Fix environment population for build package workaround

The fix in eb03bc1 calls `subprocess.run` with an empty
dict as environment to escape snap environment when installing
packages on the host system.  This does unfortunately not work in
all circumstances.

Fixes #644

charmtools/build/builder.py

@@ -786,7 +786,8 @@ def workaround_charmcraft_maybe_ensure_build_packages(self):
                      or os.environ.get('CHARMCRAFT_PART_NAME', None))):
             log.warning('Probably running as root in charmcraft, proactively '
                         'installing the `git` and `virtualenv` packages.')
-            subprocess.run(('apt', '-y', 'install', 'git', 'virtualenv'), check=True, env={})
+            subprocess.run(('apt', '-y', 'install', 'git', 'virtualenv'),
+                           check=True, env=utils.host_env())
 
     def generate(self):
         layers = self.fetch()

charmtools/build/tactics.py

@@ -1064,24 +1064,14 @@ def __str__(self):
         return "Building wheelhouse in {}".format(directory)
 
     def _get_env(self):
-        """Get environment appropriate for executing external commands.
+        """Get environment for executing commands outside snap context.
 
         :returns: Dictionary with environment variables
         :rtype: Dict[str,str]
         """
         if self.use_python_from_snap:
             return os.environ.copy()
-
-        env = os.environ.copy()
-        for key in ('PREFIX', 'PYTHONHOME', 'PYTHONPATH',
-                    'GIT_TEMPLATE_DIR', 'GIT_EXEC_PATH'):
-            if key in env:
-                del(env[key])
-        env['PATH'] = ':'.join([
-            element
-            for element in env['PATH'].split(':')
-            if not element.startswith('/snap/charm/')])
-        return env
+        return utils.host_env()
 
     def combine(self, existing):
         ""  # suppress inherited doc

charmtools/utils.py

@@ -633,3 +633,21 @@ def validate_display_name(entity, linter):
         linter.err('display-name: not in valid format. '
                    'Only letters, numbers, dashes, and hyphens are permitted.')
         return
+
+
+def host_env():
+    """Get environment appropriate for executing commands outside snap context.
+
+    :returns: Dictionary with environment variables
+    :rtype: Dict[str,str]
+    """
+    env = os.environ.copy()
+    for key in ('PREFIX', 'PYTHONHOME', 'PYTHONPATH',
+                'GIT_TEMPLATE_DIR', 'GIT_EXEC_PATH'):
+        if key in env:
+            del(env[key])
+    env['PATH'] = ':'.join([
+        element
+        for element in env['PATH'].split(':')
+        if not element.startswith('/snap/charm/')])
+    return env

tests/test_utils.py

@@ -1,5 +1,6 @@
 from __future__ import print_function
 
+import unittest
 from unittest import TestCase
 from charmtools import utils
 from six import StringIO
@@ -43,3 +44,19 @@ def react(db):
         self.assertIn("Beta", output)
         self.assertIn("@when('db.ready'", output)
         self.assertIn("bar", output)
+
+    @unittest.mock.patch("os.environ")
+    def test_host_env(self, mock_environ):
+        mock_environ.copy.return_value = {
+            'PREFIX': 'fake-prefix',
+            'PYTHONHOME': 'fake-pythonhome',
+            'PYTHONPATH': 'fake-pythonpath',
+            'GIT_TEMPLATE_DIR': 'fake-git-template-dir',
+            'GIT_EXEC_PATH': 'fake-git-exec-path',
+            'SOME_OTHER_KEY': 'fake-some-other-key',
+            'PATH': '/snap/charm/current/bin:/usr/bin:'
+                    '/snap/charm/current/usr/bin:/bin',
+        }
+        self.assertDictEqual(
+            {'SOME_OTHER_KEY': 'fake-some-other-key', 'PATH': '/usr/bin:/bin'},
+            utils.host_env())