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

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)