Release Zulip Server 2.1.8.

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="".

docs/overview/changelog.md

@@ -7,6 +7,11 @@ All notable changes to the Zulip server are documented in this file.
 This section lists notable unreleased changes; it is generally updated
 in bursts.
 
+### 2.1.8 -- 2021-08-11
+
+- Fixed possible `0257_fix_has_link_attribute.py` database migration
+  failure, which would cause errors during the upgrade process.
+
 ### 2.1.7 -- 2020-06-25
 
 - CVE-2020-15070: Fix privilege escalation vulnerability with custom

version.py

@@ -1,6 +1,6 @@
 import os
 
-ZULIP_VERSION = "2.1.7"
+ZULIP_VERSION = "2.1.8"
 # Add information on number of commits and commit hash to version, if available
 zulip_git_version_file = os.path.join(os.path.dirname(os.path.abspath(__file__)), 'zulip-git-version')
 if os.path.exists(zulip_git_version_file):
@@ -10,7 +10,7 @@ if os.path.exists(zulip_git_version_file):
             ZULIP_VERSION = version
 
 LATEST_MAJOR_VERSION = "2.1"
-LATEST_RELEASE_VERSION = "2.1.7"
+LATEST_RELEASE_VERSION = "2.1.8"
 LATEST_RELEASE_ANNOUNCEMENT = "https://blog.zulip.org/2019/12/13/zulip-2-1-released/"
 
 # Versions of the desktop app below DESKTOP_MINIMUM_VERSION will be

zerver/migrations/0257_fix_has_link_attribute.py

@@ -13,9 +13,9 @@ BATCH_SIZE = 1000
 def process_batch(apps: StateApps, id_start: int, id_end: int, last_id: int) -> None:
     Message = apps.get_model('zerver', 'Message')
     for message in Message.objects.filter(id__gte=id_start, id__lte=id_end).order_by("id"):
-        if message.rendered_content == "":
+        if message.rendered_content in ["", None]:
             # There have been bugs in the past that made it possible
-            # for a message to have "" as its rendered_content; we
+            # for a message to have "" or None as its rendered_content; we
             # need to skip those because lxml won't process them.
             #
             # They should safely already have the correct state