agent: allow to set SO_REUSEPORT on listening socket

Introduce Option.SocketReuseAddrAndPort which, if set, will lead to the
SO_REUSEPORT socket option being set on the listening socket on
Unix-like OSes. This also sets SO_REUSEADDR which is already the default
in net.Listen (see net.setDefaultSockopts).

Setting these options increases the chance to re-bind() to the same
address and port upon agent restart if Options.Addr is set.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

agent/agent.go

@@ -8,6 +8,7 @@ package agent
 
 import (
 	"bufio"
+	"context"
 	"encoding/binary"
 	"fmt"
 	"io"
@@ -55,6 +56,13 @@ type Options struct {
 	// can call Close before shutting down.
 	// Optional.
 	ShutdownCleanup bool
+
+	// ReuseSocketAddrAndPort determines whether the SO_REUSEADDR and
+	// SO_REUSEADDR socket options should be set on the listening socket of
+	// the agent. This option is only effective on unix-like OSes and if
+	// Addr is set to a fixed host:port.
+	// Optional.
+	ReuseSocketAddrAndPort bool
 }
 
 // Listen starts the gops agent on a host process. Once agent started, users
@@ -96,11 +104,14 @@ func Listen(opts Options) error {
 	if addr == "" {
 		addr = defaultAddr
 	}
-	ln, err := net.Listen("tcp", addr)
+	var lc net.ListenConfig
+	if opts.ReuseSocketAddrAndPort {
+		lc.Control = setsockoptReuseAddrAndPort
+	}
+	listener, err = lc.Listen(context.Background(), "tcp", addr)
 	if err != nil {
 		return err
 	}
-	listener = ln
 	port := listener.Addr().(*net.TCPAddr).Port
 	portfile = fmt.Sprintf("%s/%d", gopsdir, os.Getpid())
 	err = ioutil.WriteFile(portfile, []byte(strconv.Itoa(port)), os.ModePerm)

agent/agent_test.go

@@ -54,6 +54,17 @@ func TestAgentListenMultipleClose(t *testing.T) {
 	Close()
 }
 
+func TestAgentListenReuseAddrAndPort(t *testing.T) {
+	err := Listen(Options{
+		Addr:                   "127.0.0.1:50000",
+		ReuseSocketAddrAndPort: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	Close()
+}
+
 func TestFormatBytes(t *testing.T) {
 	tests := []struct {
 		val  uint64

agent/sockopt_unix.go

@@ -0,0 +1,36 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !js,!plan9,!windows
+
+package agent
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// setsockoptReuseAddrAndPort sets the SO_REUSEADDR and SO_REUSEPORT socket
+// options on c's underlying socket in order to increase the chance to re-bind()
+// to the same address and port upon agent restart.
+func setsockoptReuseAddrAndPort(network, address string, c syscall.RawConn) error {
+	var soerr error
+	if err := c.Control(func(su uintptr) {
+		sock := int(su)
+		// Allow reuse of recently-used addresses. This socket option is
+		// set by default on listeners in Go's net package, see
+		// net.setDefaultSockopts.
+		soerr = unix.SetsockoptInt(sock, unix.SOL_SOCKET, unix.SO_REUSEADDR, 1)
+		if soerr != nil {
+			return
+		}
+		// Allow reuse of recently-used ports. This gives the agent a
+		// better chance to re-bind upon restarts.
+		soerr = unix.SetsockoptInt(sock, unix.SOL_SOCKET, unix.SO_REUSEPORT, 1)
+	}); err != nil {
+		return err
+	}
+	return soerr
+}

agent/sockopt_unsupported.go

@@ -0,0 +1,13 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build js,wasm plan9 windows
+
+package agent
+
+import "syscall"
+
+func setsockoptReuseAddrAndPort(network, address string, c syscall.RawConn) error {
+	return nil
+}

go.mod

@@ -9,6 +9,6 @@ require (
 	github.com/shirou/gopsutil v2.20.4+incompatible
 	github.com/stretchr/testify v1.3.0 // indirect
 	github.com/xlab/treeprint v1.0.0
-	golang.org/x/sys v0.0.0-20200515095857-1151b9dac4a9 // indirect
+	golang.org/x/sys v0.0.0-20201207223542-d4d67f95c62d
 	rsc.io/goversion v1.2.0
 )

go.sum

@@ -15,7 +15,7 @@ github.com/stretchr/testify v1.3.0 h1:TivCn/peBQ7UY8ooIcPgZFpTNSz0Q2U6UrFlUfqbe0
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
 github.com/xlab/treeprint v1.0.0 h1:J0TkWtiuYgtdlrkkrDLISYBQ92M+X5m4LrIIMKrbDTs=
 github.com/xlab/treeprint v1.0.0/go.mod h1:IoImgRak9i3zJyuxOKUP1v4UZd1tMoKkq/Cimt1uhCg=
-golang.org/x/sys v0.0.0-20200515095857-1151b9dac4a9 h1:YTzHMGlqJu67/uEo1lBv0n3wBXhXNeUbB1XfN2vmTm0=
-golang.org/x/sys v0.0.0-20200515095857-1151b9dac4a9/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20201207223542-d4d67f95c62d h1:MiWWjyhUzZ+jvhZvloX6ZrUsdEghn8a64Upd8EMHglE=
+golang.org/x/sys v0.0.0-20201207223542-d4d67f95c62d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 rsc.io/goversion v1.2.0 h1:SPn+NLTiAG7w30IRK/DKp1BjvpWabYgxlLp/+kx5J8w=
 rsc.io/goversion v1.2.0/go.mod h1:Eih9y/uIBS3ulggl7KNJ09xGSLcuNaLgmvvqa07sgfo=