[SR-13307] Default `mode` to `open(2)` is not same between Foundation and corelibs Foundation

[niw]

Previous ID	SR-13307
Radar	None
Original Reporter	@niw
Type	Bug

Environment

macOS 10.15.6
Ubuntu 18.04 (Official swift:latest Docker image)

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: 915184f699d2e0392da90f9a7be55035

Issue Description:

For example, with following code, users expected to create a new file (if it doesn’t exist) with permission that respects current umask.

import Foundation
try! Data().write(to: URL(fileURLWithPath: "file"))

On macOS, In case umask is 022, it creates file with permission 0644 on macOS.
in case umask is 0, it creates file with permission 0666.
Therefore, default mode to open(2) seems having 0666.

However, on Linux, since corelibs Foundation is using default mode 0600 to open(2) (called via _CFOpenFileWithMode()), in case umask is 022, it creates file with permission 0600 instead of 0644.
https://github.com/apple/swift-corelibs-foundation/blame/master/Sources/Foundation/NSData.swift#L438

This behavior's inconsistency surprising Foundation library users.

[niw]

Probably this is a bug introduced by
#1876
The original code is using 0666 for mode to open(2) but it changed to 0600.
cc: @spevans

[niw]

The patch is available at #2851