optimize std.fs.Dir.makeOpenPath for the case that the path already exists

[andrewrk]

Current implementation:

zig/lib/std/fs.zig

Lines 1406 to 1413 in 79757f2

	/// This function performs `makePath`, followed by `openDir`.
	/// If supported by the OS, this operation is atomic. It is not atomic on
	/// all operating systems.
	pub fn makeOpenPath(self: Dir, sub_path: []const u8, open_dir_options: OpenDirOptions) !Dir {
	// TODO improve this implementation on Windows; we can avoid 1 call to NtClose
	try self.makePath(sub_path);
	return self.openDir(sub_path, open_dir_options);
	}

Typically, something like this will happen:

mkdirat(AT_FDCWD, "/home/andy/.cache/zig", 0755) = -1 EEXIST (File exists)
openat(AT_FDCWD, "/home/andy/.cache/zig", O_RDONLY|O_CLOEXEC|O_PATH|O_DIRECTORY) = 5

We can optimize for the common case by first attempting the openat() and then falling back to mkdirat() if we get ENOENT. This will make the common case one syscall instead of two.

As a bonus, consider solving the TODO above as well regarding Windows. NtCreateFile is capable of creating and opening a directory with a single call.

[Jarred-Sumner]

nice