Separate test builds w/o native libs and w/o gpg

Update tox and travis configuration to run tests
- with all extra dependencies and gpg (if available)
- without native dependencies (i.e. cryptography and nacl)
- without gpg

Previously the latter two were combined. The separation allows
for more fine grained testing of the gpg subpackage whose,
signature creation and key export function should fail on both,
missing gpg and missing cryptography library (with different
error messages), whereas the signature verification function should
only fail if the cryptography library is missing.

Author: Lukas Puehringer

.travis.yml

@@ -8,6 +8,8 @@ matrix:
       env: TOXENV=py27
     - python: "2.7"
       env: TOXENV=purepy27
+    - python: "2.7"
+      env: TOXENV=py27-no-gpg
       before_install:
         - sudo apt-get remove -y --allow-remove-essential gnupg gnupg2
     - python: "3.5"
@@ -20,8 +22,10 @@ matrix:
       env: TOXENV=py38
     - python: "3.8"
       env: TOXENV=purepy38
+    - python: "3.8"
+      env: TOXENV=py38-no-gpg
       before_install:
-        - sudo apt-get remove -y  --allow-remove-essential gnupg gnupg2
+        - sudo apt-get remove -y --allow-remove-essential gnupg gnupg2
 
 install:
   - pip install -U tox coveralls

tests/check_public_interfaces.py

@@ -193,20 +193,23 @@ def test_purepy_ed25519(self):
         pub, 'ed25519', bsig, data)
     self.assertEqual(False, invalid)
 
-  def test_gpg_cmds(self):
-    """Ensure functions calling GPG commands throw an appropriate error"""
+  def test_gpg_functions(self):
+    """Public GPG functions must raise error on missing cryptography lib. """
+    expected_error = securesystemslib.exceptions.UnsupportedLibraryError
+    expected_error_msg = securesystemslib.gpg.functions.NO_CRYPTO_MSG
 
-    with self.assertRaises(securesystemslib.exceptions.UnsupportedLibraryError):
+    with self.assertRaises(expected_error) as ctx:
       securesystemslib.gpg.functions.create_signature('bar')
+    self.assertEqual(expected_error_msg, str(ctx.exception))
 
-    with self.assertRaises(securesystemslib.exceptions.UnsupportedLibraryError):
+    with self.assertRaises(expected_error) as ctx:
       securesystemslib.gpg.functions.verify_signature(None, 'f00', 'bar')
+    self.assertEqual(expected_error_msg, str(ctx.exception))
 
-    with self.assertRaises(securesystemslib.exceptions.UnsupportedLibraryError):
+    with self.assertRaises(expected_error) as ctx:
       securesystemslib.gpg.functions.export_pubkey('f00')
+    self.assertEqual(expected_error_msg, str(ctx.exception))
 
-    with self.assertRaises(securesystemslib.exceptions.UnsupportedLibraryError):
-      securesystemslib.gpg.util.get_version()
 
 
 if __name__ == '__main__':

tox.ini

@@ -32,3 +32,13 @@ deps =
 
 commands =
     python -m tests.check_public_interfaces
+
+# Below test envs require manual removal of gpg on the host system (see
+# .travis.yml) and are excluded from the default tox run
+[testenv:py27-no-gpg]
+commands =
+    python -m tests.check_public_interfaces_gpg
+
+[testenv:py38-no-gpg]
+commands =
+    python -m tests.check_public_interfaces_gpg