FEATURE: Inline topic summary. Cached version accessible to everyone. (#22551)

* FEATURE:  Inline topic summary. Cached version accessible to everyone.

Anons and non-members of the `custom_summarization_allowed_groups_map` groups can see cached summaries for any accessible topic. After the first 12 hours and if the posts to summarize have changed, allowed users clicking on the button will automatically re-generate it.

* Ensure chat summaries work and prevent model hallucinations when there are no messages.

app/services/topic_summarization.rb

@@ -5,33 +5,36 @@ class TopicSummarization
     @strategy = strategy
   end
 
-  def summarize(topic)
-    DistributedMutex.synchronize("toppic_summarization_#{topic.id}") do
-      existing_summary = SummarySection.find_by(target: topic, meta_section_id: nil)
-      return existing_summary.summarized_text if existing_summary
+  def summarize(topic, user)
+    existing_summary = SummarySection.find_by(target: topic, meta_section_id: nil)
 
-      content = {
-        resource_path: "#{Discourse.base_path}/t/-/#{topic.id}",
-        content_title: topic.title,
-        contents: [],
-      }
+    # For users without permissions to generate a summary, we return what we have cached.
+    # Existing summary shouldn't be nil in this scenario because the controller checks its existence.
+    return existing_summary if !user || !Summarization::Base.can_request_summary_for?(user)
 
-      targets_data = summary_targets(topic).pluck(:post_number, :raw, :username)
+    return existing_summary if existing_summary && fresh?(existing_summary, topic)
 
-      targets_data.map do |(pn, raw, username)|
-        content[:contents] << { poster: username, id: pn, text: raw }
-      end
+    delete_cached_summaries_of(topic) if existing_summary
 
-      summarization_result = strategy.summarize(content)
+    content = {
+      resource_path: "#{Discourse.base_path}/t/-/#{topic.id}",
+      content_title: topic.title,
+      contents: [],
+    }
 
-      cached_summary(summarization_result, targets_data.map(&:first), topic)
+    targets_data = summary_targets(topic).pluck(:post_number, :raw, :username)
 
-      summarization_result[:summary]
+    targets_data.map do |(pn, raw, username)|
+      content[:contents] << { poster: username, id: pn, text: raw }
     end
+
+    summarization_result = strategy.summarize(content)
+
+    cache_summary(summarization_result, targets_data.map(&:first), topic)
   end
 
   def summary_targets(topic)
-    topic.has_summary? ? best_replies(topic) : pick_selection(topic)
+    @targets ||= topic.has_summary? ? best_replies(topic) : pick_selection(topic)
   end
 
   private
@@ -66,7 +69,18 @@ class TopicSummarization
       .order(:post_number)
   end
 
-  def cached_summary(result, post_numbers, topic)
+  def delete_cached_summaries_of(topic)
+    SummarySection.where(target: topic).destroy_all
+  end
+
+  def fresh?(summary, topic)
+    return true if summary.created_at > 12.hours.ago
+    latest_post_to_summarize = summary_targets(topic).last.post_number
+
+    latest_post_to_summarize <= summary.content_range.to_a.last
+  end
+
+  def cache_summary(result, post_numbers, topic)
     main_summary =
       SummarySection.create!(
         target: topic,
@@ -86,5 +100,7 @@ class TopicSummarization
         meta_section_id: main_summary.id,
       )
     end
+
+    main_summary
   end
 end