R/Shiny app which (re)publishes an RSS feed

app.R

# Shiny app that serves a transformed RSS feed immediately at "/".
# It fetches the source RSS on every request, appends a single space to the end
# of each line, and returns it with the correct RSS content type so feed readers
# can subscribe directly.

# This particular app fixes the annoying problem in Inoreader, which concatenates
# the words in the descriptions of new R packages.

library(shiny)

SOURCE_URL <- "https://dirk.eddelbuettel.com/cranberries/cran/new/index.rss"

fetch_and_transform <- function(url = SOURCE_URL) {
  con <- 
    url(url, "rb")
  on.exit(close(con), add = TRUE)
  lines <- 
    c(paste0('<!-- ',Sys.time(),' -->'),
      readLines(con, warn = FALSE, encoding = "UTF-8"))
  paste0(lines, " ", collapse = "\n")
}

# UI is a function so we can return a raw HTTP response instead of HTML
ui <- function() {
  
  content <- 
    tryCatch(fetch_and_transform(), error = function(e) e)
  
  if (inherits(content, "error")) 
    httpResponse(
      status = 502,
      content_type = "text/plain; charset=UTF-8",
      content = paste0("Upstream fetch failed: ", conditionMessage(content))
    ) else
    httpResponse(
      status = 200,
      content_type = "application/rss+xml; charset=UTF-8",
      content = content
    )
}

server <- function(input, output, session) {}

shinyApp(ui, server)