func (pc *persistConn) shouldRetryRequest(req *Request, err error) bool {
	if http2isNoCachedConnError(err) {
		// Issue 16582: if the user started a bunch of
		// requests at once, they can all pick the same conn
		// and violate the server's max concurrent streams.
		// Instead, match the HTTP/1 behavior for now and dial
		// again to get a new TCP connection, rather than failing
		// this request.
		return true
	}
	if err == errMissingHost {
		// User error.
		return false
	}
	if !pc.isReused() {
		// This was a fresh connection. There's no reason the server
		// should've hung up on us.
		//
		// Also, if we retried now, we could loop forever
		// creating new connections and retrying if the server
		// is just hanging up on us because it doesn't like
		// our request (as opposed to sending an error).
		return false
	}
	if _, ok := err.(nothingWrittenError); ok {
		// We never wrote anything, so it's safe to retry, if there's no body or we
		// can "rewind" the body with GetBody.
		return req.outgoingLength() == 0 || req.GetBody != nil
	}
	if !req.isReplayable() {
		// Don't retry non-idempotent requests.
		return false
	}
	if _, ok := err.(transportReadFromServerError); ok {
		// We got some non-EOF net.Conn.Read failure reading
		// the 1st response byte from the server.
		return true
	}
	if err == errServerClosedIdle {
		// The server replied with io.EOF while we were trying to
		// read the response. Probably an unfortunately keep-alive
		// timeout, just as the client was writing a request.
		return true
	}
	return false // conservatively
}
Warm-up
