Magic Folder: permissions of downloaded files are not set according to the user's umask

[dawuud]

During the demo with Zooko, Daira, Warner and Meejah last night we found yet another bug: the magic-folder downloader writes files with the original uploader's permissions intact as if the umask were 0077, but it should probably use the downloader's user's umask instead. If we do it like that then the user downloading files will have consistent permissions for all downloaded files.

[daira]

I don't think it uses the original uploader's permissions. It uses fileutil.write (​here), which uses whatever the default is for open(path, "wb") (​here). But it does this as the daemonised node process, which might not have the same umask as the user's shell (ISTR that twistd changes the umask).

[daira]

twistd sets umask to 0077 by default, unless overridden by ​--umask.

The correct approach here seems to be to save the original umask for later use by _write_downloaded_file, before passing control to twistd in tahoe start/run. (We don't want to change the umask of the node from 0077 using --umask because that would have difficult-to-review security consequences.)

[daira]

This bug is Unix-specific.