Nil pointer dereference in context

[Pratim_Roy]

Is the line https://github.com/golang/go/blob/138d2c9b88d9e3d5adcebf9cb7c356b43d6a9782/src/context/context.go#L566

return c.Context.Value(key)

Concurrent Safe?

The stack trace of error I am receiving is attached

This panic is arising when a lot of read is happening to context value.

[skillian (Sean Killian)]

Yes, Context.Value is thread/goroutine-safe. I can’t tell from this snippet of code what the issue is. Why are you redacting out one of the lines of the stack trace?

[Pratim_Roy]

The stack contains the repo information for which our organization has asked us not to expose it anywhere. That’s why i had to redact it.

I can provide some information though.
For every request, we put this in request context.

*r = *r.WithContext(ctx.WithValue(r.Context(), appData, &sync.Map{}))

And fetch it

if data, ok := ctx.Value(appData).(*sync.Map); ok {        //here is the panic happening
		data.Range(func(key, value interface{}) bool {
			if k, ok := key.(string); ok {
				appData[k] = value
			}

			return true
		})
	}

All this happens through middleware.

[skillian (Sean Killian)]

@Pratim_Roy

*r = *r.WithContext(ctx.WithValue(r.Context(), appData, &sync.Map{}))

This is not threadsafe/goroutine-safe on all machine architectures.

EDIT: This is wrong; I was thinking this was storing to a pointer, but I suspect it’s actually storing to the Request struct which is definitely not threadsafe.

Ranging over the data *sync.Map won’t access invalid memory, but it is non-deterministic, so if other goroutines are adding/removing elements, you don’t know what you’ll get.

@Pratim_Roy

appData[k] = value

It looks like appData is a (potentially global?) map and you’re writing into it. This is not threadsafe/goroutine-safe.