Issue node shutdown on empty workload

Remote pytest_runtestloop requires at least 2 tests, or a test
and a shutdown command. In case of inter test locking, for
example using a file lock, the tests could deadlock, since the
shutdown command would not be enqueued, and the last test of a
worker would never finish, and would not allow another worker
to make progress.

changelog/351.bugfix

@@ -0,0 +1 @@
+Fix scheduling deadlock in case of inter-test locking.

testing/acceptance_test.py

@@ -1132,6 +1132,56 @@ class TestFileScope:
         assert c1 == c2
 
 
+class TestLocking:
+    _test_content = """
+    class TestClassName%s(object):
+
+        @classmethod
+        def setup_class(cls):
+            FILE_LOCK.acquire()
+
+        @classmethod
+        def teardown_class(cls):
+            FILE_LOCK.release()
+
+        def test_a(self):
+            pass
+
+        def test_b(self):
+            pass
+
+        def test_c(self):
+            pass
+
+    """
+
+    test_file1 = """
+    import filelock
+
+    FILE_LOCK = filelock.FileLock("test.lock")
+
+    """ + (
+        (_test_content * 4) % ("A", "B", "C", "D")
+    )
+
+    @pytest.mark.parametrize("scope", ["each", "load", "loadscope", "loadfile", "no"])
+    def test_single_file(self, testdir, scope):
+        testdir.makepyfile(test_a=self.test_file1)
+        result = testdir.runpytest("-n2", "--dist=%s" % scope, "-v")
+        result.assert_outcomes(passed=(12 if scope != "each" else 12 * 2))
+
+    @pytest.mark.parametrize("scope", ["each", "load", "loadscope", "loadfile", "no"])
+    def test_multi_file(self, testdir, scope):
+        testdir.makepyfile(
+            test_a=self.test_file1,
+            test_b=self.test_file1,
+            test_c=self.test_file1,
+            test_d=self.test_file1,
+        )
+        result = testdir.runpytest("-n2", "--dist=%s" % scope, "-v")
+        result.assert_outcomes(passed=(48 if scope != "each" else 48 * 2))
+
+
 def parse_tests_and_workers_from_output(lines):
     result = []
     for line in lines:

tox.ini

@@ -23,6 +23,7 @@ deps =
   pytestmaster: git+https://github.com/pytest-dev/pytest.git@master
   pytestfeatures: git+https://github.com/pytest-dev/pytest.git@features
   pexpect: pexpect
+  filelock
 platform=
   pexpect: linux|darwin
 commands=

xdist/scheduler/each.py

@@ -126,6 +126,7 @@ class EachScheduling(object):
             if not pending:
                 pending[:] = range(len(self.node2collection[node]))
                 node.send_runtest_all()
+                node.shutdown()
             else:
                 node.send_runtest_some(pending)
             self._started.append(node)

xdist/scheduler/load.py

@@ -178,6 +178,9 @@ class LoadScheduling(object):
                     return
                 num_send = items_per_node_max - len(node_pending)
                 self._send_tests(node, num_send)
+        else:
+            node.shutdown()
+
         self.log("num items waiting for node:", len(self.pending))
 
     def remove_node(self, node):

xdist/scheduler/loadscope.py

@@ -306,6 +306,7 @@ class LoadScopeScheduling(object):
 
         # Check that more work is available
         if not self.workqueue:
+            node.shutdown()
             return
 
         self.log("Number of units waiting for node:", len(self.workqueue))