syscall: add a test for passing an fd over a unix socket

Re-submitting previously reverted change 160ec5506cb7.

R=golang-dev, r, r
CC=golang-dev
https://golang.org/cl/6129052

src/pkg/syscall/passfd_test.go

+// Copyright 2012 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 linux darwin
+
+package syscall_test
+
+import (
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"net"
+	"os"
+	"os/exec"
+	"syscall"
+	"testing"
+	"time"
+)
+
+// TestPassFD tests passing a file descriptor over a Unix socket.
+//
+// This test involved both a parent and child process. The parent
+// process is invoked as a normal test, with "go test", which then
+// runs the child process by running the current test binary with args
+// "-test.run=^TestPassFD$" and an environment variable used to signal
+// that the test should become the child process instead.
+func TestPassFD(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		passFDChild()
+		return
+	}
+
+	tempDir, err := ioutil.TempDir("", "TestPassFD")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	fds, err := syscall.Socketpair(syscall.AF_LOCAL, syscall.SOCK_STREAM, 0)
+	if err != nil {
+		t.Fatalf("Socketpair: %v", err)
+	}
+	defer syscall.Close(fds[0])
+	defer syscall.Close(fds[1])
+	writeFile := os.NewFile(uintptr(fds[0]), "child-writes")
+	readFile := os.NewFile(uintptr(fds[1]), "parent-reads")
+	defer writeFile.Close()
+	defer readFile.Close()
+
+	cmd := exec.Command(os.Args[0], "-test.run=^TestPassFD$", "--", tempDir)
+	cmd.Env = append([]string{"GO_WANT_HELPER_PROCESS=1"}, os.Environ()...)
+	cmd.ExtraFiles = []*os.File{writeFile}
+
+	out, err := cmd.CombinedOutput()
+	if len(out) > 0 || err != nil {
+		t.Fatalf("child process: %q, %v", out, err)
+	}
+
+	c, err := net.FileConn(readFile)
+	if err != nil {
+		t.Fatalf("FileConn: %v", err)
+	}
+	defer c.Close()
+
+	uc, ok := c.(*net.UnixConn)
+	if !ok {
+		t.Fatalf("unexpected FileConn type; expected UnixConn, got %T", c)
+	}
+
+	buf := make([]byte, 32) // expect 1 byte
+	oob := make([]byte, 32) // expect 24 bytes
+	closeUnix := time.AfterFunc(5*time.Second, func() {
+		t.Logf("timeout reading from unix socket")
+		uc.Close()
+	})
+	_, oobn, _, _, err := uc.ReadMsgUnix(buf, oob)
+	closeUnix.Stop()
+
+	scms, err := syscall.ParseSocketControlMessage(oob[:oobn])
+	if err != nil {
+		t.Fatalf("ParseSocketControlMessage: %v", err)
+	}
+	if len(scms) != 1 {
+		t.Fatalf("expected 1 SocketControlMessage; got scms = %#v", scms)
+	}
+	scm := scms[0]
+	gotFds, err := syscall.ParseUnixRights(&scm)
+	if err != nil {
+		t.Fatalf("syscall.ParseUnixRights: %v", err)
+	}
+	if len(gotFds) != 1 {
+		t.Fatalf("wanted 1 fd; got %#v", gotFds)
+	}
+
+	f := os.NewFile(uintptr(gotFds[0]), "fd-from-child")
+	defer f.Close()
+
+	got, err := ioutil.ReadAll(f)
+	want := "Hello from child process!\n"
+	if string(got) != want {
+		t.Errorf("child process ReadAll: %q, %v; want %q", got, err, want)
+	}
+}
+
+// passFDChild is the child process used by TestPassFD.
+func passFDChild() {
+	defer os.Exit(0)
+
+	// Look for our fd. It should be fd 3, but we work around an fd leak
+	// bug here (http://golang.org/issue/2603) to let it be elsewhere.
+	var uc *net.UnixConn
+	for fd := uintptr(3); fd <= 10; fd++ {
+		f := os.NewFile(fd, "unix-conn")
+		var ok bool
+		netc, _ := net.FileConn(f)
+		uc, ok = netc.(*net.UnixConn)
+		if ok {
+			break
+		}
+	}
+	if uc == nil {
+		fmt.Println("failed to find unix fd")
+		return
+	}
+
+	// Make a file f to send to our parent process on uc.
+	// We make it in tempDir, which our parent will clean up.
+	flag.Parse()
+	tempDir := flag.Arg(0)
+	f, err := ioutil.TempFile(tempDir, "")
+	if err != nil {
+		fmt.Printf("TempFile: %v", err)
+		return
+	}
+
+	f.Write([]byte("Hello from child process!\n"))
+	f.Seek(0, 0)
+
+	rights := syscall.UnixRights(int(f.Fd()))
+	dummyByte := []byte("x")
+	n, oobn, err := uc.WriteMsgUnix(dummyByte, rights, nil)
+	if err != nil {
+		fmt.Printf("WriteMsgUnix: %v", err)
+		return
+	}
+	if n != 1 || oobn != len(rights) {
+		fmt.Printf("WriteMsgUnix = %d, %d; want 1, %d", n, oobn, len(rights))
+		return
+	}
+}