While it should be an invariant that message.rendered_content is never
None for a row saved to the database, it is possible for that
invariant to be violated, likely including due to bugs in previous
versions of data import/export tools.
While it'd be ideal for such messages to be rendered to fix the
invariant, it doesn't make sense for this has_link migration to crash
because of such a corrupted row, so we apply the similar policy we
already have for rendered_content="".
This avoids risk of OOM issues on servers with relatively limited RAM
and millions of messages of history; apparently, fetching all messages
ordered by ID could be quite memory-intensive even with an iterator
usage model.
Fortunately, we have other migrations that already follow this pattern
of iterating over messages, so it's easy to borrow existing code to
make this migration run reasonably.
This fixes a few minor issues with the migration:
* Skips messages with empty rendered_content, fixing an exception that
affected 4 messages on chat.zulip.org.
* Accesses messages in order.
* Provides some basic output on the progress made.
This should make life substantially better for any organizations that
run into trouble with this migration, either due to it taking a long
time to run or due to any new exceptions.
