Semaphores don't get created on Alpine Linux

[Mark Heppner]

Using Alpine Linux 3.6, semaphores don't seem to be getting created in /dev/shm:

# ls -l /dev/shm
total 8
-rw-------    1 root     root      20971520 Oct 18 15:34 asgi-groups
-rw-------    1 root     root     104857600 Oct 18 15:34 asgi-messages

Using the same versions listed below on Debian Jessie, the semaphores and shared memory appear in /dev/shm:

# ls -l /dev/shm
total 12
-rw------- 1 root root  20971520 Oct 18 15:38 asgi-groups
-rw------- 1 root root 104857600 Oct 18 15:38 asgi-messages
-rw------- 1 root root        32 Oct 18 15:38 sem.asgi-groups
-rw------- 1 root root        32 Oct 18 15:38 sem.asgi-messages

I'm not sure if it's related to the missing semaphore, but the generated value ends up being None. In Debian, I can at least see the value as {}.

Once daphne or a second worker are started, then the value can't be unpickled and results in the stack trace below.

---

On Alpine:

Python 3.6.1 (default, May  2 2017, 15:16:41)
[GCC 6.3.0] on linux

On Debian:

Python 3.6.3 (default, Oct 10 2017, 02:29:16)
[GCC 4.9.2] on linux

Both:

asgi-ipc==1.4.1
asgiref==1.1.
channels==1.1.8
daphne==1.3.0
Django==1.11.6
posix-ipc==1.0.0

From Alpine:

  File "./manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "/usr/lib/python3.6/site-packages/django/core/management/__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/usr/lib/python3.6/site-packages/channels/management/commands/runworker.py", line 83, in handle
    worker.run()
  File "/usr/lib/python3.6/site-packages/channels/worker.py", line 87, in run
    channel, content = self.channel_layer.receive_many(channels, block=True)
  File "/usr/lib/python3.6/site-packages/asgiref/base_layer.py", line 43, in receive_many
    return self.receive(channels, block)
  File "/usr/lib/python3.6/site-packages/asgi_ipc/core.py", line 83, in receive
    message = self.message_store.pop(channel)
  File "/usr/lib/python3.6/site-packages/asgi_ipc/store.py", line 123, in pop
    with self.mutate_value() as value:
  File "/usr/lib/python3.6/contextlib.py", line 82, in __enter__
    return next(self.gen)
  File "/usr/lib/python3.6/site-packages/asgi_ipc/store.py", line 57, in mutate_value
    value = pickle.load(self.mmap)
_pickle.UnpicklingError: invalid load key, '\xff'

[Mark Heppner]

Found the problem. In Alpine, semaphores don't get prefixed with sem., so the path is being overwritten in the constructor when it creates the shared memory. Maybe this is a difference between musl and glibc?

Can something maybe be added to further distinguish the two? Even though it's redundant on glibc-based Linux version, maybe just append '-semaphore' to the end of the path?

[Andrew Godwin]

Shared memory stuff is so underspecified I'm sure it's just a difference. Fix shouldn't be too hard, but it might have to wait a bit until I get to rewriting the channel layers to be async.