How to Properly Set Context Values Within an `http.HandleFunc`?

Setting Context Values within HTTP Handlers

Context values provide a convenient way to pass data between handlers in a HTTP application. This Q&A demonstrates the best practice for setting context values within an http.HandleFunc.

Question:

How to effectively set a context value inside an http.HandleFunc?

Answer:

The approach presented in the question, which involves overwriting the request object, is not optimal. Instead, use the Request.WithContext method to create a shallow copy of the request and set the context value in the new copy. Return a pointer to this shallow copy.

Code:

func setValue(r *http.Request, val string) *http.Request {
  return r.WithContext(context.WithValue(r.Context(), myContext, val))
}

Handler Invocation:

When invoking another handler, pass the modified request along:

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    r = setValue(r, "foobar")
    anotherHandler.ServeHTTP(w, r)
})

This approach ensures that:

• The original request is not overwritten, avoiding potential issues with downstream code.
• The context value is correctly propagated to handlers that are called later.

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