server: fix fd leak in DialTCP() on Linux

A new socket was created so we must close it before this function
returns _either_ on failure or success. On success, net.FileConn() in
newTCPConn() increases the refcount of the socket so this fi.Close()
doesn't destroy the socket; the caller can use it (and needs to close
it later).

Signed-off-by: FUJITA Tomonori <[email protected]>

server/sockopt_linux.go

@@ -177,6 +177,15 @@ func (d *TCPDialer) DialTCP(addr string, port int) (*net.TCPConn, error) {
 	if err != nil {
 		return nil, err
 	}
+	fi := os.NewFile(uintptr(fd), "")
+	defer fi.Close()
+	// A new socket was created so we must close it before this
+	// function returns either on failure or success. On success,
+	// net.FileConn() in newTCPConn() increases the refcount of
+	// the socket so this fi.Close() doesn't destroy the socket.
+	// The caller must call Close() with the file later.
+	// Note that the above os.NewFile() doesn't play with the
+	// refcount.
 
 	if err = syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_BROADCAST, 1); err != nil {
 		return nil, os.NewSyscallError("setsockopt", err)
@@ -208,10 +217,7 @@ func (d *TCPDialer) DialTCP(addr string, port int) (*net.TCPConn, error) {
 		return nil, os.NewSyscallError("bind", err)
 	}
 
-	newTCPConn := func(fd int) (*net.TCPConn, error) {
-		fi := os.NewFile(uintptr(fd), "")
-		defer fi.Close()
-
+	newTCPConn := func(fi *os.File) (*net.TCPConn, error) {
 		if conn, err := net.FileConn(fi); err != nil {
 			return nil, err
 		} else {
@@ -224,7 +230,7 @@ func (d *TCPDialer) DialTCP(addr string, port int) (*net.TCPConn, error) {
 	case syscall.EINPROGRESS, syscall.EALREADY, syscall.EINTR:
 		// do timeout handling
 	case nil, syscall.EISCONN:
-		return newTCPConn(fd)
+		return newTCPConn(fi)
 	default:
 		return nil, os.NewSyscallError("connect", err)
 	}
@@ -259,7 +265,7 @@ func (d *TCPDialer) DialTCP(addr string, port int) (*net.TCPConn, error) {
 			switch err := syscall.Errno(nerr); err {
 			case syscall.EINPROGRESS, syscall.EALREADY, syscall.EINTR:
 			case syscall.Errno(0), syscall.EISCONN:
-				return newTCPConn(fd)
+				return newTCPConn(fi)
 			default:
 				return nil, os.NewSyscallError("getsockopt", err)
 			}

server/sockopt_linux_test.go

@@ -18,8 +18,12 @@ package server
 
 import (
 	"bytes"
+	"fmt"
+	"net"
+	"os"
 	"syscall"
 	"testing"
+	"time"
 	"unsafe"
 )
 
@@ -64,3 +68,39 @@ func Test_buildTcpMD5Sigv6(t *testing.T) {
 		t.Error("Something wrong v6")
 	}
 }
+
+func Test_DialTCP_FDleak(t *testing.T) {
+	openFds := func() int {
+		pid := os.Getpid()
+		f, err := os.OpenFile(fmt.Sprintf("/proc/%d/fdinfo", pid), os.O_RDONLY, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer f.Close()
+		names, err := f.Readdirnames(0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return len(names)
+	}
+
+	before := openFds()
+
+	for i := 0; i < 10; i++ {
+		laddr, _ := net.ResolveTCPAddr("tcp", net.JoinHostPort("127.0.0.1", "0"))
+		d := TCPDialer{
+			Dialer: net.Dialer{
+				LocalAddr: laddr,
+				Timeout:   1 * time.Second,
+			},
+		}
+		if _, err := d.DialTCP("127.0.0.1", 1); err == nil {
+			t.Fatalf("should not succeed")
+		}
+
+	}
+
+	if after := openFds(); before != after {
+		t.Fatalf("could be fd leak, %d %d", before, after)
+	}
+}