BaseLogFile should not call os.umask

	@mithrandi reported
Trac ID	trac#9215
Type	defect
Created	2017-07-08 06:11:24Z

BaseLogFile sets and restores umask here: https://github.com/twisted/twisted/blob/3d11cba9fe1f50fc55cb5622d4f47dfcc9c9f204/src/twisted/python/logfile.py#L79-L83

Unfortunately, umask is process-global state, which means that any other threads doing file-related operations at the same time are affected, which is bad. We should instead rely on something like #9214 to set the mode directly (the underlying open(2) libc call supports this).

The way I ran into this: in our application, when certain types of external request are made, we dump the request and response contents to a file on disk, in a directory named after the date. We also have Eliot logs going to a DailyLogFile, which are written out in a separate thread for performance reasons. This means that for the first operation of every new day, there is a race between DailyLogFile setting the umask to 0 briefly as the first log message triggers log rotation, and our dumping code creating the new day's dumps directory, resulting in this directory randomly having 0o0000 permissions occasionally.

trac-id__9215 9215
type__defect defect
reporter__mithrandi mithrandi
priority__normal normal
milestone__None None
branch__ 
branch_author__ 
status__new new
resolution__None None
component__core core
keywords__None None
time__1499494284859101 1499494284859101
changetime__1499494284859101 1499494284859101
version__None None
owner__None None

Start in src/twisted/python/logfile.py at BaseLogFile and inspect the umask handling linked in the issue. Review issue #9214 for the proposed direct mode-setting approach; done means BaseLogFile no longer changes process-global umask while creating log files and the existing behavior remains covered.