Is there a way to list keys in context.Context?

Question

So, I have a context.Context(https://golang.org/pkg/context/) variable with me, is there a way I can list all the keys this variable holds?

Answer 1

It is possible to list the internals of context.Context using unsafe reflection and using that information to figure out the keys and/or see if the information you want is in the context.

There are some pitfall such as if the context implementation returns a hardcoded value for a key it wont show up here, and it can be quite unclear on how to actually access the values using keys.

This is nothing I run in production. But in my case I needed to be able to inspect the context.Context to understand better what information it contains.

func printContextInternals(ctx interface{}, inner bool) {
    contextValues := reflect.ValueOf(ctx).Elem()
    contextKeys := reflect.TypeOf(ctx).Elem()

    if !inner {
        fmt.Printf("\nFields for %s.%s\n", contextKeys.PkgPath(), contextKeys.Name())
    }

    if contextKeys.Kind() == reflect.Struct {
        for i := 0; i < contextValues.NumField(); i++ {
            reflectValue := contextValues.Field(i)
            reflectValue = reflect.NewAt(reflectValue.Type(), unsafe.Pointer(reflectValue.UnsafeAddr())).Elem()

            reflectField := contextKeys.Field(i)

            if reflectField.Name == "Context" {
                printContextInternals(reflectValue.Interface(), true)
            } else {
                fmt.Printf("field name: %+v\n", reflectField.Name)
                fmt.Printf("value: %+v\n", reflectValue.Interface())
            }
        }
    } else {
        fmt.Printf("context is empty (int)\n")
    }
}

Examples:

func Ping(w http.ResponseWriter, r *http.Request) {
    printContextInternals(r.Context(), false)
    /* Prints
        Fields for context.valueCtx
        context is empty (int)
        field name: key
        value: net/http context value http-server
        field name: val
        value: &{Addr::20885 Handler:0xc00001c000 TLSConfig:0xc000001c80 ReadTimeout:0s ReadHeaderTimeout:0s WriteTimeout:0s IdleTimeout:0s MaxHeaderBytes:0 TLSNextProto:map[h2:0x12db010] ConnState:<nil> ErrorLog:<nil> BaseContext:<nil> ConnContext:<nil> disableKeepAlives:0 inShutdown:0 nextProtoOnce:{done:1 m:{state:0 sema:0}} nextProtoErr:<nil> mu:{state:0 sema:0} listeners:map[0xc00015a840:{}] activeConn:map[0xc000556fa0:{}] doneChan:<nil> onShutdown:[0x12e9670]}
        field name: key
        value: net/http context value local-addr
        field name: val
        value: [::1]:20885
        field name: mu
        value: {state:0 sema:0}
        field name: done
        value: 0xc00003c2a0
        field name: children
        value: map[context.Background.WithValue(type *http.contextKey, val <not Stringer>).WithValue(type *http.contextKey, val [::1]:20885).WithCancel.WithCancel:{}]
        field name: err
        value: <nil>
        field name: mu
        value: {state:0 sema:0}
        field name: done
        value: <nil>
        field name: children
        value: map[]
        field name: err
        value: <nil>
        field name: key
        value: 0
        field name: val
        value: map[]
        field name: key
        value: 1
        field name: val
        value: &{handler:0x151cf50 buildOnly:false name: err:<nil> namedRoutes:map[] routeConf:{useEncodedPath:false strictSlash:false skipClean:false regexp:{host:<nil> path:0xc0003d78f0 queries:[]} matchers:[0xc0003d78f0 [GET POST]] buildScheme: buildVarsFunc:<nil>}}

    */


    printContextInternals(context.Background(), false)
    /* Prints
        Fields for context.emptyCtx
        context is empty (int)
    */
}