StartTransaction returns transaction with outdated context, when the transaction already exists

[far4599]

Summary

StartTransaction function returns currentTransaction with outdated context.

Steps To Reproduce

1. Create transaction with StartTransaction: tr := sentry.StartTransaction(context.TODO(),...)
2. Add a new value to its context: ctx := context.WithValue(tc.Context(), "key", "value")
3. Receive transaction from the context like it is done somewhere down the stack: tr := sentry.StartTransaction(ctx,...)
4. Check if "key" exists in the transaction context: _, exist := tc.Context().Value("key").(string) // exist == false

Expected Behavior

StartTransaction may create a new transaction or return the one from the context, it is expected that tc.Context() returns the latest provided context, but not the outdated one.

Additional context

I found a tricky bug here.

func StartTransaction(ctx context.Context, name string, options ...SpanOption) *Span {
	currentTransaction, exists := ctx.Value(spanContextKey{}).(*Span)
	if exists {
		return currentTransaction
	}
	...

StartTransaction function returns currentTransaction when it exists in the provided context. And currentTransaction contains a pointer to the context inside, but the older version of the context. It is expected that a newly started transaction will provide access to the modified context through transaction.Context() method.

To resolve the issue we need to update the pointer to the context, stored in currentTransaction

func StartTransaction(ctx context.Context, name string, options ...SpanOption) *Span {
	currentTransaction, exists := ctx.Value(spanContextKey{}).(*Span)
	if exists {
		currentTransaction.ctx = ctx
		return currentTransaction
	}
	...

I found it when I used the grpc gateway with implemented interceptors. As the service supports HTTP and grpc the code reuses interceptors for both protocols. Sentry tracing is added in both HTTP middleware and grpc interceptor layers, and when the StartTransaction function is called second type the content for transaction.Context() contains the previous version of the content which was saved at the moment of object creation.