reminders: Stop interpolating user-provided values in format string.

We must not intermix Markdown strings which are ready for the message,
with format strings which we intend to interpolate on.

(cherry picked from commit 575dd10f99)

zerver/lib/reminders.py

@@ -88,11 +88,12 @@ def get_reminder_formatted_content(
         content += "\n"
         fence = get_unused_fence(content)
         quoted_message = "{fence}quote\n{msg_content}\n{fence}"
-        content += quoted_message
-        length_without_message_content = len(content.format(fence=fence, msg_content=""))
+        length_without_message_content = len(
+            content + quoted_message.format(fence=fence, msg_content="")
+        )
         max_length = settings.MAX_MESSAGE_LENGTH - length_without_message_content
         msg_content = truncate_content(message.content, max_length, "\n[message truncated]")
-        content = content.format(
+        content += quoted_message.format(
             fence=fence,
             msg_content=msg_content,
         )

zerver/tests/test_reminders.py

@@ -463,3 +463,20 @@ class RemindersTest(ZulipTestCase):
                 f"Maximum reminder note length: {len(note) - 1} characters",
                 status_code=400,
             )
+
+        # Test with note containing formatting characters
+        note = "{123}"
+        content = "{456}"
+        message_id = self.send_stream_message(
+            self.example_user("hamlet"), "Verona", content, topic_name="{789}"
+        )
+        result = self.do_schedule_reminder(message_id, scheduled_delivery_timestamp, note)
+        self.assert_json_success(result)
+        scheduled_message = self.last_scheduled_reminder()
+        self.assertEqual(
+            scheduled_message.content,
+            "You requested a reminder for #**Verona>{789}@"
+            + str(message_id)
+            + "**. Note:\n > {123}\n\n"
+            f"@_**King Hamlet|10** [said](http://zulip.testserver/#narrow/channel/3-Verona/topic/.7B789.7D/near/{message_id}):\n```quote\n{content}\n```",
+        )