net/http: http.Client Timeout incompatible with writable bodies

[drigz]

As part of #26937, Transport started returning writable bodies for 101 Switching Protocol responses. This is great, thank you!

However, if you have a non-zero Timeout on your http.Client, you can't use this, because of the following lines from client.go:

	if !deadline.IsZero() {
		resp.Body = &cancelTimerBody{
			stop:          stopTimer,
			rc:            resp.Body,
			reqDidTimeout: didTimeout,
		}
	}

This wraps the writable body in a non-writable body.

Go version: 1.12
Observed behavior: res.Body.(io.ReadWriteCloser) fails when client.Timeout > 0.
Expected behavior: timeouts and writeable bodies are compatible.

Although we have some streaming requests, we would like to set a long (eg one hour) timeout on our requests to prevent resource exhaustion. #30876 suggests that we can't even do this manually with contexts.

[agnivade]

@bradfitz

[nhooyr]

How about we change the context cancellation to also apply to the request body. Then, when the body is first written to, both the context cancellation and Timeout are reset to give the application full control over the protocol being spoken now?