Target a method call inside a function body

[adrien-f]

Greetings !

I would like to know if it's possible to target a method call inside a function body like this:

func (c *SomeType) Run() {
...
resp, err := c.httpClient.Do(req)
...
}

Where httpClient is net/http.*Client. I would then like to use wrap-expression to intercept the result and enrich the parent trace.

Thank you for your help 🙌

[RomainMuller]

Hey!

We had tried method-call instrumentation at some point, but it turned out to be extremely expensive due to the need to evaluate every selector expression that is the parent of a call expression; and this particular node combination happens to be extremely frequent. On top of this, it requires determining the apparent (i.e, static) type of the selector expression's expr part; and that runs the risk of not catching calls that are made to the correct receiver type, but through an interface.

In all practical use-cases we have had so far where method-call would have been useful, we have been able to move the instrumentation to the function side (i.e, instrumenting the body of *net/http.Client.Do instead of calls to it). Maybe this is also an option in your case here? Of course, if your intention is to do so only in certain context, then this is more challenging (as there is very little information provided to the Do method, not even a context.Context). A bit of difficulty here also comes from the fact net/http is part of the standard lib and aspects have to be written carefully to avoid creating dependency cycles.

I shall note that -- when using the Datadog instrumentation (I'm assuming this is what you're using), you can retrieve the current span from the goroutine's context without having a context.Context value around; as we inject the necessary elements to be able to do trace propagation via goroutine-local storage (GLS) -- using tracer.SpanFromContext(context.TODO()) will actually pull a span from the GLS if there is one.