fix(model): Ensure Secret.get_content returns a copy of the dict

This avoids the caller modifying the Secret instance's dict.

Fixes canonical#999

ops/model.py

@@ -1181,6 +1181,9 @@ def _on_secret_changed(self, event):
     def get_content(self, *, refresh: bool = False) -> Dict[str, str]:
         """Get the secret's content.
 
+        Returns:
+            A copy of the secret's content dictionary.
+
         Args:
             refresh: If true, fetch the latest revision's content and tell
                 Juju to update to tracking that revision. The default is to
@@ -1190,7 +1193,7 @@ def get_content(self, *, refresh: bool = False) -> Dict[str, str]:
         if refresh or self._content is None:
             self._content = self._backend.secret_get(
                 id=self.id, label=self.label, refresh=refresh)
-        return self._content
+        return self._content.copy()
 
     def peek_content(self) -> Dict[str, str]:
         """Get the content of the latest revision of this secret.

test/test_model.py

@@ -3104,6 +3104,18 @@ def test_get_content_uncached(self):
         self.assertEqual(fake_script_calls(self, clear=True),
                          [['secret-get', 'secret:z', '--format=json']])
 
+    def test_get_content_copies_dict(self):
+        fake_script(self, 'secret-get', """echo '{"foo": "bar"}'""")
+
+        secret = self.make_secret(id='z')
+        content = secret.get_content()
+        self.assertEqual(content, {'foo': 'bar'})
+        content['new'] = 'value'
+        self.assertEqual(secret.get_content(), {'foo': 'bar'})
+
+        self.assertEqual(fake_script_calls(self, clear=True),
+                         [['secret-get', 'secret:z', '--format=json']])
+
     def test_peek_content(self):
         fake_script(self, 'secret-get', """echo '{"foo": "peeked"}'""")