os: ModeSetgid has no effect while using with Mkdir() on Linux

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.9.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64" GOBIN="" GOEXE="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOOS="linux" GOPATH="/root/go" GORACE="" GOROOT="/usr/lib/golang" GOTOOLDIR="/usr/lib/golang/pkg/tool/linux_amd64" GCCGO="gccgo" CC="gcc" GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build203196509=/tmp/go-build -gno-record-gcc-switches" CXX="g++" CGO_ENABLED="1" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config"

What did you do?

package main

import (
	"os"
)

func main() {
	os.Mkdir("test", 0770 | os.ModeSetgid)
}

What did you expect to see?

# ls -la test
insgesamt 4
drwxr-s---.  2 root root    6 24. Mai 04:09 .
dr-xr-x---. 13 root root 4096 24. Mai 04:09 ..
#

What did you see instead?

# ls -la test
insgesamt 4
drwxr-x---.  2 root root    6 24. Mai 04:09 .
dr-xr-x---. 13 root root 4096 24. Mai 04:09 ..
#

Why did this happen?

According to strace -f ./mkdir the Go stdlib behaves as expected...

[pid  6782] mkdirat(AT_FDCWD, "test", 02770 <unfinished ...>
[pid  6782] <... mkdirat resumed> )     = 0

... but on Linux this is not enough, see mkdirat(2):

The mkdirat() system call operates in exactly the same way as mkdir(2), (...)

... and mkdir(2):

The  argument  mode  specifies the permissions to use.
It is modified (...): the permissions of the created directory are (mode & ~umask & 0777).
Other mode bits of the created directory depend on the operating system.
For Linux, see below.
(...)
That is, under Linux the created directory actually gets mode (mode & ~umask & 01777)

How could this be fixed?

Similar to #8383, via Chmod.