How do I correctly close a connection in R, so its connection 'slot' gets released?

The easiest way to avoid problems like this is to explicitly close the connection when you're done with it. In R, the easiest way to do that is to use on.exit() which will ensure the url gets closed even if an error occurs in your code

read_url <- function(url, ...) {  on.exit(close(url))  readLines(url, ...)}showConnections()g <- read_url("http://www.google.com")showConnections()

For me Hadley's answer did not work because of two reasons: 1) close is an S3 generic and has no method for character objects (maybe it has changed since the answer?); 2) My connection was not in the table returned by showConnections. It was a curl type connection left open by reader::read_tsv after encountering an SSL expired certificate error. If we get the warnings about connections created by third party packages, we have to obtain the connection object to be able to close them. I wrote two little function for this purpose. You see these import more packages than definitely needed, this is because I designed them for a package, but you can easily remove these dependencies.

library(magrittr)library(tibble)library(dplyr)library(purrr)#' Retrieve the open connection(s) pointing to URI#'#' @param uri Character: path or URL the connection points to.#'#' @return A list of connection objects.#'#' @importFrom magrittr %>%#' @importFrom tibble rownames_to_column#' @importFrom dplyr filter pull#' @importFrom purrr map#' @noRdget_connections <- function(uri){    showConnections(all = TRUE) %>%    as.data.frame %>%    rownames_to_column('con_id') %>%    filter(description == uri) %>%    pull(con_id) %>%    as.integer %>%    map(getConnection)}#' Closes the open connection(s) pointing to URI#'#' @param uri Character: path or URL the connection points to.#'#' @return Invisible `NULL`.#'#' @importFrom magrittr %>%#' @importFrom purrr walk#' @noRdclose_connection <- function(uri){    uri %>%    get_connections %>%    walk(close)    invisible(NULL)}

Having the functions above, you can do as Hadley has shown:

my_function <- function(my_url, ...) {    on.exit(close_connection(my_url))}