net/http: document r.Context().Done() behaviour on cancelled POST request

[novalagung]

What version of Go are you using (go version)?

$ go version

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env

What did you do?

What did you expect to see?

What did you see instead?

---

Recently I found this question on stack overflow, the OP asked why the r.Context().Done() channel does not receive any data on cancelled POST request. But it does receive data on cancelled GET request.

I tried to find the relevant explanation on the go doc, but I found nothing. Is this not documented?

[julieqiu]

A better place to ask this kind of question is golang-nuts. See https://golang.org/wiki/Questions.

[novalagung]

@julieqiu I think you are missing my point. I'm not asking about the answer. I have the answer already, and even in the [stackoverflow]https://stackoverflow.com/questions/57246852 it's also answered.

But the things it the answer is not documented in any golang doc. This might cause some confusion.

[novalagung]

The net/http server checks for closed connections by reading the connection. No reads are started until the application starts reading the request body (if any).

What I mean is above statement. I think it's need to be documented.

On GET client request, when the request is cancelled from the client side then immediately r.Context().Done() channel receives a value. But on POST request, it does not.

On POST client request, the r.Context().Done() never receive any value, unless in the handler there is a read process from r.Body and during that reading process the request is cancelled by the client. Then only that the the channel will receive a value.

[julieqiu]

@novalgung sorry for the confusion. It seems like this is the correct behavior,
but perhaps someone with net/http expertise should decide if this needs to be
documented.

/cc @dmitshur @bradfitz