Reject yum gather backend on Python 3

It will not run, and having a nice error message is better than a
cryptic crash.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

doc/configuration.rst

@@ -575,10 +575,12 @@ Options
         * With ``greedy_method = "build" ``pkg-b-provider-1`` and
           ``pkg-b-provider-2`` will be pulled in.
 
-**gather_backend** = ``yum``
+**gather_backend**
-    (*str*) -- Either ``yum`` or ``dnf``. This changes the entire codebase
+    (*str*) --This changes the entire codebase doing dependency solving, so it
-    doing dependency solving, so it can change the result in unpredictable
+    can change the result in unpredictable ways.
-    ways.
+
+    On Python 2, the choice is between ``yum`` or ``dnf`` and defaults to
+    ``yum``. On Python 3 ``dnf`` is the only option and default.
 
     Particularly the multilib work is performed differently by using
     ``python-multilib`` library. Please refer to ``multilib`` option to see the

pungi/checks.py

@@ -581,8 +581,8 @@ def make_schema():
             "gather_source_mapping": {"type": "string"},
             "gather_backend": {
                 "type": "string",
-                "enum": ["yum", "dnf"],
+                "enum": _get_gather_backends(),
-                "default": "yum",
+                "default": _get_default_gather_backend(),
             },
             "gather_profiler": {
                 "type": "boolean",
@@ -1203,3 +1203,13 @@ CONFIG_DEPS = {
         ],
     },
 }
+
+
+def _get_gather_backends():
+    if six.PY2:
+        return ['yum', 'dnf']
+    return ['dnf']
+
+
+def _get_default_gather_backend():
+    return 'yum' if six.PY2 else 'dnf'

tests/test_config.py

@@ -10,6 +10,7 @@ except ImportError:
 import os
 import six
 import sys
+import mock
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
 
@@ -250,6 +251,38 @@ class GatherConfigTestCase(ConfigTestCase):
             cfg,
             [checks.REQUIRES.format('gather_source', 'json', 'gather_source_mapping')])
 
+    def test_dnf_backend_is_default_on_py3(self):
+        cfg = load_config(
+            pkgset_source='koji',
+            pkgset_koji_tag='f27',
+        )
+
+        with mock.patch('six.PY2', new=False):
+            self.assertValidation(cfg, [])
+        self.assertEqual(cfg['gather_backend'], 'dnf')
+
+    def test_yum_backend_is_default_on_py2(self):
+        cfg = load_config(
+            pkgset_source='koji',
+            pkgset_koji_tag='f27',
+        )
+
+        with mock.patch('six.PY2', new=True):
+            self.assertValidation(cfg, [])
+        self.assertEqual(cfg['gather_backend'], 'yum')
+
+    def test_yum_backend_is_rejected_on_py3(self):
+        cfg = load_config(
+            pkgset_source='koji',
+            pkgset_koji_tag='f27',
+            gather_backend='yum',
+        )
+
+        with mock.patch('six.PY2', new=False):
+            self.assertValidation(
+                cfg,
+                ["Failed validation in gather_backend: 'yum' is not one of ['dnf']"])
+
 
 class OSBSConfigTestCase(ConfigTestCase):
     def test_validate(self):