Creating a rvest::html_session to a servr::httd http server - r

Question

I'm working on a project that requires accessing webpages and I do this via
rvest::html_session(). For documentation and training I would like to set
up a reproducible example and have considered the following.
Use servr::httd(system.file("egwebsite", package = "<pkgname>"), daemon =
TRUE, browser = FALSE) to set up a simple HTTP server
Use rvest::html_session("http://127.0.0.1:4321") to set up the html
session.
However, the following simple example behaves differently on Linux (Debian 9)
and Windows 10. (I do not have easy access to OSx and have not tested on
that OS).
# On Windows
servr::httd(daemon = TRUE, browser = FALSE, port = 4321)
## Serving the directory /home/dewittpe/so/my-servr-question at http://127.0.0.1:4321
## To stop the server, run servr::daemon_stop("94019719908480") or restart your R session
R.utils::withTimeout(
{
s <- rvest::html_session("http://127.0.0.1:4321")
},
timeout = 3,
onTimeout = "error")
s
## <session> http://127.0.0.1:4321/
## Status: 200
## Type: text/html
## Size: 2352
servr::daemon_stop()
However, on my Linux box (Debian 9) I get the following
servr::httd(daemon = TRUE, browser = FALSE, port = 4321)
## Serving the directory /home/dewittpe/so/my-servr-question at http://127.0.0.1:4321
## To stop the server, run servr::daemon_stop("94019719908480") or restart your R session
R.utils::withTimeout(
{
s <- rvest::html_session("http://127.0.0.1:4321")
},
timeout = 3,
onTimeout = "error")
## Error: reached elapsed time limit
## Error in curl::curl_fetch_memory(url, handle = handle) :
## Operation was aborted by an application callback
That is, I am unable to create a html_session in the same R interactive
session that spawned the http server. If, however, I start a second R
session while the leaving the initial session running, I am able to create
the html_session without error.
What can I do so that I can create an html_session based on a servr::httd
HTTP server within the same R session on Linux?
Edit 1
If I add httr::verbose() to the html_session call I get the following when the session is created successfully. When the process hangs and fails to create the session the output stops on the last -> and none of the lines with <- are shown.
> s <- html_session("http://127.0.0.1:4321", httr::verbose())
-> GET / HTTP/1.1
-> Host: 127.0.0.1:4321
-> User-Agent: libcurl/7.52.1 r-curl/3.1 httr/1.3.1
-> Accept-Encoding: gzip, deflate
-> Accept: application/json, text/xml, application/xml, */*
->
<- HTTP/1.1 200 OK
<- Content-Type: text/html
<- Content-Length: 61303
<-

Answer 1

I have found a solution to my problem, run servr::httd in a subprocess. This solution requires the subprocess package.
First, a helper function R_binary will return the file path for the R binary on Windows or unix based OS.
R_binary <- function () {
R_exe <- ifelse (tolower(.Platform$OS.type) == "windows", "R.exe", "R")
return(file.path(R.home("bin"), R_exe))
}
Next, start R vanilla as a subprocess.
subR <- subprocess::spawn_process(R_binary(), c("--vanilla"))
Then start the HTTP server in the subprocess
subprocess::process_write(subR, 'servr::httd(".", browser = FALSE, port = 4321)\n')
## [1] 47
subprocess::process_read(subR)$stderr
## [1] "Serving the directory /home/dewittpe/so/my-servr-question at http://127.0.0.1:4321"
A quick test to show that there is communication between the active R session and the HTTP server:
session <- rvest::html_session("http://127.0.0.1:4321")
session
## <session> http://127.0.0.1:4321/
## Status: 200
## Type: text/html
## Size: 1054
And finally, kill the subprocess
subprocess::process_kill(subR)