Default reactive domain not accessible in downloadHandler callback

[wch]

The function getDefaultReactiveDomain() should return a session object, which is an environment. This is what happens in regular reactives and observers, but in a downloadHandler callback, it simply returns NULL.

Example:

shinyApp(
  ui = bootstrapPage(
    downloadLink('download'),
    textOutput('txt')
  ),
  server = function(input, output) {
    output$download <- downloadHandler(
      'foo.txt',
      function(file) {
        # domain is NULL
        domain <- getDefaultReactiveDomain()
        print(domain)

        writeLines(format(domain), con = file)
      }
    )

    # Works here
    output$txt <- renderPrint({
      getDefaultReactiveDomain()
    })
  }
)

[wch]

A workaround for now is to pass session to withProgress and incProgress:

shinyApp(
  ui = bootstrapPage(
    downloadLink('download')
  ),
  server = function(input, output, session) {
    output$download <- downloadHandler(
      'foo.txt',
      function(file) {
        withProgress({
          incProgress(session = session)

          writeLines("test test", con = file)

          incProgress(session=session)
        }, session = session)
      }
    )
  }
)