Plumber messages are sent to stderr instead of stdout

[david-cortes]

Steps to reproduce:

• Configure stdout and stderr to output to different places.
• Launch plumber through Rscript with a file like this:

library(plumber)
r <- plumb("some_file.R")
r$run(host = host, port = port)

• Check the standard output and standard error that is produced.

Expected behavior: information messages such as

Starting server to listen on port 1234

should go into standard output.

Actual behavior: information messages go into standard error instead.

[schloerke]

Good catch!

I believe message produces stderr output. We should probably change them to cat statements where appropriate.

[aronatkins]

Prior art: shiny emits a "Listening on ..." message.
https://github.com/rstudio/shiny/blob/29d24d7e08f161d79c8f227723fbec67353b6fc9/R/server.R#L590

[atheriel]

It's also possible to catch message() signals at the application level and write them to standard output, if that's what you absolutely need:

withCallingHandlers(
  message("Informational message."),
  message = function(m) {
    cat(m$message)
    invokeRestart("muffleMessage")
  }
)

This can also be used to e.g. catch library messages and reformat them for structured logging output, something that becomes impossible when using cat() directly as proposed.