portlist: reuse bufio.Reader between files

name         old time/op    new time/op    delta
ListPorts-6    1.18ms ± 5%    1.16ms ± 5%     ~     (p=0.075 n=10+10)

name         old alloc/op   new alloc/op   delta
ListPorts-6    27.2kB ± 0%    14.9kB ± 0%  -45.14%  (p=0.001 n=8+9)

name         old allocs/op  new allocs/op  delta
ListPorts-6      90.0 ± 0%      84.0 ± 0%   -6.67%  (p=0.000 n=10+10)

Updates tailscale/corp#2566

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

portlist/portlist_linux.go

@@ -40,8 +40,9 @@ func listPorts() (List, error) {
 	if sawProcNetPermissionErr.Get() {
 		return nil, nil
 	}
-	l := []Port{}
+	var ret []Port
 
+	var br *bufio.Reader
 	for _, fname := range sockfiles {
 		// Android 10+ doesn't allow access to this anymore.
 		// https://developer.android.com/about/versions/10/privacy/changes#proc-net-filesystem
@@ -59,17 +60,19 @@ func listPorts() (List, error) {
 		if err != nil {
 			return nil, fmt.Errorf("%s: %s", fname, err)
 		}
-		defer f.Close()
-		r := bufio.NewReader(f)
-
-		ports, err := parsePorts(r, filepath.Base(fname))
+		if br == nil {
+			br = bufio.NewReader(f)
+		} else {
+			br.Reset(f)
+		}
+		ports, err := parsePorts(br, filepath.Base(fname))
+		f.Close()
 		if err != nil {
 			return nil, fmt.Errorf("parsing %q: %w", fname, err)
 		}
-
-		l = append(l, ports...)
+		ret = append(ret, ports...)
 	}
-	return l, nil
+	return ret, nil
 }
 
 // fileBase is one of "tcp", "tcp6", "udp", "udp6".

portlist/portlist_linux_test.go

@@ -129,3 +129,13 @@ func BenchmarkParsePorts(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkListPorts(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_, err := listPorts()
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}