如何适当地增加上下文

Question

I'm trying to create an authentication middleware in my Lambda, which basically injects a property user in the ctx struct, and call the handler function. How I'm doing:

middlewares/authentication.go:

package middlewares

import (
    "context"

    "github.com/aws/aws-lambda-go/events"
    "github.com/passus/api/models"
)

func Authentication(next MiddlewareSignature) MiddlewareSignature {
    user := models.User{}

    return func(ctx context.Context, request events.APIGatewayProxyRequest) (events.APIGatewayProxyResponse, error) {
        claims := request.RequestContext.Authorizer["claims"]

        // Find user by claims properties.
        if err := user.Current(claims); err != nil {
            return events.APIGatewayProxyResponse{}, err
        }

        // Augment ctx with user property.
        ctx = context.WithValue(ctx, "user", user)
        return next(ctx, request)
    }
}

my-lambda.go:

package main

import (
    "context"
    "fmt"

    "github.com/aws/aws-lambda-go/events"
    "github.com/aws/aws-lambda-go/lambda"
    "github.com/passus/api/middlewares"
)

func Handler(ctx context.Context, request events.APIGatewayProxyRequest) (events.APIGatewayProxyResponse, error) {
    fmt.Println(ctx.user)

    return events.APIGatewayProxyResponse{}, nil
}

func main() {
    lambda.Start(
        middlewares.Authentication(Handler),
    )
}

The problem with this approach is that: it doesn't work. I see the following error when I try to build it: create/main.go:13:17: ctx.user undefined (type context.Context has no field or method user)

Thank you in advance.

Answer 1

You cannot access values added to a context directly—you need to use the Value(key interface{}) interface{} API.

This is because any value added to a Context needs to be immutable in order to be thread safe. Any changes to existing values on a Context, is accomplished by creating a new Context.

This is the updated my-lambda.go:

func Handler(ctx context.Context, request events.APIGatewayProxyRequest) (events.APIGatewayProxyResponse, error) {
    fmt.Println(ctx.value("user").(models.User))

    return events.APIGatewayProxyResponse{}, nil
}

Value returns an interface, so you need to use type assertion.

NB: the use of plain strings as keys on a Context is not recommended, as this could result in key collision.