Index: tools/testrunner/local/command.py
--- tools/testrunner/local/command.py.orig
+++ tools/testrunner/local/command.py
@@ -223,7 +223,10 @@ class PosixCommand(DesktopCommand):
         shell=True,
         # Make the new shell create its own process group. This allows to kill
         # all spawned processes reliably (https://crbug.com/v8/8292).
-        preexec_fn=os.setsid,
+        # Use subprocess' native session setup instead of preexec_fn=os.setsid:
+        # on OpenBSD the worker was observed blocking in Popen before the test
+        # timeout could be armed.
+        start_new_session=True,
       )
     except Exception as e:
       sys.stderr.write('Error executing: %s\n' % self)
