const maxPostCloseReadBytes = 256 << 10
…
const maxPostCloseReadTime = 50 * time.Millisecond

func maybeDrainBody(r io.Reader) bool {
	drainedCh := make(chan bool, 1)
	go func() {
…
		if _, err := io.CopyN(io.Discard, r, maxPostCloseReadBytes+1); err == io.EOF {
			drainedCh <- true
		} else {
			drainedCh <- false
		}
	}()
	select {
	case drained := <-drainedCh:
		return drained
	case <-time.After(maxPostCloseReadTime):
		return false
	}
}
…
		select {
		case err := <-waitForBodyRead:
			tryPutIdle := func() {
				alive = alive &&
					!pc.sawEOF &&
					pc.wroteRequest() &&
					tryPutIdleConn(rc.treq)
			}
			switch err {
			case io.EOF:
				tryPutIdle()
				eofc <- struct{}{}
			case errClosedEarly:
				// Read resp before signaling eofc: the send lets the caller's
				// Close return, and resp belongs to the caller after that.
				tryDrain := alive && resp.ContentLength <= maxPostCloseReadBytes
				eofc <- struct{}{}
				if tryDrain && maybeDrainBody(body.body) {
					tryPutIdle()
				} else {
					alive = false
				}
			default:
				alive = false
			}

Warm-up