Fix failing tests of hid.write on MacOS by pulling target functions t…

…o top of module (#1344)

Resolves #1343.

On MacOS, the tests of `hid.write.ProcessWithResult` fail because the
multiprocessing module tries to pickle the inlined target functions so
it can then spawn child processes. To be picklable, functions need to be
defined with `def` in the top level of a
module[[1](https://docs.python.org/3/library/pickle.html#what-can-be-pickled-and-unpickled)].
On MacOS, multiprocessing uses spawn instead of
fork[[2](python/cpython@17a5588)],
so this is a problem that does not show up on
Linux[[3](https://docs.python.org/3.11/library/multiprocessing.html#the-spawn-and-forkserver-start-methods)].

After moving the definitions of these functions to the top level of the
module, the tests pass both on MacOS and on Linux running on CircleCI.

<a data-ca-tag
href="https://codeapprove.com/pr/tiny-pilot/tinypilot/1344"><img
src="https://codeapprove.com/external/github-tag-allbg.png" alt="Review
on CodeApprove" /></a>

---------

Co-authored-by: Andrew M. Farrell <armorsmith42@gmail.com>

app/hid/write_test.py

@@ -5,15 +5,40 @@
 
 import hid.write
 
+# Dummy functions to represent what can happen when a Human Interface Device
+# writes.
+#
+# On some MacOS systems, the multiprocessing module spawns rather than forks new
+# processes[1], which pickles these functions[2]. So, they must be defined
+# using `def` at the top level of a module[3].
+#
+# This was observed on a 2021 Macbook Pro M1 Max running OSX Ventura 13.2.1.
+#
+# [1] https://github.com/python/cpython/commit/17a5588740b3d126d546ad1a13bdac4e028e6d50
+# [2] https://docs.python.org/3.11/library/multiprocessing.html#the-spawn-and-forkserver-start-methods
+# [3] https://docs.python.org/3/library/pickle.html#what-can-be-pickled-and-unpickled:~:text=(using%20def%2C%20not%20lambda)
 
-class WriteTest(unittest.TestCase):
 
-    def test_process_with_result_child_completed(self):
+def do_nothing():
+    pass
+
+
+def sleep_1_second():
+    time.sleep(1)
+
 
-        def target():
-            pass
+def raise_exception():
+    raise Exception('Child exception')
 
-        process = hid.write.ProcessWithResult(target=target, daemon=True)
+
+def return_string():
+    return 'Done!'
+
+
+class WriteTest(unittest.TestCase):
+
+    def test_process_with_result_child_completed(self):
+        process = hid.write.ProcessWithResult(target=do_nothing, daemon=True)
         process.start()
         process.join()
         result = process.result()
@@ -22,11 +47,8 @@ def target():
             hid.write.ProcessResult(return_value=None, exception=None), result)
 
     def test_process_with_result_child_not_completed(self):
-
-        def target():
-            time.sleep(1)
-
-        process = hid.write.ProcessWithResult(target=target, daemon=True)
+        process = hid.write.ProcessWithResult(target=sleep_1_second,
+                                              daemon=True)
         process.start()
         # Get the result before the child process has completed.
         self.assertIsNone(process.result())
@@ -35,14 +57,11 @@ def target():
         process.kill()
 
     def test_process_with_result_child_exception(self):
-
-        def target():
-            raise Exception('Child exception')
-
         # Silence stderr while the child exception is being raised to avoid
         # polluting the terminal output.
         with mock.patch('sys.stderr', io.StringIO()):
-            process = hid.write.ProcessWithResult(target=target, daemon=True)
+            process = hid.write.ProcessWithResult(target=raise_exception,
+                                                  daemon=True)
             process.start()
             process.join()
         result = process.result()
@@ -53,11 +72,7 @@ def target():
         self.assertEqual('Child exception', str(result.exception))
 
     def test_process_with_result_return_value(self):
-
-        def target():
-            return 'Done!'
-
-        process = hid.write.ProcessWithResult(target=target, daemon=True)
+        process = hid.write.ProcessWithResult(target=return_string, daemon=True)
         process.start()
         process.join()
         result = process.result()