unix: add Dup3 on FreeBSD

Other BSDs provide dup3(2) syscall, on FreeBSD it is implemented as libc
function using fcntl(2). This CL adds similar Go implementation.

Fixes golang/go#55935

Change-Id: I9c6d762415c7bed5442966a7fcbf9a6f8dfdaf2a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/470675
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

unix/dup3_test.go

@@ -0,0 +1,65 @@
+// Copyright 2023 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.
+
+//go:build freebsd || linux || netbsd || openbsd
+// +build freebsd linux netbsd openbsd
+
+package unix_test
+
+import (
+	"os"
+	"runtime"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestDup3(t *testing.T) {
+	tempFile, err := os.CreateTemp("", "TestDup")
+	if err != nil {
+		t.Fatalf("CreateTemp failed: %v", err)
+	}
+	defer os.Remove(tempFile.Name())
+	defer tempFile.Close()
+	oldFd := int(tempFile.Fd())
+
+	// On NetBSD, it is not an error if oldFd == newFd
+	if runtime.GOOS != "netbsd" {
+		if got, want := unix.Dup3(oldFd, oldFd, 0), unix.EINVAL; got != want {
+			t.Fatalf("Dup3: expected err %v, got %v", want, got)
+		}
+	}
+
+	// Create and reserve a file descriptor.
+	// Dup3 automatically closes it before reusing it.
+	nullFile, err := os.Open("/dev/null")
+	if err != nil {
+		t.Fatalf("Open failed: %v", err)
+	}
+	defer nullFile.Close()
+	newFd := int(nullFile.Fd())
+
+	err = unix.Dup3(oldFd, newFd, 0)
+	if err != nil {
+		t.Fatalf("Dup3: %v", err)
+	}
+
+	b1 := []byte("Test123")
+	b2 := make([]byte, 7)
+	_, err = unix.Write(newFd, b1)
+	if err != nil {
+		t.Fatalf("Write to Dup3 fd failed: %v", err)
+	}
+	_, err = unix.Seek(oldFd, 0, 0)
+	if err != nil {
+		t.Fatalf("Seek failed: %v", err)
+	}
+	_, err = unix.Read(oldFd, b2)
+	if err != nil {
+		t.Fatalf("Read back failed: %v", err)
+	}
+	if string(b1) != string(b2) {
+		t.Errorf("Dup3: read %q from file, want %q", string(b2), string(b1))
+	}
+}

unix/syscall_freebsd.go

@@ -325,6 +325,18 @@ func PtraceSingleStep(pid int) (err error) {
 	return ptrace(PT_STEP, pid, 1, 0)
 }
 
+func Dup3(oldfd, newfd, flags int) error {
+	if oldfd == newfd || flags&^O_CLOEXEC != 0 {
+		return EINVAL
+	}
+	how := F_DUP2FD
+	if flags&O_CLOEXEC != 0 {
+		how = F_DUP2FD_CLOEXEC
+	}
+	_, err := fcntl(oldfd, how, newfd)
+	return err
+}
+
 /*
  * Exposed directly
  */