Merge pull request #4 from dwave-examples/fix/dash-process-forking

Use `spawn` instead of `fork` method for Dash callbacks

app.py

@@ -46,6 +46,17 @@
 cache = diskcache.Cache("./cache")
 background_callback_manager = DiskcacheManager(cache)
 
+# Fix Dash long callbacks crashing on macOS 10.13+ (also potentially not working
+# on other POSIX systems), caused by https://bugs.python.org/issue33725
+# (aka "beware of multithreaded process forking").
+#
+# Note: default start method has already been changed to "spawn" on darwin in
+# the `multiprocessing` library, but its fork, `multiprocess` still hasn't caught up.
+# (see docs: https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods)
+import multiprocess
+if multiprocess.get_start_method(allow_none=True) is None:
+    multiprocess.set_start_method('spawn')
+
 from app_configs import (
     APP_TITLE,
     CLASSICAL_TAB_LABEL,