DEV: Move array type custom fields to JSON type in automation (#26939)

The automation plugin has 4 custom field types that are array typed. However, array typed custom fields are deprecated and should be migrated to JSON type.

This commit does a couple of things:

1. Migrate all four custom fields to JSON
2. Fix a couple of small bugs that have been discovered while migrating the custom fields to JSON (see the comments on this commit's PR for details https://github.com/discourse/discourse/pull/26939)

plugins/automation/app/models/discourse_automation/automation.rb

@@ -34,41 +34,38 @@ module DiscourseAutomation
     MAX_NAME_LENGTH = 30
     validates :name, length: { in: MIN_NAME_LENGTH..MAX_NAME_LENGTH }
 
-    def attach_custom_field(target)
+    def add_id_to_custom_field(target, custom_field_key)
       if ![Topic, Post, User].any? { |m| target.is_a?(m) }
         raise "Expected an instance of Topic/Post/User."
       end
 
-      now = Time.now
-      fk = target.custom_fields_fk
-      row = {
-        fk => target.id,
-        :name => DiscourseAutomation::CUSTOM_FIELD,
-        :value => id,
-        :created_at => now,
-        :updated_at => now,
-      }
-
-      relation = "#{target.class.name}CustomField".constantize
-      relation.upsert(
-        row,
-        unique_by:
-          "idx_#{target.class.name.downcase}_custom_fields_discourse_automation_unique_id_partial",
-      )
+      change_automation_ids_custom_field_in_mutex(target, custom_field_key) do
+        target.reload
+        ids = Array(target.custom_fields[custom_field_key])
+        if !ids.include?(self.id)
+          ids << self.id
+          ids = ids.compact.uniq
+          target.custom_fields[custom_field_key] = ids
+          target.save_custom_fields
+        end
+      end
     end
 
-    def detach_custom_field(target)
+    def remove_id_from_custom_field(target, custom_field_key)
       if ![Topic, Post, User].any? { |m| target.is_a?(m) }
         raise "Expected an instance of Topic/Post/User."
       end
 
-      fk = target.custom_fields_fk
-      relation = "#{target.class.name}CustomField".constantize
-      relation.where(
-        fk => target.id,
-        :name => DiscourseAutomation::CUSTOM_FIELD,
-        :value => id,
-      ).delete_all
+      change_automation_ids_custom_field_in_mutex(target, custom_field_key) do
+        target.reload
+        ids = Array(target.custom_fields[custom_field_key])
+        if ids.include?(self.id)
+          ids = ids.compact.uniq
+          ids.delete(self.id)
+          target.custom_fields[custom_field_key] = ids
+          target.save_custom_fields
+        end
+      end
     end
 
     def trigger_field(name)
@@ -187,5 +184,12 @@ module DiscourseAutomation
     def validate_trigger_fields
       !triggerable || triggerable.valid?(self)
     end
+
+    def change_automation_ids_custom_field_in_mutex(target, key)
+      DistributedMutex.synchronize(
+        "automation_custom_field_#{key}_#{target.class.table_name}_#{target.id}",
+        validity: 5.seconds,
+      ) { yield }
+    end
   end
 end