Pass and use original sys.argv to/with workers

This gets used e.g. by argparse for the "prog" part.

We could explicitly pass it through and/or set it on
config._parser.prog, but that is a bit tedious just for this use case,
and it looks like "simulating" the main prog here appears to not be that
bad of a hack after all.

testing/test_remote.py

@@ -1,6 +1,8 @@
+import os
 import py
 import pprint
 import pytest
+import sys
 
 from xdist.workermanage import WorkerController, unserialize_report
 from xdist.remote import serialize_report
@@ -397,3 +399,55 @@ def test():
     )
     result = testdir.runpytest("-n2", "--max-worker-restart=0")
     assert result.ret == 0
+
+
+def test_remote_inner_argv(testdir):
+    """Test/document the behavior due to execnet using `python -c`."""
+    testdir.makepyfile(
+        """
+        import sys
+        def test():
+            assert sys.argv == ["-c"]
+    """
+    )
+    result = testdir.runpytest("-n1")
+    assert result.ret == 0
+
+
+def test_remote_usage_prog(testdir):
+    outer_argv = sys.argv[0]
+    prog = os.path.basename(outer_argv)
+
+    testdir.makeconftest(
+        """
+        import pytest
+
+        config_parser = None
+
+        @pytest.fixture
+        def get_config_parser():
+            return config_parser
+
+        def pytest_configure(config):
+            global config_parser
+            config_parser = config._parser
+    """
+    )
+    testdir.makepyfile(
+        """
+        import sys
+
+        def test(get_config_parser, request):
+            get_config_parser._getparser().error("my_usage_error")
+
+            assert request.config.workerinput["mainargv"] == {!r}
+    """.format(
+            outer_argv
+        )
+    )
+
+    result = testdir.runpytest_subprocess("-n1")
+    assert result.ret == 1
+    result.stdout.fnmatch_lines(
+        ["usage: %s *" % prog, "%s: error: my_usage_error" % prog]
+    )

xdist/remote.py

@@ -261,6 +261,7 @@ def remote_initconfig(option_dict, args):
     import py
 
     config = remote_initconfig(option_dict, args)
+    config._parser.prog = os.path.basename(workerinput["mainargv"][0])
     config.workerinput = workerinput
     config.workeroutput = {}
     # TODO: deprecated name, backward compatibility only. Remove it in future

xdist/workermanage.py

@@ -2,6 +2,7 @@
 import fnmatch
 import os
 import re
+import sys
 import threading
 
 import py
@@ -213,6 +214,7 @@ def __init__(self, nodemanager, gateway, config, putevent):
             "workercount": len(nodemanager.specs),
             "slaveid": gateway.id,
             "slavecount": len(nodemanager.specs),
+            "mainargv": sys.argv,
         }
         # TODO: deprecated name, backward compatibility only. Remove it in future
         self.slaveinput = self.workerinput