Set start method to forkserver for MacOS (#3919)

* Set start method to forkserver for MacOS

* Polished up changes

* Added release note

RELEASE-NOTES.md

@@ -21,9 +21,10 @@
 - `pm.sample` now takes 1000 draws and 1000 tuning samples by default, instead of 500 previously (see [#3855](https://github.com/pymc-devs/pymc3/pull/3855)).
 - Moved argument division out of `NegativeBinomial` `random` method. Fixes [#3864](https://github.com/pymc-devs/pymc3/issues/3864) in the style of [#3509](https://github.com/pymc-devs/pymc3/pull/3509).
 - The Dirichlet distribution now raises a ValueError when it's initialized with <= 0 values (see [#3853](https://github.com/pymc-devs/pymc3/pull/3853)).
-- Dtype bugfix in `MvNormal` and `MvStudentT` (see [3836](https://github.com/pymc-devs/pymc3/pull/3836))
+- Dtype bugfix in `MvNormal` and `MvStudentT` (see [3836](https://github.com/pymc-devs/pymc3/pull/3836)).
 - End of sampling report now uses `arviz.InferenceData` internally and avoids storing
-  pointwise log likelihood (see [#3883](https://github.com/pymc-devs/pymc3/pull/3883))
+  pointwise log likelihood (see [#3883](https://github.com/pymc-devs/pymc3/pull/3883)).
+- The multiprocessing start method on MacOS is now set to "forkserver", to avoid crashes (see issue [#3849](https://github.com/pymc-devs/pymc3/issues/3849), solved by [#3919](https://github.com/pymc-devs/pymc3/pull/3919)).
 
 ### Deprecations
 - Remove `sample_ppc` and `sample_ppc_w` that were deprecated in 3.6.

pymc3/__init__.py

@@ -16,6 +16,8 @@
 __version__ = "3.8"
 
 import logging
+import multiprocessing as mp
+import platform
 
 _log = logging.getLogger("pymc3")
 
@@ -25,10 +27,15 @@ if not logging.root.handlers:
         handler = logging.StreamHandler()
         _log.addHandler(handler)
 
+# Set start method to forkserver for MacOS to enable multiprocessing
+# Closes issue https://github.com/pymc-devs/pymc3/issues/3849
+sys = platform.system()
+if sys == "Darwin":
+    new_context = mp.get_context("forkserver")
+
 
 def __set_compiler_flags():
     # Workarounds for Theano compiler problems on various platforms
-    import platform
     import theano
 
     system = platform.system()