Reuse authorized read pages without stale presentation or CSRF masks

Transfer the C completed-response cache lesson into Rails, keeping authentication, room checks and cookies per request. A persistent read-only SQLite observer detects local and foreign commits and rejects racing admission. Whole-page misses render fresh to avoid stale nested fragments; message ETags reflect token-neutral presentation.

app/models/response_cache.rb

@@ -0,0 +1,89 @@
+require "sqlite3"
+
+# A read-only observer sees commits from every writer, including other runtimes.
+# PRAGMA data_version can only be compared on the same persistent connection.
+class ResponseCache
+  MAX_ENTRY_BYTES = 1.megabyte
+  MAX_KEY_BYTES = 2.kilobytes
+
+  def self.instance
+    @instance ||= new
+  end
+
+  def initialize
+    @mutex = Mutex.new
+    @entries = {}
+    @bytes = 0
+  end
+
+  def budget
+    [ ENV.fetch("CAMPFIRE_RESPONSE_CACHE_MB", "64").to_i, 0 ].max.megabytes
+  end
+
+  def version
+    @mutex.synchronize { current_version }
+  rescue SQLite3::Exception
+    clear
+    nil
+  end
+
+  def read(key, version)
+    @mutex.synchronize do
+      @entries[key]&.first if current_version == version
+    end
+  rescue SQLite3::Exception
+    clear
+    nil
+  end
+
+  def write(key, version, entry)
+    size = key.bytesize + entry[:body].bytesize + entry[:headers].sum { |name, value| name.bytesize + value.bytesize } + 256
+    return if key.bytesize > MAX_KEY_BYTES || size > [ budget, MAX_ENTRY_BYTES ].min
+
+    @mutex.synchronize do
+      return unless current_version == version
+      return if @entries.key?(key)
+
+      while @entries.any? && @bytes + size > budget
+        _, (_, removed_size) = @entries.shift
+        @bytes -= removed_size
+      end
+      @entries[key] = [ entry, size ]
+      @bytes += size
+    end
+  rescue SQLite3::Exception
+    clear
+  end
+
+  def clear
+    @mutex.synchronize do
+      @entries.clear
+      @bytes = 0
+      @observer&.close
+      @observer = @database = @version = nil
+    end
+  end
+
+  private
+    def current_version
+      database = File.expand_path(ActiveRecord::Base.connection_db_config.database)
+      if @database != database || !@observer
+        @entries.clear
+        @bytes = 0
+        @observer&.close
+        @observer = nil
+        @observer = SQLite3::Database.new(database, readonly: true)
+        @database = database
+        @namespace = SecureRandom.hex(16)
+        @version = nil
+      end
+
+      version = @observer.get_first_value("PRAGMA data_version")
+      if @version != version
+        @entries.clear
+        @bytes = 0
+        @version = version
+      end
+      [ @database, @namespace, @version ]
+    end
+end