#434

backend/app/controllers/counts_exports_controller.rb

@@ -24,6 +24,14 @@ class CountsExportsController < ApplicationController
     job_state(cancel: true)
   end
 
+  def start_download
+    result = CountsExportJobs.download_attempt(params[:token], session[:counts_export_owner])
+    return head :not_found unless result
+
+    response.headers['Cache-Control'] = 'private, no-store'
+    render json: result
+  end
+
   def prepare
     locale = resolve_locale!
     payload = request.request_parameters.merge(request.query_parameters)
@@ -45,6 +53,11 @@ class CountsExportsController < ApplicationController
   end
 
   def download
+    if params[:job].present? || params[:attempt].present?
+      return head :method_not_allowed unless request.get?
+
+      return confirmed_download
+    end
     file = CountsExportDownload.find(params[:token], session[:counts_export_owner])
     return head :not_found unless file
 
@@ -55,6 +68,47 @@ class CountsExportsController < ApplicationController
               disposition: 'attachment'
   end
 
+  def confirmed_download
+    owner = session[:counts_export_owner]
+    arguments = { file_token: params[:token], attempt: params[:attempt] }
+    return head :not_found unless arguments[:attempt].present? &&
+        CountsExportJobs.download_attempt(params[:job], owner, **arguments)
+
+    file = CountsExportDownload.find(params[:token], owner)
+    unless file
+      CountsExportJobs.download_attempt(params[:job], owner, **arguments, status: 'unavailable')
+      return head :not_found
+    end
+    # Open before committing the response, but never buffer the entire export.
+    stream = File.open(file.fetch('path'), 'rb')
+    response.headers['Cache-Control'] = 'private, no-store'
+    send_file_headers! filename: "counts-#{ file.fetch('resource') }.#{ file.fetch('format') }",
+        type: file['format'] == 'json' ? 'application/json' : 'text/csv',
+        disposition: 'attachment'
+    job = params[:job]
+    authority = HistoryAccess.user
+    self.response_body = Enumerator.new do |output|
+      begin
+        first = true
+        while (chunk = stream.read(64 * 1024))
+          output << chunk
+          if first
+            HistoryAccess.set(user: authority) do
+              CountsExportJobs.download_attempt(job, owner, **arguments, status: 'started')
+            end
+            first = false
+          end
+        end
+      ensure
+        stream.close
+      end
+    end
+  rescue IOError, SystemCallError
+    CountsExportJobs.download_attempt(params[:job], owner, **arguments, status: 'failed')
+    head :service_unavailable
+  end
+  private :confirmed_download
+
   def job_state(cancel:)
     result = CountsExportJobs.status(params[:token], session[:counts_export_owner], cancel:)
     return head :not_found unless result