proposal: net/http: add context cancelation reason for server handlers

[hummerd]

Proposal Details

While serving HTTP request we have req.Context() that can be propagated down the pipe. This context can be canceled either by standard library - by http.Server itself in case of communication issues or by http.TimeoutHandler. But also it can be canceled by user code.
It would be nice to have ability to clarify the reason why context was canceled (in case it was canceled by standard library). Logged reason will make you 100% sure and should make investigation of such context cancellation errors easier.

I'm ready to make a PR in case this proposal is considered useful.

[mauri870]

You can make use of the Cause apis to do that, context.WithDeadlineCause, context.WithCancelCause, context.WithTimeoutCause and so on, and then look at context.Cause for the cancellation cause.

[hummerd]

You can make use of the Cause apis to do that, context.WithDeadlineCause, context.WithCancelCause, context.WithTimeoutCause and so on, and then look at context.Cause for the cancellation cause.

The idea of the proposal is to add cause for cases when context is canceled within standard library itself. And yes it can be accomplished with context.WithCancelCause and context.WithTimeoutCause.

[kszafran]

This would be very useful. Nowadays, when I see that a request failed due to context canceled, I have no idea happened. It could be a timeout on a SQL query, some internal request, etc. And if the context was canceled, because the client closed the connection (which happens in (*connReader).backgrounRead() -> cr.handleReadError(err), as far as I'm able to tell), I'd like to be able to distinguish it and not log it as an error.

[seankhliao]

cc @neild