[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2535: Magic Folder: permissions of downloaded files are not set according to the user's umask

[Tahoe-LAFS]

#2535: Magic Folder: permissions of downloaded files are not set according to the
user's umask
-------------------------------------+-------------------------------------
     Reporter:  dawuud               |      Owner:  daira
         Type:  defect               |     Status:  closed
     Priority:  normal               |  Milestone:  undecided
    Component:  code-frontend-       |    Version:  1.10.1
  magic-folder                       |   Keywords:  magic-folder
   Resolution:  fixed                |  permissions security usability unix
Launchpad Bug:                       |  docs-needed
-------------------------------------+-------------------------------------

Comment (by daira):

 The current approach is undesirable because the `os.umask` in the finally
 block could fail, leaving an unsafe umask for the whole node process.

 Using `os.open` with a mode argument wouldn't work because the umask
 specified by twistd (0077) would still be masked off, both when creating
 the file and when creating any ancestor directories that do not already
 exist. It appears that the only safe way for a Unix process to atomically
 set an arbitrary umask when creating a file, without setting its own
 process-wide umask, is to create a subprocess that will have the desired
 umask.

 Here, we don't need atomicity for setting the umask of the replacement
 file, so we could use `chmod` to relax the permissions from the ones that
 result from the twistd umask. However we would also need to chmod any
 created directories. Reopening as a reminder to change it to use this
 method.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2535#comment:20>
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