trun: Add test for how /etc/{passwd,group} is setup for spawned job

Even if we don't want to spawn full sshd to verify that ptys work, we can
still minimally test that inside user/group database is setup as expected.

/reviewed-by @kirr
/reviewed-on nexedi/nxdtest!13

nxdtest/nxdtest_test.py

@@ -19,7 +19,9 @@
 
 # verify general functionality
 
+import grp
 import os
+import pwd
 import sys
 import re
 import time
@@ -261,6 +263,36 @@ TestCase('TESTCASE', ['mount', '-t', 'tmpfs', 'none', '/etc'])
     assert "# leaked mount: none /etc tmpfs" in captured.out
 
 
+# verify that inside environment, that nxdtest creates, user/group database is
+# minimal.
+@userns_only
+def test_run_usermap(run_nxdtest, capsys):
+    tdumpusergroups = "%s/testprog/tdumpusergroups" % (dirname(__file__),)
+
+    run_nxdtest(
+        """
+TestCase('TESTCASE', %r)
+""" % [tdumpusergroups])
+    captured = capsys.readouterr()
+    assert captured.err == ''
+
+    # we expect only current user, root and nobody/nogroup to be present
+    uok = [repr(u) for u in [
+        pwd.getpwuid(os.getuid()),
+        pwd.getpwnam('root'),
+        pwd.getpwnam('nobody')]]
+    gok = [repr(g) for g in [
+        grp.getgrgid(os.getgid()),
+        grp.getgrnam('root'),
+        grp.getgrnam('nogroup')]]
+    want = '---- 8< ----\n' # XXX won't need this scissors, if we would test trun directly
+    for _ in sorted(uok) + sorted(gok):
+        want += _+'\n'
+    want += '---- 8< ----'
+
+    assert want in captured.out
+
+
 # verify that inside environment, that nxdtest creates, file permissions are
 # still respected.
 def test_run_writero(run_nxdtest, capsys):

nxdtest/testprog/tdumpusergroups

+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+# Copyright (C) 2021  Nexedi SA and Contributors.
+#
+# This program is free software: you can Use, Study, Modify and Redistribute
+# it under the terms of the GNU General Public License version 3, or (at your
+# option) any later version, as published by the Free Software Foundation.
+#
+# You can also Link and Combine this program with other software covered by
+# the terms of any of the Free Software licenses or any of the Open Source
+# Initiative approved licenses and Convey the resulting work. Corresponding
+# source of such a combination shall include the source code for all other
+# software used.
+#
+# This program is distributed WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See COPYING file for full licensing terms.
+# See https://www.nexedi.com/licensing for rationale and options.
+"""Program tdumpusergroups helps to verify that nxdtest maps users and groups
+in namespace.
+
+It dumps content of user and group databases in raw form.
+Database entries are printed in sorted order.
+"""
+
+from __future__ import absolute_import, print_function
+
+import grp
+import pwd
+import sys
+
+def main():
+    print('---- 8< ----')
+    uall = [repr(u) for u in pwd.getpwall()]
+    gall = [repr(g) for g in grp.getgrall()]
+    for u in sorted(uall):
+      print(u)
+    for g in sorted(gall):
+      print(g)
+    print('---- 8< ----')
+
+if __name__ == '__main__':
+    main()