Ignore user site packages when running buildout

I had a Theia instance where I installed buildout with `pip install -e --user /path/to/checkout` and later deleted that `/path/to/checkout` and this made it impossible to run instance buildout, failing with import error when importing zc.buildout  in buildout script.

This is because python load user site packages by default. I believe we don't want this, a broken user site package should not prevent slapos from running buildout.

These changes are about running buildout with `PYTHONNOUSERSITE` set, so that python ignores user site packages when running buildout.

references: 

 * https://docs.python.org/3/using/cmdline.html#envvar-PYTHONNOUSERSITE
 * https://docs.python.org/3/library/site.html#site.ENABLE_USER_SITE

See merge request nexedi/slapos.core!266

slapos/grid/utils.py

@@ -40,7 +40,6 @@ import logging
 import psutil
 import time
 
-import six
 
 from slapos.grid.exception import BuildoutFailedError, WrongPermissionError
 
@@ -57,8 +56,6 @@ PYTHON_ENVIRONMENT_REMOVE_LIST = [
   'PYTHONDEBUG',
   'PYTHONDONTWRITEBYTECODE',
   'PYTHONINSPECT',
-  'PYTHONNOUSERSITE',
-  'PYTHONNOUSERSITE',
   'PYTHONUNBUFFERED',
   'PYTHONVERBOSE',
 ]
@@ -150,10 +147,11 @@ def getCleanEnvironment(logger, home_path='/tmp'):
     if old is not None:
       removed_env.append(k)
   changed_env['HOME'] = env['HOME'] = home_path
-  for k in sorted(six.iterkeys(changed_env)):
-    logger.debug('Overridden %s = %r' % (k, changed_env[k]))
+  changed_env['PYTHONNOUSERSITE'] = env['PYTHONNOUSERSITE'] = 'true'
+  for k, v in sorted(changed_env.items()):
+    logger.debug('Overridden %s = %r', k, v)
   if removed_env:
-    logger.debug('Removed from environment: %s' % ', '.join(sorted(removed_env)))
+    logger.debug('Removed from environment: %s',  ', '.join(sorted(removed_env)))
   return env
 
 

slapos/tests/test_slapgrid.py

@@ -283,6 +283,49 @@ class TestBasicSlapgridCP(BasicMixin, unittest.TestCase):
     os.mkdir(self.instance_root)
     self.assertRaises(ConnectionError, self.grid.processComputerPartitionList)
 
+  def test_environment_variable_HOME(self):
+    # When running instance, $HOME is set to the partition path
+    computer = ComputerForTest(self.software_root, self.instance_root)
+    partition = computer.instance_list[0]
+    partition.requested_state = 'started'
+    partition.software.setBuildout('#!/bin/sh\n echo $HOME > env_HOME')
+    with httmock.HTTMock(computer.request_handler):
+      self.assertEqual(self.grid.processComputerPartitionList(), slapgrid.SLAPGRID_SUCCESS)
+    with open(os.path.join(partition.partition_path, 'env_HOME')) as f:
+      self.assertEqual(f.read().strip(), partition.partition_path)
+
+  def test_no_user_site_packages(self):
+    # When running instance buildout, python's user site packages are ignored
+    computer = ComputerForTest(self.software_root, self.instance_root)
+    partition = computer.instance_list[0]
+    partition.requested_state = 'started'
+
+    # Make a broken user site package in this partition home, that will error
+    # as soon as site is initialized, making python unusable. While this is
+    # a bit unrealistic, real life problems were observed after installing a
+    # broken buildout with `pip install --editable --user`, because user site
+    # packages have priority over sys.path assignments in buildout generated
+    # scripts.
+    home_site_packages_dir = os.path.join(
+        partition.partition_path,
+        '.local',
+        'lib',
+        'python{version[0]}.{version[1]}'.format(version=sys.version_info),
+        'site-packages',
+    )
+    os.makedirs(home_site_packages_dir)
+    with open(os.path.join(home_site_packages_dir, 'dummy-nspkg.pth'),
+              'w') as f:
+      f.write('import sys; raise SystemExit("ahaha")')
+
+    partition.software.setBuildout(textwrap.dedent('''\
+      #!{sys.executable}
+      print("ok")
+    '''.format(sys=sys)))
+
+    with httmock.HTTMock(computer.request_handler):
+      self.assertEqual(self.grid.processComputerPartitionList(), slapgrid.SLAPGRID_SUCCESS)
+
 
 class MasterMixin(BasicMixin):