Postgres ssl mode prefer (#21498)

* refactor postgres,
* adds a basic unit test module
* first step towards a common utils module
* set postgresql_db doc argument defaults to what the code actually uses

* unit tests that actually test a missing/found psycopg2, no dependency needed

* add doc fragments, use common args, ansible2ify the imports

* update dict

* add AnsibleModule import

* mv AnsibleModule import to correct file

* restore some database utils we need

* rm some more duplicated pg doc fragments

* change ssl_mode from disable to prefer, add update docs

* use LibraryError pattern for import verification

per comments on #21435. basically LibraryError and touching up its usage in pg_db and the tests.

test/units/module_utils/test_postgresql.py

@@ -0,0 +1,75 @@
+import json
+import sys
+
+from ansible.compat.tests import unittest
+from ansible.compat.tests.mock import patch, MagicMock
+from ansible.compat.six.moves import builtins
+
+from ansible.module_utils._text import to_native
+from ansible.module_utils.basic import AnsibleModule
+from units.mock.procenv import swap_stdin_and_argv
+
+
+import pprint
+
+realimport = builtins.__import__
+
+class TestPostgres(unittest.TestCase):
+    def clear_modules(self, mods):
+        for mod in mods:
+            if mod in sys.modules:
+                del sys.modules[mod]
+
+    @patch.object(builtins, '__import__')
+    def test_postgres_pg2_missing_ensure_libs(self, mock_import):
+        def _mock_import(name, *args, **kwargs):
+            if name == 'psycopg2':
+                raise ImportError
+            return realimport(name, *args, **kwargs)
+
+        self.clear_modules(['psycopg2', 'ansible.module_utils.postgres'])
+        mock_import.side_effect = _mock_import
+        mod = builtins.__import__('ansible.module_utils.postgres')
+
+        self.assertFalse(mod.module_utils.postgres.HAS_PSYCOPG2)
+
+        with self.assertRaises(mod.module_utils.postgres.LibraryError) as context:
+            mod.module_utils.postgres.ensure_libs(sslrootcert=None)
+        self.assertIn('psycopg2 is not installed', to_native(context.exception))
+
+    @patch.object(builtins, '__import__')
+    def test_postgres_pg2_found_ensure_libs(self, mock_import):
+        def _mock_import(name, *args, **kwargs):
+            if 'psycopg2' in name:
+                return MagicMock()
+            return realimport(name, *args, **kwargs)
+
+        self.clear_modules(['psycopg2', 'ansible.module_utils.postgres'])
+        mock_import.side_effect = _mock_import
+        mod = builtins.__import__('ansible.module_utils.postgres')
+
+        self.assertTrue(mod.module_utils.postgres.HAS_PSYCOPG2)
+
+        ensure_ret = mod.module_utils.postgres.ensure_libs(sslrootcert=None)
+        self.assertFalse(ensure_ret)
+        pprint.pprint(ensure_ret)
+
+    @patch.object(builtins, '__import__')
+    def test_postgres_pg2_found_ensure_libs_old_version(self, mock_import):
+        def _mock_import(name, *args, **kwargs):
+            if 'psycopg2' in name:
+                m = MagicMock()
+                m.__version__ = '2.4.1'
+                return m
+            return realimport(name, *args, **kwargs)
+
+        self.clear_modules(['psycopg2', 'ansible.module_utils.postgres'])
+        mock_import.side_effect = _mock_import
+        mod = builtins.__import__('ansible.module_utils.postgres')
+
+        self.assertTrue(mod.module_utils.postgres.HAS_PSYCOPG2)
+
+        with self.assertRaises(mod.module_utils.postgres.LibraryError) as context:
+            mod.module_utils.postgres.ensure_libs(sslrootcert='yes')
+        self.assertIn('psycopg2 must be at least 2.4.3 in order to use', to_native(context.exception))
+