Tests GPG interfaces when gpg command isn't installed

Signed-off-by: Joshua Lock <jlock@vmware.com>

Author: joshuagl

.travis.yml

@@ -8,6 +8,10 @@ matrix:
       env: TOXENV=py27
     - python: "2.7"
       env: TOXENV=purepy27
+      before_install:
+        - sudo apt-get remove -y gnupg gnupg2
+      after_script:
+        - sudo apt-get install -y gnupg gnupg2
     - python: "3.5"
       env: TOXENV=py35
     - python: "3.6"
@@ -18,6 +22,10 @@ matrix:
       env: TOXENV=py38
     - python: "3.8"
       env: TOXENV=purepy38
+      before_install:
+        - sudo apt-get remove -y gnupg gnupg2
+      after_script:
+        - sudo apt-get install -y gnupg gnupg2
 
 install:
   - pip install -U tox coveralls

tests/check_public_interfaces.py

@@ -43,6 +43,9 @@
 
 
 import securesystemslib.exceptions
+import securesystemslib.gpg.constants
+import securesystemslib.gpg.functions
+import securesystemslib.gpg.util
 import securesystemslib.interface
 import securesystemslib.keys
 
@@ -194,6 +197,18 @@ 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"""
+
+    with self.assertRaises(securesystemslib.exceptions.UnsupportedLibraryError):
+      securesystemslib.gpg.functions.create_signature('bar')
+
+    with self.assertRaises(securesystemslib.exceptions.UnsupportedLibraryError):
+      securesystemslib.gpg.functions.export_pubkey('f00')
+
+    with self.assertRaises(securesystemslib.exceptions.UnsupportedLibraryError):
+      securesystemslib.gpg.util.get_version()
+
 
 if __name__ == '__main__':
   suite = unittest.TestLoader().loadTestsFromTestCase(TestPublicInterfaces)