fix(model): make Secret.set_content invalidate local cache (#1001)

This is so that if get_content() is called after set_content(), it fetches and
returns the expected new content. We could also set self._content to
the new content, but re-fetching it from Juju seems better.

ops/model.py

@@ -1225,6 +1225,7 @@ def set_content(self, content: Dict[str, str]):
         if self._id is None:
             self._id = self.get_info().id
         self._backend.secret_set(typing.cast(str, self.id), content=content)
+        self._content = None  # invalidate cache so it's refetched next get_content()
 
     def set_info(self, *,
                  label: Optional[str] = None,

test/test_model.py

@@ -3116,6 +3116,24 @@ def test_get_content_copies_dict(self):
         self.assertEqual(fake_script_calls(self, clear=True),
                          [['secret-get', 'secret:z', '--format=json']])
 
+    def test_set_content_invalidates_cache(self):
+        fake_script(self, 'secret-get', """echo '{"foo": "bar"}'""")
+        fake_script(self, 'secret-set', """exit 0""")
+
+        secret = self.make_secret(id='z')
+        old_content = secret.get_content()
+        self.assertEqual(old_content, {'foo': 'bar'})
+        secret.set_content({'new': 'content'})
+        fake_script(self, 'secret-get', """echo '{"new": "content"}'""")
+        new_content = secret.get_content()
+        self.assertEqual(new_content, {'new': 'content'})
+
+        self.assertEqual(fake_script_calls(self, clear=True), [
+            ['secret-get', 'secret:z', '--format=json'],
+            ['secret-set', 'secret:z', 'new=content'],
+            ['secret-get', 'secret:z', '--format=json'],
+        ])
+
     def test_peek_content(self):
         fake_script(self, 'secret-get', """echo '{"foo": "peeked"}'""")