Add integration test to cover the fix for pantsbuild#5169.

tests/python/pants_test/pantsd/BUILD

@@ -66,6 +66,7 @@ python_tests(
   name = 'pantsd_integration',
   sources = ['test_pantsd_integration.py'],
   dependencies = [
+    '3rdparty/python:futures',
     'src/python/pants/pantsd:process_manager',
     'src/python/pants/util:collections',
     'tests/python/pants_test:int-test',

tests/python/pants_test/pantsd/test_pantsd_integration.py

@@ -9,11 +9,15 @@
 import itertools
 import os
 import signal
+import threading
 import time
 from contextlib import contextmanager
 
+from concurrent.futures import ThreadPoolExecutor
+
 from pants.pantsd.process_manager import ProcessManager
 from pants.util.collections import combined_dict
+from pants.util.dirutil import touch
 from pants_test.pants_run_integration_test import PantsRunIntegrationTest
 from pants_test.testutils.process_test_util import check_process_exists_by_command
 
@@ -55,6 +59,25 @@ def read_pantsd_log(workdir):
       yield line.strip()
 
 
+def launch_file_toucher(f):
+  """Launch a loop to touch the given file, and return a function to call to stop and join it."""
+  executor = ThreadPoolExecutor(max_workers=1)
+  halt = threading.Event()
+
+  def file_toucher():
+    while not halt.isSet():
+      touch(f)
+      time.sleep(1)
+
+  future = executor.submit(file_toucher)
+
+  def join():
+    halt.set()
+    future.result(timeout=10)
+
+  return join
+
+
 class TestPantsDaemonIntegration(PantsRunIntegrationTest):
   @contextmanager
   def pantsd_test_context(self, log_level='info'):
@@ -189,3 +212,20 @@ def test_pantsd_lifecycle_invalidation(self):
         # Run with an env var.
         pantsd_run(cmd[1:], {'GLOBAL': {'level': cmd[0]}})
         checker.assert_running()
+
+  def test_pantsd_filesystem_invalidation(self):
+    """Runs with pantsd enabled, in a loop, while another thread invalidates files."""
+    with self.pantsd_successful_run_context() as (pantsd_run, checker, workdir):
+      cmd = ['list', '::']
+      pantsd_run(cmd)
+      checker.await_pantsd()
+
+      # Launch a separate thread to poke files in 3rdparty.
+      join = launch_file_toucher('3rdparty/BUILD')
+
+      # Repeatedly re-list 3rdparty while the file is being invalidated.
+      for _ in range(0, 8):
+        pantsd_run(cmd)
+        checker.assert_running()
+
+      join()