Silence stderr in tests (#1708)

Resolves #1642: our test
output [is nice and tidy
again](https://app.circleci.com/pipelines/github/tiny-pilot/tinypilot/4286/workflows/253f8f8b-b3af-47f8-82dc-b88e22d1122b/jobs/29448?invite=true#step-103-8652_129).

Contrary to what I initially mentioned in the ticket, one of the two
stacktraces didn’t come from
[`test_process_with_result_child_exception`](https://github.com/tiny-pilot/tinypilot/blob/cd3f5ff30a04659a750aec6dbf60ca49927f10f0/app/execute_test.py#L58-L71),
which already had the “[stderr
silencing](https://github.com/tiny-pilot/tinypilot/blob/cd3f5ff30a04659a750aec6dbf60ca49927f10f0/app/execute_test.py#L61)”,
but rather from
[`test_background_thread_ignores_function_exception`](https://github.com/tiny-pilot/tinypilot/blob/cd3f5ff30a04659a750aec6dbf60ca49927f10f0/app/execute_test.py#L99-L100).
(I’ve updated the ticket accordingly.)

The “stderr silencing” technique is working fine, we just didn’t have it
on the other two functions. These were added later, so we likely just
forgot about this issue.

I’ve extracted a context manager function, to make it easier to discover
and re-use this technique in the tests, and to avoid us having to repeat
the explanatory comment + exact invocation again and again.
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---------

Co-authored-by: Jan Heuermann <[email protected]>

app/execute_test.py

@@ -1,3 +1,4 @@
+import contextlib
 import io
 import time
 import unittest
@@ -12,11 +13,16 @@
 # processes[1], which pickles these functions[2]. So, they must be defined
 # using `def` at the top level of a module[3].
 #
+# Another by-effect of this is that the `silence_stderr` context manager
+# doesn’t have any effect on MacOS systems, so we cannot prevent the stacktrace
+# printing there.[4]
+#
 # 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/3.9/library/multiprocessing.html#the-spawn-and-forkserver-start-methods
 # [3] https://docs.python.org/3.9/library/pickle.html#what-can-be-pickled-and-unpickled:~:text=(using%20def%2C%20not%20lambda)
+# [4] https://github.com/tiny-pilot/tinypilot/issues/1713
 
 
 def do_nothing():
@@ -35,6 +41,14 @@ def return_string():
     return 'Done!'
 
 
+@contextlib.contextmanager
+def silence_stderr():
+    """Silences stderr to avoid polluting the terminal output of the tests."""
+    # Note: on MacOS systems, this doesn’t have an effect (see comment above).
+    with mock.patch('sys.stderr', io.StringIO()):
+        yield None
+
+
 class ExecuteTest(unittest.TestCase):
 
     def test_process_with_result_child_completed(self):
@@ -58,7 +72,7 @@ def test_process_with_result_child_not_completed(self):
     def test_process_with_result_child_exception(self):
         # Silence stderr while the child exception is being raised to avoid
         # polluting the terminal output.
-        with mock.patch('sys.stderr', io.StringIO()):
+        with silence_stderr():
             process = execute.ProcessWithResult(target=raise_exception,
                                                 daemon=True)
             process.start()
@@ -89,12 +103,14 @@ def test_execute_with_timeout_return_value(self):
         self.assertEqual('Done!', return_value)
 
     def test_execute_with_timeout_child_exception(self):
-        with self.assertRaises(Exception) as ctx:
-            execute.with_timeout(raise_exception, timeout_in_seconds=0.5)
+        with silence_stderr():
+            with self.assertRaises(Exception) as ctx:
+                execute.with_timeout(raise_exception, timeout_in_seconds=0.5)
         self.assertEqual('Child exception', str(ctx.exception))
 
     def test_background_thread_ignores_function_successful(self):
         self.assertEqual(None, execute.background_thread(return_string))
 
     def test_background_thread_ignores_function_exception(self):
-        self.assertEqual(None, execute.background_thread(raise_exception))
+        with silence_stderr():
+            self.assertEqual(None, execute.background_thread(raise_exception))