
 				continue
 			}
-			fddir, err := os.Open(fmt.Sprintf("/proc/%s/fd", pid))
-			if err != nil {
-				// Can't open fd list for this pid. Maybe
-				// don't have access. Ignore it.
-				continue
-			}
-			defer fddir.Close()
-
-			for {
-				fds, err := fddir.Readdirnames(100)
-				if err == io.EOF {
-					break
-				}
-				if err != nil {
-					return nil, fmt.Errorf("readdir: %s", err)
-				}
-				for _, fd := range fds {
-					target, err := os.Readlink(fmt.Sprintf("/proc/%s/fd/%s", pid, fd))
-					if err != nil {
-						// Not a symlink or no permission.
-						// Skip it.
-						continue
-					}
-
-					// TODO(apenwarr): use /proc/*/cmdline instead of /comm?
-					// Unsure right now whether users will want the extra detail
-					// or not.
-					pe := pm[target]
-					if pe != nil {
-						comm, err := ioutil.ReadFile(fmt.Sprintf("/proc/%s/comm", pid))
-						if err != nil {
-							// Usually shouldn't happen. One possibility is
-							// the process has gone away, so let's skip it.
-							continue
-						}
-						pe.Process = strings.TrimSpace(string(comm))
-					}
-				}
+			if err := fn(pid); err != nil {
+				return err
 			}
 		}
 	}
-
-	return pl, nil
 }

portlist/portlist_other.go

@@ -6,8 +6,10 @@
 
 package portlist
 
+import "time"
+
 // We have to run netstat, which is a bit expensive, so don't do it too often.
-const POLL_SECONDS = 5
+const pollInterval = 5 * time.Second
 
 func listPorts() (List, error) {
 	return listPortsNetstat("-na")

portlist/portlist_test.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package portlist
+
+import "testing"
+
+func TestGetList(t *testing.T) {
+	pl, err := GetList(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, p := range pl {
+		t.Logf("[%d] %+v", i, p)
+	}
+	t.Logf("As String: %v", pl.String())
+}
+
+func BenchmarkGetList(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_, err := GetList(nil)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}

portlist/portlist_windows.go

@@ -4,8 +4,10 @@
 
 package portlist
 
+import "time"
+
 // Forking on Windows is insanely expensive, so don't do it too often.
-const POLL_SECONDS = 5
+const pollInterval = 5 * time.Second
 
 func listPorts() (List, error) {
 	return listPortsNetstat("-na")