os: Mkdir and OpenFile permission inconsistency on BSD when using sticky bit

[matthijskooijman]

From review of the code, I found an issue with Mkdir and OpenFile on the BSD platforms. Both functions contain an exception on these platforms when the sticky bit is set in the permissions passed:

go/src/os/file.go

Lines 220 to 223 in 8776be1

	// mkdir(2) itself won't handle the sticky bit on *BSD and Solaris
	if !supportsCreateWithStickyBit && perm&ModeSticky != 0 {
	Chmod(name, perm)
	}

go/src/os/file_unix.go

Lines 159 to 163 in 8776be1

	if !supportsCreateWithStickyBit && flag&O_CREATE != 0 && perm&ModeSticky != 0 {
	if _, err := Stat(name); IsNotExist(err) {
	chmod = true
	}
	}

Normally, the permissions of the created file or directory are influenced using the umask.
But these exceptions mean that, on BSD platforms, an explicit Chmod happens using the passed permissions, completely bypassing umask. This is at least inconsistent and surprising, and could be a security problem in some cases.

[gopherbot]

Change https://golang.org/cl/84477 mentions this issue: os: respect umask in Mkdir and OpenFile on BSD systems when perm has ModeSticky set