macOS: analytics daemon crashes due to fork related error

[pmrowla]

After the daemon changes introduced in #4262 (f799e1e), DVC on macOS crashes when we try to spawn the analytics daemon.

dvc git:f799e1e3 ✩  py:dvc ❯ dvc version
WARNING: Unable to detect supported link types, as cache directory '.dvc/cache' doesn't exist. It is usually auto-created by commands such as `dvc add/fetch/pull/run/import`, but you could create it manually to enable this check.
DVC version: 1.1.11+f799e1
Python version: 3.8.5
Platform: macOS-10.15.6-x86_64-i386-64bit
Binary: False
Package: None
Supported remotes: azure, gdrive, gs, hdfs, http, https, s3, ssh, oss
Repo: dvc, git
Filesystem type (workspace): ('apfs', '/dev/disk1s1')
objc[34576]: +[__NSCFConstantString initialize] may have been in progress in another thread when fork() was called.
objc[34576]: +[__NSCFConstantString initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.

Disabling analytics avoids the crash

dvc git:f799e1e3 ✩  py:dvc ❯ dvc config --local core.analytics false
dvc git:f799e1e3 ✩  py:dvc ❯ dvc version
WARNING: Unable to detect supported link types, as cache directory '.dvc/cache' doesn't exist. It is usually auto-created by commands such as `dvc add/fetch/pull/run/import`, but you could create it manually to enable this check.
DVC version: 1.1.11+f799e1
Python version: 3.8.5
Platform: macOS-10.15.6-x86_64-i386-64bit
Binary: False
Package: None
Supported remotes: azure, gdrive, gs, hdfs, http, https, s3, ssh, oss
Repo: dvc, git
Filesystem type (workspace): ('apfs', '/dev/disk1s1')

This problem appears to be related to fork being unsafe in macOS

See related Python issue: https://bugs.python.org/issue33725 and Python 3.8 multiprocessing docs

[pmrowla]

As mentioned in the python issue, setting the OBJC_DISABLE_INITIALIZE_FORK_SAFETY env variable before running python/dvc can be used as a workaround, but that doesn't seem like an ideal solution for us

[efiop]

@pmrowla Thank you for the investigation! 🙏 Quick solution is to continue using Popen on mac and windows. Will take a look.