Allow Foundation to create file with permissions respects process umask

**Problem**

`Data.write(to:)` is a only method in the Foundation that can create a
regular file.
However, it ignores `uamask` and always set 0600 permission unlike
macOS Foundation, which respects process `umask`.

**Solution**

1. With `.atomic` write option

It uses `mkstemp(3)` in `_NSCreateTemporaryFile`, which is always
creating a file with 0600 permission, if the system follows
the latest POSIX specification or the permission is undefined.

On macOS Foundation, therefore `_NSCreateTemporaryFile` uses
`mktemp(3)` and `open(2)` instead to respect `umask`.

2. Without `.atomic` write option

It uses `0o600` even if it uses `open(2)` that respects `umask`.
Simply gives `0o666` instead.

This is a bug caused by previous commit in
swiftlang#1876.

JIRA: https://bugs.swift.org/browse/SR-13307

Author: niw

Sources/Foundation/NSData.swift

@@ -434,10 +434,10 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
         }
 
         let fm = FileManager.default
-        // The destination file path may not exist so provide a default file permissions of RW user only
-        let permissions = (try? fm._permissionsOfItem(atPath: path)) ?? 0o600
+        let permissions = try? fm._permissionsOfItem(atPath: path)
 
         if writeOptionsMask.contains(.atomic) {
+            // Okay... how to use umask to create tempfile?
             let (newFD, auxFilePath) = try _NSCreateTemporaryFile(path)
             let fh = FileHandle(fileDescriptor: newFD, closeOnDealloc: true)
             do {
@@ -446,7 +446,9 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
                 // requires that there be no open handles to the file
                 fh.closeFile()
                 try _NSCleanupTemporaryFile(auxFilePath, path)
-                try fm.setAttributes([.posixPermissions: NSNumber(value: permissions)], ofItemAtPath: path)
+                if let permissions = permissions {
+                    try fm.setAttributes([.posixPermissions: NSNumber(value: permissions)], ofItemAtPath: path)
+                }
             } catch {
                 let savedErrno = errno
                 try? fm.removeItem(atPath: auxFilePath)
@@ -457,11 +459,15 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
             if writeOptionsMask.contains(.withoutOverwriting) {
                 flags |= O_EXCL
             }
+            let createMode = Int(S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)
 
-            guard let fh = FileHandle(path: path, flags: flags, createMode: permissions) else {
+            guard let fh = FileHandle(path: path, flags: flags, createMode: createMode) else {
                 throw _NSErrorWithErrno(errno, reading: false, path: path)
             }
             try doWrite(fh)
+            if let permissions = permissions {
+                try fm.setAttributes([.posixPermissions: NSNumber(value: permissions)], ofItemAtPath: path)
+            }
         }
     }
 

Sources/Foundation/NSPathUtilities.swift

@@ -760,10 +760,15 @@ internal func _NSCreateTemporaryFile(_ filePath: String) throws -> (Int32, Strin
     let maxLength = Int(PATH_MAX) + 1
     var buf = [Int8](repeating: 0, count: maxLength)
     let _ = template._nsObject.getFileSystemRepresentation(&buf, maxLength: maxLength)
-    let fd = mkstemp(&buf)
-    if fd == -1 {
+
+    guard let name = mktemp(&buf) else {
+        throw _NSErrorWithErrno(errno, reading: false, path: filePath)
+    }
+    let fd = open(name, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)
+    if fd != 1 {
         throw _NSErrorWithErrno(errno, reading: false, path: filePath)
     }
+
     let pathResult = FileManager.default.string(withFileSystemRepresentation: buf, length: Int(strlen(buf)))
 #endif
     return (fd, pathResult)