Pre Parsed Multi Value Headers not supported

[ChronosMasterOfAllTime]

Problem

HTTP headers by nature are multi value in the http.Request type.

When you are doing the Access-Control-Request-Headers parse in the preflight checks, you assume it will always be a singleton. However, when using libraries such as the aws-lambda-go-api-proxy, it parses the headers into a multi value format before passing the request to chi. See splitSingletonHeaders as this tells the proxy how to handle the headers (The actual splitting work is done on requestV2#180). Once it does this, the values are already pre-split before it reaches the CORS middleware.

The cors library always expects things to be in a single string, comma delimited format. By this nature, the Get call on the http.Header type will only return the first value and ignore the rest. This was causing our CORS preflight to fail as the response Access-Control-Allow-Headers were missing all the values.

Solution Use the Values method and concatenate all the requestHeaders. It guarantees backwards compatibility and won't be a breaking change. I added test cases to fix this in PR #35 .

[ChronosMasterOfAllTime]

@VojtechVitek @pkieltyka I have created an issue and already pushed the fix PR. Can we please get this reviewed? I ensured this would maintain the existing functionality to not be a breaking change.

[ChronosMasterOfAllTime]

For anyone that comes here, here is the current workaround:

1. set OptionsPassthrough: true on your cors.Options input. What does this do? It tells the Chi CORS middleware to let your own downstream processor to continue to process the request after it has run.
2. Add this middleware function immediately after the cors middleware as defined below

func(next http.Handler) http.Handler {
    // This middleware is temporary until chi cors middleware is fixed
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        if r.Method == http.MethodOptions {
            headers := r.Header.Values("Access-Control-Request-Headers")

            for _, header := range headers {
                if !utils.SliceContains(allowedHeaders, strings.ToLower(header)) {
                    logrus.Error("Invalid header: ", header)
                    w.Header().Del("Access-Control-Allow-Headers")
                    w.WriteHeader(http.StatusForbidden)
                    return
                }
            }

            w.Header().Set("Access-Control-Allow-Headers", strings.Join(headers, ","))
            w.WriteHeader(http.StatusOK)
        } else {
            next.ServeHTTP(w, r)
        }
    })
})