Make xdist work even if looponfail or boxed are disabled

Fix #32

xdist/looponfail.py

@@ -25,6 +25,10 @@ def pytest_addoption(parser):
 def pytest_cmdline_main(config):
 
     if config.getoption("looponfail"):
+        usepdb = config.getoption('usepdb')  # a core option
+        if usepdb:
+            raise pytest.UsageError(
+                "--pdb incompatible with --looponfail.")
         looponfail_main(config)
         return 2  # looponfail only can get stop with ctrl-C anyway
 

xdist/plugin.py

@@ -94,12 +94,8 @@ def pytest_cmdline_main(config):
         config.option.dist = "load"
     val = config.getvalue
     if not val("collectonly"):
-        usepdb = config.option.usepdb  # a core option
-        if val("looponfail"):
-            if usepdb:
-                raise pytest.UsageError(
-                    "--pdb incompatible with --looponfail.")
-        elif val("dist") != "no":
+        usepdb = config.getoption('usepdb')  # a core option
+        if val("dist") != "no":
             if usepdb:
                 raise pytest.UsageError(
                     "--pdb incompatible with distributing tests.")