Re: "mode" and varios processes sharing the same queue for writting [SOLVED]

["Iñaki Baz Castillo" <ibc@aliax.net>]

El Viernes, 8 de Enero de 2010, Iñaki Baz Castillo escribió:
> Hi, I'll have an external process (running as root) which creates a
>  posix_mq queue just for read-only. As it runs as root I can set a longer
>  "maxmsg" value.
> 
> Then I've other processes (Unicor workers) running as non privileged users
>  and must write in that queue.
> 
> So in the external process I do:
> 
>   File.umask(0000)
>   MQ = POSIX_MQ.new "/mq", IO::RDONLY | IO::CREAT, 00002
> 
> "mode" = 00002 which means "others have write permission" (in combination
>  with the modified umask, of course, if not it doesn't work).
> 
> Is it the correct approach?

The already working solution:

----------------------------------
group_name   = ARGV[0]

# Change the effective group (just if we are root).
if Process.euid == 0 and group_name
  gid = Etc.getgrnam(group_name).gid
  puts "setting effective group to '#{group_name}' (gid #{gid})..."
  Process::GID.change_privilege(gid)
end

# Change umask to 0047 so the group has write permissions for the posix_mq.
File.umask(0047)  # => When creating a file permissions are:  -rw--w----

MQ = POSIX_MQ.new "/my_mq", IO::RDONLY | IO::CREAT, 00620
--------------------------------

In this way a user whose primary group is "group_name" will have write access 
(but no read access) to the mqueue. :)



> And the other question: Being posix_mq process/thread safe, does it mean
>  that I could open the queue writter in the master process so it could be
>  safely shared by all the workers? or shall I open the queue writter for
>  each worker?

After some testing I would say that it's totally safe (it works for me by 
creating the mqueue writter in the master process).



Best regards.


-- 
Iñaki Baz Castillo <ibc@aliax.net>