How to Properly Pass Context in Middleware and Handlers in Go?

Passing Context in Middleware and Handlers in Go

Introduction

Understanding context in Go can be confusing. Let's explore how to effectively pass context to middleware and handlers.

Passing Context in Middleware

To pass context to middleware, follow these steps:

1. Derive a new context from the request context using context.WithTimeout() or context.WithValue().
2. Call ServeHTTP with the updated request, which now contains the new context.
3. In the authorization checker, add the user's information to the context before calling ServeHTTP.

For example, to add a timeout to the request:

ctx, cancel := context.WithTimeout(r.Context(), time.Duration(60*time.Second))
defer cancel()
r = r.WithContext(ctx)

Passing Context in Handlers

To pass context to handlers:

1. Use context.WithValue() to add values to the request's context.
2. In the handler, use request.Context().Value() to access the values.

For example, to add the user ID to the context:

ctx := context.WithValue(r.Context(), ContextUserKey, "theuser")
h.ServeHTTP(w, r.WithContext(ctx))

Example Code

Here's an example implementation of middleware and a handler using context:

func checkAuth(authToken string) util.Middleware {
    return func(h http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            if r.Header.Get("Auth") != authToken {
                util.SendError(w, "...", http.StatusForbidden, false)
                return
            }
            // Add authentication-specific context here
            h.ServeHTTP(w, r)
        })
    }
}

type Handler struct {
    ...
    ...
}

func (h *HandlerW) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    // Get context values here
    decoder := json.NewDecoder(r.Body)

    // ...
}

func main() {
    router := mux.NewRouter()
    authToken, ok := getAuthToken()
    if !ok {
        panic("...")
    }
    authCheck := checkAuth(authToken)
    h := Handler{
       ...
   } 
   router.Handle("/hello", util.UseMiddleware(authCheck, Handler, ...))
}

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