#434

backend/app/services/counts_export_download.rb

@@ -15,26 +15,38 @@ class CountsExportDownload
 
   # 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
+    def initialize body, &state
+      @body = body
       @closed = false
       @state = state
     end
 
-    def each
+    def each &block
+      return enum_for(:each) unless block
       return if @closed
 
-      @stream = File.open(@path, 'rb')
+      transmit(&block)
+    end
+
+    def close
+      return if @closed
+
+      @closed = true
+      @body.close if @body.respond_to?(:close)
+    end
+
+    private
+
+    def transmit
       first = true
-      while (chunk = @stream.read(64 * 1024))
+      @body.each do |chunk|
         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')
+          # Rack accepted a chunk; browser receipt and saving are not guaranteed.
+          raise IOError, 'Download acknowledgement failed' unless @state.call('started')
+
           first = false
         end
       end
@@ -46,11 +58,6 @@ class CountsExportDownload
     ensure
       close
     end
-
-    def close
-      @closed = true
-      @stream.close if @stream && !@stream.closed?
-    end
   end
 
   def self.directory