Passing a string to a handler function in Go - http

Question

I have a generic Webserver which I want to use on different domains / servers.
For setting up each server I simply read a JSON config file with all necessary information. One would be for example the redirect for all traffic which reaches port 80 and forward it to a TLS service. Since I don't want to make the config object global. How can I pass the content from my inputFromConfigFile to the redirectTLS function?
Here is an example:
func main(){
var inputFromConfigFile = "https://www.example.com:443"
go func() {
if err := http.ListenAndServe(":80", http.HandlerFunc(redirectTLS)); err != nil {
log.Fatalf("ListenAndServe error: %v", err)
}
}()
}
//Pass the above string to this function:
func redirectTLS(w http.ResponseWriter, r *http.Request) {
http.Redirect(w, r, "https://www.example.com:443"+r.RequestURI,http.StatusMovedPermanently)
}

Answer 1

You can define a custom Handler (could be implemented as a struct) as long as it matches the http.Handler interface. The config could be saved inside the Handler as a struct field.
type Handler struct {
// config goes here
}
func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
// anything that handler needs to do here
}
Example: https://pkg.go.dev/net/http#example-Handle

Answer 2

You can define redirectTLS as an inline closure function directly in main:
var inputFromConfigFile = "https://www.example.com:443"
go func() {
err := http.ListenAndServe(":80", func(w http.ResponseWriter, r *http.Request) {
http.Redirect(w, r, inputFromConfigFile+r.RequestURI, http.StatusMovedPermanently)
})
if err != nil {
log.Fatalf("ListenAndServe error: %v", err)
}
}()

Answer 3

I would make the config object global.
Otherwise, you can define a function that accepts the config as an argument, and returns a handler function that closes over the configuration object:
var inputFromConfigFile = "https://www.example.com:443"
http.ListenAndServe(":80", createHandler(inputFromConfigFile))
// ...
func createHandler(config string) http.HandlerFunc {
return http.HandlerFunc(func (w http.ResponseWriter, r *http.Request) {
http.Redirect(w, r, config+r.RequestURI,http.StatusMovedPermanently)
})
}