Use execnet main_thread_only execmodel (#1027)

Use the execnet main_thread_only execmodel so that code which expects
to run in the main thread will just work.  This execmodel has been
merged to the execnet master branch via pytest-dev/execnet#243, so this
patch should not be merged until there is a released version of execnet
supporting the main_thread_only execmodel.

Closes #620

changelog/1027.improvement.rst

@@ -0,0 +1,3 @@
+``pytest-xdist`` workers now always execute the tests in the main thread.
+
+Previously some tests might end up executing in a separate thread other than ``main`` in the workers, due to some internal `èxecnet`` details. This can cause problems specially with async frameworks where the event loop is running in the ``main`` thread (for example `#620 <https://github.com/pytest-dev/pytest-xdist/issues/620>`__).

changelog/620.bugfix

@@ -0,0 +1 @@
+Use the ``execnet`` new ``main_thread_only`` "execmodel" so that code which expects to only run in the main thread will now work as expected.

src/xdist/looponfail.py

@@ -82,7 +82,7 @@ class RemoteControl:
             print("RemoteControl:", msg)
 
     def initgateway(self) -> execnet.Gateway:
-        return execnet.makegateway("popen")
+        return execnet.makegateway("execmodel=main_thread_only//popen")
 
     def setup(self) -> None:
         if hasattr(self, "gateway"):

src/xdist/workermanage.py

@@ -56,13 +56,15 @@ class NodeManager:
         self.testrunuid = self.config.getoption("testrunuid")
         if self.testrunuid is None:
             self.testrunuid = uuid.uuid4().hex
-        self.group = execnet.Group()
+        self.group = execnet.Group(execmodel="main_thread_only")
         if specs is None:
             specs = self._getxspecs()
         self.specs: list[execnet.XSpec] = []
         for spec in specs:
             if not isinstance(spec, execnet.XSpec):
                 spec = execnet.XSpec(spec)
+            if getattr(spec, "execmodel", None) != "main_thread_only":
+                spec = execnet.XSpec(f"execmodel=main_thread_only//{spec}")
             if not spec.chdir and not spec.popen:
                 spec.chdir = defaultchdir
             self.group.allocate_id(spec)
@@ -90,6 +92,8 @@ class NodeManager:
         spec: execnet.XSpec,
         putevent: Callable[[tuple[str, dict[str, Any]]], None],
     ) -> WorkerController:
+        if getattr(spec, "execmodel", None) != "main_thread_only":
+            spec = execnet.XSpec(f"execmodel=main_thread_only//{spec}")
         gw = self.group.makegateway(spec)
         self.config.hook.pytest_xdist_newgateway(gateway=gw)
         self.rsync_roots(gw)

testing/test_remote.py

@@ -49,7 +49,7 @@ class WorkerSetup:
     def setup(self) -> None:
         self.pytester.chdir()
         # import os ; os.environ['EXECNET_DEBUG'] = "2"
-        self.gateway = execnet.makegateway()
+        self.gateway = execnet.makegateway("execmodel=main_thread_only//popen")
         self.config = config = self.pytester.parseconfigure()
         putevent = self.events.put if self.use_callback else None
 

testing/test_workermanage.py

@@ -83,7 +83,7 @@ class TestNodeManagerPopen:
         assert len(call.specs) == 2
 
         call = hookrecorder.popcall("pytest_xdist_newgateway")
-        assert call.gateway.spec == execnet.XSpec("popen")
+        assert call.gateway.spec == execnet.XSpec("execmodel=main_thread_only//popen")
         assert call.gateway.id == "gw0"
         call = hookrecorder.popcall("pytest_xdist_newgateway")
         assert call.gateway.id == "gw1"
@@ -177,7 +177,7 @@ class TestHRSync:
         assert names == {"dir", "file.txt", "somedir"}
 
     def test_hrsync_one_host(self, source: Path, dest: Path) -> None:
-        gw = execnet.makegateway("popen//chdir=%s" % dest)
+        gw = execnet.makegateway("execmodel=main_thread_only//popen//chdir=%s" % dest)
         finished = []
         rsync = HostRSync(source)
         rsync.add_target_host(gw, finished=lambda: finished.append(1))