Foundation: dynamically allocate temporary path

Query the desired buffer size and allocate an appropriate buffer for the
temporary path query.  This should prevent a truncated path from being
given.

Author: compnerd

Foundation/NSPathUtilities.swift

@@ -11,11 +11,14 @@ import CoreFoundation
 
 public func NSTemporaryDirectory() -> String {
 #if os(macOS) || os(iOS)
-    var buf = [Int8](repeating: 0, count: 100)
-    let r = confstr(_CS_DARWIN_USER_TEMP_DIR, &buf, buf.count)
-    if r != 0 && r < buf.count {
-        return String(cString: buf, encoding: .utf8)!
-    }
+    var length: Int = confstr(_CS_DARWIN_USER_TEMP_DIR, nil, 0)
+    guard length > 0 else { return "/tmp" }
+
+    var buffer: [Int8] = Array<Int8>(repeating: 0, count: length)
+    length = confstr(_CS_DARWIN_USER_TEMP_DIR, &buffer, buffer.count)
+    precondition(length > 0 && length < buffer.count,
+                 "_CS_DARWIN_USER_TEMP_DIR mutation race")
+    return String(cString: buffer, encoding: .utf8)!
 #elseif os(Windows)
     var cchLength: DWORD = GetTempPathW(0, nil)
     var wszPath: UnsafeMutableBufferPointer = UnsafeMutableBufferPointer<WCHAR>.allocate(capacity: Int(cchLength + 1))