Prevent inserting subsequent topic visits

This should prevent a few strange behaviors related to topic visits,
such as "losing" new comments if you accidentally double-click when
entering a topic's comments.

Currently, the "grace period" is set to 2 minutes, and no new visits
will be stored until the previous visit is at least that old.

tildes/alembic/versions/34753d8124b4_prevent_inserting_subsequent_topic_.py

@@ -0,0 +1,51 @@
+"""Prevent inserting subsequent topic_visits
+
+Revision ID: 34753d8124b4
+Revises: 19400b1efe8b
+Create Date: 2020-01-30 19:37:55.095357
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "34753d8124b4"
+down_revision = "19400b1efe8b"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.execute(
+        """
+        create or replace function prevent_recent_repeat_visits() returns trigger as $$
+        begin
+            perform * from topic_visits
+                where user_id = NEW.user_id
+                    and topic_id = NEW.topic_id
+                    and visit_time >= now() - interval '2 minutes';
+
+            if (FOUND) then
+                return null;
+            else
+                return NEW;
+            end if;
+        end;
+        $$ language plpgsql;
+    """
+    )
+
+    op.execute(
+        """
+        create trigger prevent_recent_repeat_visits_insert
+            before insert on topic_visits
+            for each row
+            execute procedure prevent_recent_repeat_visits();
+    """
+    )
+
+
+def downgrade():
+    op.execute("drop trigger prevent_recent_repeat_visits_insert on topic_visits")
+    op.execute("drop function prevent_recent_repeat_visits")

tildes/sql/init/triggers/topic_visits/topic_visits.sql

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+-- Copyright (c) 2020 Tildes contributors <code@tildes.net>
+-- SPDX-License-Identifier: AGPL-3.0-or-later
+
+-- prevent inserting another visit immediately after an existing one
+create or replace function prevent_recent_repeat_visits() returns trigger as $$
+begin
+    perform * from topic_visits
+        where user_id = NEW.user_id
+            and topic_id = NEW.topic_id
+            and visit_time >= now() - interval '2 minutes';
+
+    if (FOUND) then
+        return null;
+    else
+        return NEW;
+    end if;
+end;
+$$ language plpgsql;
+
+
+create trigger prevent_recent_repeat_visits_insert
+    before insert on topic_visits
+    for each row
+    execute procedure prevent_recent_repeat_visits();