Provide a more helpful error message on spawn() failures

Make a good-faith effort to catch the `RuntimeError` thrown when
improperly invoking the YumFinder on non-Linux platforms, and present
a useful error message to the user.

Drive-by: update `tox.ini` to work with 4.x

Fixes: Issue #31

soufi/finders/yum.py

@@ -4,6 +4,7 @@
 import abc
 import lzma
 import pickle  # nosec
+import sys
 import warnings
 from multiprocessing import Process, Queue
 from types import SimpleNamespace
@@ -230,7 +231,20 @@ def do_task(target, *args):
     """Run the target callable in a subprocess and return its response."""
     queue = Queue()
     process = Process(target=target, args=(queue,) + args)
-    process.start()
+    try:
+        process.start()
+    except RuntimeError:
+        sys.exit(
+            """
+FATAL: Running this finder directly from the global scope is not supported on
+       this platform.  To use this finder, call it instead from the main
+       module, e.g.:
+
+           if __name__ == '__main__':
+               soufi.finder.factory(*args, **kwargs).find()
+
+Aborting."""
+        )
     # We don't want to wait *forever*, but jobs can take several minutes to
     # complete, so wait a relatively long time
     response = queue.get(timeout=600)

soufi/tests/finders/test_yum_finder.py

@@ -478,6 +478,16 @@ def test_do_task_reraises_exceptions(self):
         err = self.assertRaises(RuntimeError, yum.do_task, kaboom, data)
         self.assertEqual(data, str(err))
 
+    def test_do_task_handles_spawn_errors_on_silly_platforms(self):
+        # This simulates calling `process.start()` from the global scope on
+        # platforms that do not support such things.  The default traceback
+        # is not intrinsically helpful, so test that we kick back a more
+        # useful error message.  See issue #31.
+        process = self.patch(yum, 'Process')
+        process.return_value.start.side_effect = RuntimeError
+        err = self.assertRaises(SystemExit, yum.do_task, None)
+        self.assertIn('FATAL: ', str(err))
+
 
 # A simple subprocess function that throws a test exception.  Used by
 # TestYumFinderHelpers.test_do_task_reraises_exceptions

tox.ini

@@ -13,7 +13,7 @@ setenv =
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
-whitelist_externals =
+allowlist_externals =
     bash
     echo
     mkdir
@@ -94,10 +94,14 @@ exclude =
     .eggs
 show-source = true
 ignore =
-    D10  # ignore missing docstrings (for now)
-    D202  # No blank lines allowed after function docstring (caused by Black)
-    E203  # Whitespace before ':' (caused by Black)
-    W503  # line breaks after operators (caused by Black)
+    # ignore missing docstrings (for now)
+    D10
+    # No blank lines allowed after function docstring (caused by Black)
+    D202
+    # Whitespace before ':' (caused by Black)
+    E203
+    # line breaks after operators (caused by Black)
+    W503
 
 [testenv:functional]
 commands =