Set cache control headers to avoid caching UI

[schloerke]

Fixes #3806

A PR to get the conversation started.

Root of issue:

When the UI isn't re-rendered in the R process that's serving the Shiny app, we don't have access to global theme information in the server logic. This drives the behavior seen in this issue: the server doesn't know that the UI is expecting BS4+ markup for the nav items and instead emits BS3 markup, and the mismatch results in broken client logic.

Approach:

• If a theme has been set while creating the UI and the current UI response does not have a caching policy
  • Do not allow caching of the UI

TODO:

• NEWS entry
• Test

[jcheng5]

I think we should always add the cache-busting headers. Without it, Connect and ShinyApps.io configurations will have problems in that the workerId could be a stale value.

Also, here's a little app that, while unrealistic, does break without the cache header (if you navigate away, restart the app, then navigate back). After you've done that, the UI uses a (dynamically generated) id that the server knows nothing about.

library(shiny)

id <- paste0("plot", sample.int(100000, 1))

ui <- fluidPage(
  plotOutput(id)
)

server <- function(input, output, session) {
  output[[id]] <- renderPlot({
    invalidateLater(500)
    plot(rnorm(100))
  })
}

shinyApp(ui, server)