os: handle the sticky bit separately for *BSD and Solaris

open(2) and mkdir(2) won't set the sticky bit on *BSD and Solaris.
This behavior is mentioned on sticky(8).
see also: moby/moby#6587

Fixes golang#8383.

Change-Id: Ic4733700f9926b9fc2b6fd1f998acec34e518764
Reviewed-on: https://go-review.googlesource.com/1673
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/file.go

@@ -203,6 +203,12 @@ func (f *File) WriteString(s string) (ret int, err error) {
 // If there is an error, it will be of type *PathError.
 func Mkdir(name string, perm FileMode) error {
 	e := syscall.Mkdir(name, syscallMode(perm))
+
+	// mkdir(2) itself won't handle the sticky bit on *BSD and Solaris
+	if !supportsCreateWithStickyBit && e == nil && perm&ModeSticky != 0 {
+		e = Chmod(name, perm)
+	}
+
 	if e != nil {
 		return &PathError{"mkdir", name, e}
 	}

src/os/file_unix.go

@@ -75,11 +75,23 @@ const DevNull = "/dev/null"
 // methods on the returned File can be used for I/O.
 // If there is an error, it will be of type *PathError.
 func OpenFile(name string, flag int, perm FileMode) (file *File, err error) {
+	chmod := false
+	if !supportsCreateWithStickyBit && flag&O_CREATE != 0 && perm&ModeSticky != 0 {
+		if _, err := Stat(name); IsNotExist(err) {
+			chmod = true
+		}
+	}
+
 	r, e := syscall.Open(name, flag|syscall.O_CLOEXEC, syscallMode(perm))
 	if e != nil {
 		return nil, &PathError{"open", name, e}
 	}
 
+	// open(2) itself won't handle the sticky bit on *BSD and Solaris
+	if chmod && e == nil {
+		e = Chmod(name, perm)
+	}
+
 	// There's a race here with fork/exec, which we are
 	// content to live with.  See ../syscall/exec_unix.go.
 	if !supportsCloseOnExec {

src/os/sticky_bsd.go

@@ -0,0 +1,11 @@
+// Copyright 2014 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 darwin dragonfly freebsd netbsd openbsd solaris
+
+package os
+
+// According to sticky(8), neither open(2) nor mkdir(2) will create
+// a file with the sticky bit set.
+const supportsCreateWithStickyBit = false

src/os/sticky_notbsd.go

@@ -0,0 +1,14 @@
+// Copyright 2014 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 !darwin
+// +build !dragonfly
+// +build !freebsd
+// +build !netbsd
+// +build !openbsd
+// +build !solaris
+
+package os
+
+const supportsCreateWithStickyBit = true