Prevent top-level comments in old scheduled topics

By default, new top-level comments will only be allowed in the latest
topic from a particular set of scheduled topics. Replies to existing
comments in old topics will still be allowed - this is just intended to
prevent the cases where an old scheduled topic gets bumped back up due
to a reply and people inadvertently start adding new top-level comments
to it instead of the latest one.

This should be the correct behavior for most scheduled topics, but it
can be disabled for a particular schedule if needed.

tildes/alembic/versions/0435c46f64d8_topic_schedule_add_only_new_top_level_.py

@@ -0,0 +1,32 @@
+"""topic_schedule: add only_new_top_level_comments_in_latest
+
+Revision ID: 0435c46f64d8
+Revises: 468cf81f4a6b
+Create Date: 2020-07-05 19:33:17.746617
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "0435c46f64d8"
+down_revision = "468cf81f4a6b"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        "topic_schedule",
+        sa.Column(
+            "only_new_top_level_comments_in_latest",
+            sa.Boolean(),
+            server_default="true",
+            nullable=False,
+        ),
+    )
+
+
+def downgrade():
+    op.drop_column("topic_schedule", "only_new_top_level_comments_in_latest")

tildes/tildes/models/topic/topic.py

@@ -315,6 +315,9 @@ class Topic(DatabaseModel):
         # comment:
         #  - removed topics can only be commented on by users who can remove
         #  - locked topics can only be commented on by users who can lock
+        #  - topics posted by the scheduler can only be commented in if they're the
+        #    latest topic from that schedule, or only_new_top_level_comments_in_latest
+        #    is False
         #  - otherwise, logged-in users can comment
         if self.is_removed:
             acl.extend(
@@ -336,6 +339,13 @@ class Topic(DatabaseModel):
             )
             acl.append((Deny, Everyone, "comment"))
 
+        if (
+            self.was_posted_by_scheduler
+            and self.schedule.only_new_top_level_comments_in_latest
+            and self.topic_id != self.schedule.latest_topic_id
+        ):
+            acl.append((Deny, Everyone, "comment"))
+
         acl.append((Allow, Authenticated, "comment"))
 
         # edit:

tildes/tildes/models/topic/topic_schedule.py

@@ -8,7 +8,15 @@ from typing import List, Optional
 
 from dateutil.rrule import rrule
 from jinja2.sandbox import SandboxedEnvironment
-from sqlalchemy import CheckConstraint, Column, ForeignKey, Integer, Text, TIMESTAMP
+from sqlalchemy import (
+    Boolean,
+    CheckConstraint,
+    Column,
+    ForeignKey,
+    Integer,
+    Text,
+    TIMESTAMP,
+)
 from sqlalchemy.orm import relationship
 from sqlalchemy.orm.session import Session
 from sqlalchemy.sql.expression import text
@@ -52,6 +60,9 @@ class TopicSchedule(DatabaseModel):
         TIMESTAMP(timezone=True), nullable=True, index=True
     )
     recurrence_rule: Optional[rrule] = Column(RecurrenceRule, nullable=True)
+    only_new_top_level_comments_in_latest: bool = Column(
+        Boolean, nullable=False, server_default="true"
+    )
     latest_topic_id: int = Column(Integer, ForeignKey("topics.topic_id"), nullable=True)
 
     group: Group = relationship("Group", innerjoin=True)

tildes/tildes/templates/topic.jinja2

@@ -288,6 +288,13 @@
       </div>
     </form>
   </section>
+{% elif topic.was_posted_by_scheduler
+    and topic.schedule.only_new_top_level_comments_in_latest
+    and topic != topic.schedule.latest_topic %}
+  <div class="divider"></div>
+  <p class="text-small">This is an older topic in a scheduled series, and is closed to new top-level comments.</p>
+
+  <p class="text-small">To respond, <a href="{{ topic.schedule.latest_topic.permalink }}">go to the most recent topic in the series</a> instead.</p>
 {% endif %}
 
 </article>