renv fails when HTTPUserAgent is improperly set

[arcresu]

I encountered this issue when using the rstudio binary builds of R on Debian unstable, which is not an officially supported platform. As such this is a bit niche and might not be worth addressing.

The result is the equivalent of running options(HTTPUserAgent = character(0)) (see rstudio/r-builds#196 for background).

In this case, every attempt to run install() with renv 1.0.3 fails with:

options(HTTPUserAgent = character(0))
renv::install("tidyverse")
#> Error in if (!grepl("renv", agent)) {: argument is of length zero

^{Created on 2024-01-09 with reprex v2.0.2}

This is because of this internal function:

renv/R/http.R

Lines 2 to 5 in de3af61

	renv_http_useragent <- function() {
	agent <- getOption("renv.http.useragent", default = getOption("HTTPUserAgent"))
	agent %||% renv_http_useragent_default()
	}

The fix could be to set agent to NULL if it is equal to character(0). However, feel free to close this since it's probably not a likely scenario that others will run into unless they are using the rstudio R binaries on Debian testing or unstable.

[kevinushey]

In the places where the user agent is used internally, R seems to expect it to be either NULL or a length one character vector, e.g.

https://github.com/wch/r-source/blob/e13caf6d43653e2b3fbc513bbaf6fdd6ee93b455/src/library/utils/R/readhttp.R#L39-L48

We could make renv robust against this, but I think this needs to be changed upstream.