API design for fire and forget endpoints - go

Question

I’m currently maintaining a few HTTP APIs based on the standard library and gorilla mux and running in kubernetes (GKE).
We’ve adopted the http.TimeoutHandler as our “standard” way to have a consistent timeout error management.
A typical endpoint implementation will use the following “chain”:
MonitoringMiddleware => TimeoutMiddleware => … => handler
so that we can monitor a few key metrics per endpoint.
One of our API is typically used in a “fire and forget” mode meaning that clients will push some data and not care for the API response. We are facing the issue that
the Golang standard HTTP server will cancel a request context when the client connection is no longer active (godoc)
the TimeoutHandler will return a “timeout” response whenever the request context is done (see code)
This means that we are not processing requests to completion when the client disconnects which is not what we want and I’m therefore looking for solutions.
The only discussion I could find that somewhat relates to my issue is https://github.com/golang/go/issues/18527; however
The workaround is your application can ignore the Handler's Request.Context()
would mean that the monitoring middleware would not report the "proper" status since the Handler would perform the request processing in its goroutine but the TimeoutHandler would be enforcing the status and observability would be broken.
For now, I’m not considering removing our middlewares as they’re helpful to have consistency across our APIs both in terms of behaviours and observability. My conclusion so far is that I need to “fork” the TimeoutHandler and use a custom context for when an handler should not depend on the client waiting for the response or not.
The gist of my current idea is to have:
type TimeoutHandler struct {
handler Handler
body string
dt time.Duration
// BaseContext optionally specifies a function that returns
// the base context for controling if the server request processing.
// If BaseContext is nil, the default is req.Context().
// If non-nil, it must return a non-nil context.
BaseContext func(*http.Request) context.Context
}
func (h *TimeoutHandler) ServeHTTP(w ResponseWriter, r *Request) {
reqCtx := r.Context()
if h.BaseContext != nil {
reqCtx = h.BaseContext(r)
}
ctx, cancelCtx := context.WithTimeout(reqCtx, h.dt)
defer cancelCtx()
r = r.WithContext(ctx)
...
case <-reqCtx.Done():
tw.mu.Lock()
defer tw.mu.Unlock()
w.WriteHeader(499) // write status for monitoring;
// no need to write a body since no client is listening.
case <-ctx.Done():
tw.mu.Lock()
defer tw.mu.Unlock()
w.WriteHeader(StatusServiceUnavailable)
io.WriteString(w, h.errorBody())
tw.timedOut = true
}
The middleware BaseContext callback would return context.Background() for requests to the “fire and forget” endpoint.
One thing I don’t like is that in doing so I’m losing any context keys written so this new middleware would have strong usage constraints. Overall I feel like this is more complex than it should be.
Am I completely missing something obvious?
Any feedback on API instrumentation (maybe our middlewares are an antipattern) /fire and forget implementations would be welcomed!
EDIT: as most comments are that a request for which the client does not wait for a response has unspecified behavior, I checked for more information on typical clients for which this happens.
From our logs, this happens for user agents that seem to be mobile devices. I can imagine that connections can be much more unstable and the problem will likely not disappear.
I would therefore not conclude that I shouldn't find a solution since this is currently creating false-positive alerts.