DEV: re-implement bulk sentiment classifier (#1449)

New implementation uses core concurrent job queue, it is more
robust and predictable than the one shipped in Concurrent.

Additionally:

- Trickles through updates during bulk classification
- Reports errors if we fail during a bulk classification

* push concurrency down to 40. 100 feels quite high.

lib/sentiment/post_classification.rb

@@ -44,57 +44,69 @@ module DiscourseAi
         Post.from(Arel.sql("(#{unioned_queries}) as posts"))
       end
 
+      CONCURRENT_CLASSFICATIONS = 40
+
       def bulk_classify!(relation)
-        http_pool_size = 100
         pool =
-          Concurrent::CachedThreadPool.new(
+          Scheduler::ThreadPool.new(
             min_threads: 0,
-            max_threads: http_pool_size,
-            idletime: 30,
+            max_threads: CONCURRENT_CLASSFICATIONS,
+            idle_time: 30,
           )
 
         available_classifiers = classifiers
         return if available_classifiers.blank?
 
-        promised_classifications =
-          relation
-            .map do |record|
-              text = prepare_text(record)
-              next if text.blank?
+        results = Queue.new
+        queued = 0
 
-              Concurrent::Promises
-                .fulfilled_future({ target: record, text: text }, pool)
-                .then_on(pool) do |w_text|
-                  results = Concurrent::Hash.new
-                  already_classified = w_text[:target].sentiment_classifications.map(&:model_used)
+        relation.each do |record|
+          text = prepare_text(record)
+          next if text.blank?
 
-                  classifiers_for_target =
-                    available_classifiers.reject do |ac|
-                      already_classified.include?(ac[:model_name])
-                    end
+          already_classified = record.sentiment_classifications.pluck(&:model_used)
+          missing_classifiers =
+            available_classifiers.reject { |ac| already_classified.include?(ac[:model_name]) }
 
-                  promised_target_results =
-                    classifiers_for_target.map do |cft|
-                      Concurrent::Promises.future_on(pool) do
-                        results[cft[:model_name]] = request_with(cft[:client], w_text[:text])
-                      end
-                    end
-
-                  Concurrent::Promises
-                    .zip(*promised_target_results)
-                    .then_on(pool) { |_| w_text.merge(classification: results) }
-                end
-                .flat(1)
+          missing_classifiers.each do |classifier|
+            pool.post do
+              result = { target: record, classifier: classifier, text: text }
+              begin
+                result[:classification] = request_with(classifier[:client], text)
+              rescue StandardError => e
+                result[:error] = e
+              end
+              results << result
             end
-            .compact
+            queued += 1
+          end
+        end
 
-        Concurrent::Promises
-          .zip(*promised_classifications)
-          .value!
-          .each { |r| store_classification(r[:target], r[:classification]) }
+        errors = []
+
+        while queued > 0
+          result = results.pop
+          if result[:error]
+            errors << result
+          else
+            store_classification(
+              result[:target],
+              [[result[:classifier][:model_name], result[:classification]]],
+            )
+          end
+          queued -= 1
+        end
+
+        if errors.any?
+          example_posts = errors.map { |e| e[:target].id }.take(5).join(", ")
+          Discourse.warn_exception(
+            errors[0][:error],
+            "Discourse AI: Errors during bulk classification: Failed to classify #{errors.count} posts (example ids: #{example_posts})",
+          )
+        end
       ensure
         pool.shutdown
-        pool.wait_for_termination
+        pool.wait_for_termination(timeout: 30)
       end
 
       def classify!(target)