Handle disabled pytest-xdist gracefully (#178)

* Fix loading the pytest plugin when xdist is disabled

Fix the pytest plugin to run xdist-specific logic only when the plugin
is actually enabled.  This avoids pytest crashing when xdist is
installed but explicitly disabled, e.g. via "-p no:xdist".

Fixes #177

* Explicitly pass "-p xdist" to the test requiring xdist

Explicitly pass "-p xdist" to ensure that the plugin is enabled when
running the test requiring it.  This fixes test failure when
PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 is used.  This option is very helpful
since other pytest plugins tend to break this one's test suite.

Author: mgorny

CHANGES.rst

@@ -4,7 +4,11 @@ Changelog
 10.3 (unreleased)
 -----------------
 
-- Nothing changed yet.
+Bug fixes
++++++++++
+
+- Fix crash when pytest-xdist is installed but disabled.
+  (Thanks to `@mgorny <https://github.com/mgorny>`_ for the PR.)
 
 
 10.2 (2021-09-17)

pytest_rerunfailures.py

@@ -316,7 +316,8 @@ def pytest_configure(config):
         "between re-runs.",
     )
 
-    if HAS_PYTEST_HANDLECRASHITEM:
+    if config.pluginmanager.hasplugin("xdist") and HAS_PYTEST_HANDLECRASHITEM:
+        config.pluginmanager.register(XDistHooks())
         if is_master(config):
             config.failures_db = ServerStatusDB()
         else:
@@ -325,13 +326,12 @@ def pytest_configure(config):
         config.failures_db = StatusDB()  # no-op db
 
 
-if HAS_PYTEST_HANDLECRASHITEM:
-
-    def pytest_configure_node(node):
+class XDistHooks:
+    def pytest_configure_node(self, node):
         """xdist hook"""
         node.workerinput["sock_port"] = node.config.failures_db.sock_port
 
-    def pytest_handlecrashitem(crashitem, report, sched):
+    def pytest_handlecrashitem(self, crashitem, report, sched):
         """
         Return the crashitem from pending and collection.
         """

test_pytest_rerunfailures.py

@@ -194,7 +194,7 @@ def test_crash():
         def test_pass():
             pass"""
     )
-    result = testdir.runpytest("-n", "1", "--reruns", "1", "-r", "R")
+    result = testdir.runpytest("-p", "xdist", "-n", "1", "--reruns", "1", "-r", "R")
     assert_outcomes(result, passed=2, rerun=1)