Concurrency crash

[duijf]

Operating system: Ubuntu Xenial (on Travis)
Python version: 3.6.8
Black version: 19.3b0
Does also happen on master: Haven't tested but when through the commits since the release. No commits seem to touch the codepath we're seeing here.

black crashes on certain system configurations due to multiprocessing. Here is the traceback we're seeing.

|Traceback (most recent call last):
|  File "/usr/local/lib/XXX/virtualenv/bin/black", line 11, in <module>
|    sys.exit(patched_main())
|  File "/usr/local/lib/XXX/virtualenv/lib/python3.6/site-packages/black.py", line 3754, in patched_main
|    main()
|  File "/usr/local/lib/XXX/virtualenv/lib/python3.6/site-packages/click/core.py", line 764, in __call__
|    return self.main(*args, **kwargs)
|  File "/usr/local/lib/XXX/virtualenv/lib/python3.6/site-packages/click/core.py", line 717, in main
|    rv = self.invoke(ctx)
|  File "/usr/local/lib/XXX/virtualenv/lib/python3.6/site-packages/click/core.py", line 956, in invoke
|    return ctx.invoke(self.callback, **ctx.params)
|  File "/usr/local/lib/XXX/virtualenv/lib/python3.6/site-packages/click/core.py", line 555, in invoke
|    return callback(*args, **kwargs)
|  File "/usr/local/lib/XXX/virtualenv/lib/python3.6/site-packages/click/decorators.py", line 17, in new_func
|    return f(get_current_context(), *args, **kwargs)
|  File "/usr/local/lib/XXX/virtualenv/lib/python3.6/site-packages/black.py", line 435, in main
|    executor = ProcessPoolExecutor(max_workers=os.cpu_count())
|  File "/usr/lib/python3.6/concurrent/futures/process.py", line 402, in __init__
|    EXTRA_QUEUED_CALLS)
|  File "/usr/lib/python3.6/multiprocessing/context.py", line 102, in Queue
|    return Queue(maxsize, ctx=self.get_context())
|  File "/usr/lib/python3.6/multiprocessing/queues.py", line 42, in __init__
|    self._rlock = ctx.Lock()
|  File "/usr/lib/python3.6/multiprocessing/context.py", line 67, in Lock
|    return Lock(ctx=self.get_context())
|  File "/usr/lib/python3.6/multiprocessing/synchronize.py", line 162, in __init__
|    SemLock.__init__(self, SEMAPHORE, 1, 1, ctx=ctx)
|  File "/usr/lib/python3.6/multiprocessing/synchronize.py", line 59, in __init__
|    unlink_now)
|OSError: [Errno 30] Read-only file system

This only happens when running black on more than 1 file at a time. There should probably be an option (or codepath which tries to correct this issue) for serial execution.

[ichard26]

This should be closed as PR #1141 has been merged. I think the PR author forgot that the GitHub keyword has be behind every issue you want to auto-close:

should fix #1138 and #776