File source for mran url not supported

[mungojam]

I tried to repoint checkpoint to a file url ('file:///'), but it failed in two places: is.404 function and getValidSnapshots because both make use of readLines which doesn't work for file urls (when they are pointing to folders anyway). This code gives the error: cannot open file 'C:/Local/R/MRAN/': Permission denied:

options(warn = 2) #treat warnings as errors since we don't expect any

#Sets up the checkpoint package which ensures that we
#get packages from a tested snapshot date i.e. different
#users of the script all use exactly the same package versions
SetupCheckpoint <- function() {
  message("Setting up checkpoint")

  options(checkpoint.mranUrl = "file:///C:/Local/R/MRAN/")

    #Get packages as of the following, tested snapshot date
  librarySnapshot <- "2016-03-09"
  R.version <- "3.2.3"

  checkpointLocation <- file.path(Sys.getenv("USERPROFILE"), "R")

  if(!dir.exists(file.path(checkpointLocation, ".checkpoint"))) {
    dir.create(file.path(checkpointLocation, ".checkpoint"), recursive = TRUE)
  }

  checkpoint(librarySnapshot, 
             R.version = R.version, 
             checkpointLocation = checkpointLocation)

}

SetupCheckpoint()

[mungojam]

#218 Pull request created