Fix flaky 'test_signals' in test_subprocess.py

Fixes python-triogh-1170

trio/tests/test_subprocess.py

@@ -14,9 +14,9 @@
 
 posix = os.name == "posix"
 if posix:
- from signal import SIGKILL, SIGTERM, SIGINT
+ from signal import SIGKILL, SIGTERM, SIGUSR1
 else:
- SIGKILL, SIGTERM, SIGINT = None, None, None
+ SIGKILL, SIGTERM, SIGUSR1 = None, None, None
 
 
 # Since Windows has very few command-line utilities generally available,
@@ -362,8 +362,15 @@ async def test_one_signal(send_it, signum):
 
  await test_one_signal(Process.kill, SIGKILL)
  await test_one_signal(Process.terminate, SIGTERM)
+ # Test that we can send arbitrary signals.
+ #
+ # We used to use SIGINT here, but it turns out that the Python interpreter
+ # has race conditions that can cause it to explode in weird ways if it
+ # tries to handle SIGINT during startup. SIGUSR1's default disposition is
+ # to terminate the target process, and Python doesn't try to do anything
+ # clever to handle it.
  if posix:
- await test_one_signal(lambda proc: proc.send_signal(SIGINT), SIGINT)
+ await test_one_signal(lambda proc: proc.send_signal(SIGUSR1), SIGUSR1)
 
 
 @pytest.mark.skipif(not posix, reason="POSIX specific")