#434

backend/app/services/counts_export_download.rb

@@ -13,6 +13,46 @@ class CountsExportDownload
   class TimedOut < StandardError; end
   class StorageFull < StandardError; end
 
+  # Rack body deliberately has no to_path: sendfile middleware must not bypass ACK.
+  class ResponseBody
+    def initialize(path, &state)
+      File.open(path, 'rb') { |file| file.stat }
+      @path = path
+      @stream = nil
+      @closed = false
+      @state = state
+    end
+
+    def each
+      return if @closed
+
+      @stream = File.open(@path, 'rb')
+      first = true
+      while (chunk = @stream.read(64 * 1024))
+        raise IOError, 'Download no longer authorised' unless @state.call('active')
+
+        yield chunk
+        if first
+          # This confirms yielding to Rack, never browser receipt or saving.
+          @state.call('started')
+          first = false
+        end
+      end
+      @state.call('failed') if first
+    rescue StandardError => error
+      Rails.logger.warn("counts_download_stream_failure: #{ error.class }")
+      @state.call('failed')
+      raise
+    ensure
+      close
+    end
+
+    def close
+      @closed = true
+      @stream.close if @stream && !@stream.closed?
+    end
+  end
+
   def self.directory
     Rails.root.join(ENV.fetch('COUNTS_EXPORT_DIRECTORY', 'tmp/counts_exports'))
   end