Add an error page for non-existent groups

Uses the pg_trgm extension (trigrams) to look up groups with similar
names to the one that the user tried to visit and offers them as
suggestions.

salt/salt/postgresql/site-db.sls

@@ -39,6 +39,13 @@ site-db-enable-pg_stat_statements:
     - require:
       - postgres_database: tildes
 
+site-db-enable-pg_trgm:
+  postgres_extension.present:
+    - name: pg_trgm
+    - maintenance_db: tildes
+    - require:
+      - postgres_database: tildes
+
 site-db-pg_hba-lines:
   file.accumulated:
     - name: pg_hba_lines

salt/salt/postgresql/test-db.sls

@@ -26,6 +26,13 @@ test-db-enable-intarray:
     - require:
       - postgres_database: tildes_test
 
+test-db-enable-pg_trgm:
+  postgres_extension.present:
+    - name: pg_trgm
+    - maintenance_db: tildes_test
+    - require:
+      - postgres_database: tildes_test
+
 test-db-pg_hba-lines:
   file.accumulated:
     - name: pg_hba_lines

tildes/tildes/templates/error_group_not_found.jinja2

@@ -0,0 +1,27 @@
+{% extends 'base_no_sidebar.jinja2' %}
+
+{% from 'macros/links.jinja2' import group_linked %}
+
+{% block title %}
+Group not found
+{% endblock %}
+
+{% block content %}
+
+<div class="empty">
+  <h2 class="empty-title">No group named '{{ supplied_name }}'</h2>
+  {% if group_suggestions %}
+    <p class="empty-subtitle">
+      Did you mean one of these groups instead?
+      <br><br>
+      {% for group in group_suggestions %}
+        {{ group_linked(group) }}
+        <br>
+      {% endfor %}
+    </p>
+  {% endif %}
+  <br><br>
+  <a href="/groups" class="btn btn-primary">Browse the list of groups</a>
+</div>
+
+{% endblock %}

tildes/tildes/views/exceptions.py

@@ -1,7 +1,11 @@
 """Views used by Pyramid when an exception is raised."""
 
+from pyramid.httpexceptions import HTTPNotFound
 from pyramid.request import Request
-from pyramid.view import forbidden_view_config
+from pyramid.view import forbidden_view_config, exception_view_config
+from sqlalchemy import cast, desc, func, Text
+
+from tildes.models.group import Group
 
 
 @forbidden_view_config(xhr=False, renderer="error_403.jinja2")
@@ -9,3 +13,21 @@ def forbidden(request: Request) -> dict:
     """403 Forbidden page."""
     request.response.status_int = 403
     return {}
+
+
+@exception_view_config(
+    HTTPNotFound, route_name="group", renderer="error_group_not_found.jinja2"
+)
+def group_not_found(request: Request) -> dict:
+    """Show the user a customized 404 page for group names."""
+    request.response.status_int = 404
+    supplied_name = request.matchdict.get("group_path")
+    # the 'word_similarity' function here is from the 'pg_trgm' extension
+    group_suggestions = (
+        request.query(Group)
+        .filter(func.word_similarity(cast(Group.path, Text), supplied_name) > 0)
+        .order_by(desc(func.word_similarity(cast(Group.path, Text), supplied_name)))
+        .limit(3)
+        .all()
+    )
+    return {"group_suggestions": group_suggestions, "supplied_name": supplied_name}