How to simulate error for http requests in R application

I was working on shiny R application and the app was using xmlParse(path) to parse url from web API I was using. I needed to add error handling to respond to 404 and 500 errors, so I’ve changed my code to use httr library. That way I could use two steps process of getting xml from API.

My code look like this:

      res <- GET(url)
      if (res$status_code == 500) {
        message <- "Error From API" 
        simpleFatalError(message)
        stop(message)
      } else if (res$status_code == 404) {
        message <- paste("Study", .session$sid, "not found")
        simpleFatalError(message)
        stop(message)
      } else if (res$status_code != 200) {
        message <- paste(
          "Unknown response code",
          res$status_code,
          "for Study",
          .session$sid
        )
        simpleFatalError(message)
        stop(message)
      }
      studyXML <- tryCatch(
        xmlRoot(xmlTreeParse(
          content(res, "text", encoding = "UTF-8"),
          useInternalNodes=TRUE
        )),
        error = function(e) {
           simpleFatalError("parse error")
        }
      )

and now I had the problem how to simulate 500 error because that error in the API that was needed to be handled was fixed by the API team. So what I’ve did was installing Fiddler I work on windows if you work on Linux better would be to use Burp Suite.

And I used Fiddler script which seems simpler solution because it was just one line of code:

    static function OnBeforeResponse(oSession: Session) {
        if (m_Hide304s && oSession.responseCode == 304) {
            oSession["ui-hide"] = "true";
        }
        if (oSession.url.Contains("study.xml")) { // this was part of the url that returned xml from API
            oSession.oRequest.FailSession(500, "Blocked", "Fiddler blocked this file");
        }
    }

But I had the problem that Fiddler was not handling my backed requests. And it turn out that I needed to enable proxy for httr which is as simple as:

res <- GET(path, use_proxy("127.0.0.1", 8888))

Then I was able to test my connecting to the API, and get proper error in shiny app.