Use execnet main_thread_only execmodel

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
proposed in pytest-dev/execnet#243, so this
patch should not be merged until there is a release version of execnet
supporting the main_thread_only execmodel.

Closes: pytest-dev#620

src/xdist/looponfail.py

@@ -76,7 +76,7 @@ def trace(self, *args):
             print("RemoteControl:", msg)
 
     def initgateway(self):
-        return execnet.makegateway("popen")
+        return execnet.makegateway("execmodel=main_thread_only//popen")
 
     def setup(self, out=None):
         if out is None:

src/xdist/workermanage.py

@@ -41,13 +41,15 @@ def __init__(self, config, specs=None, defaultchdir="pyexecnetcache") -> None:
         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 = []
         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)
@@ -68,6 +70,10 @@ def setup_nodes(self, putevent):
         return [self.setup_node(spec, putevent) for spec in self.specs]
 
     def setup_node(self, spec, putevent):
+        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}")
         gw = self.group.makegateway(spec)
         self.config.hook.pytest_xdist_newgateway(gateway=gw)
         self.rsync_roots(gw)

testing/test_remote.py

@@ -38,7 +38,7 @@ def __init__(self, request, pytester: pytest.Pytester) -> None:
     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")
         self.config = config = self.pytester.parseconfigure()
         putevent = self.events.put if self.use_callback else None
 

testing/test_workermanage.py

@@ -162,7 +162,7 @@ def test_hrsync_filter(self, source: Path, dest: Path) -> None:
         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))