net/http: CancelRequest twice on the same request panics

[yichengq]

If i call CancelRequest twice on the same request, CancelRequest will panic because of close a closed channel at here:
https://golang.org/src/net/http/transport.go#L517

I argue that CancelRequest should not panic here:

1. There is no need to panic when cancelling a cancelled request. And no document says that is not allowed.
2. It is hard to ensure a request is canceled without CancelRequest retry:

    go func() { c <- f(client.Do(req)) }()
    select {
    case <-ctx.Done():
        tr.CancelRequest(req)
        <-c // Wait for f to return.
        return ctx.Err()
    case err := <-c:
        return err
    }

(code from https://blog.golang.org/context)
It is possible that the request is canceled before Do is called, and it needs CancelRequest retry to ensure it is canceled.

from https://github.com/coreos/go-etcd/issues/197

[bradfitz]

https://go-review.googlesource.com/8550