client.go
func (c *HostClient) Do(req *Request, resp *Response) error {
	var err error
	var retry bool
	maxAttempts := c.MaxIdemponentCallAttempts
	if maxAttempts <= 0 {
		maxAttempts = DefaultMaxIdemponentCallAttempts
	}
	attempts := 0

	atomic.AddInt32(&c.pendingRequests, 1)
	for {
		retry, err = c.do(req, resp)
		if err == nil || !retry {
			break
		}

		if !isIdempotent(req) {
			// Retry non-idempotent requests if the server closes
			// the connection before sending the response.
			//
			// This case is possible if the server closes the idle
			// keep-alive connection on timeout.
			//
			// Apache and nginx usually do this.
			if err != io.EOF {
				break
			}
		}
		attempts++
		if attempts >= maxAttempts {
			break
		}
	}
	atomic.AddInt32(&c.pendingRequests, -1)

	if err == io.EOF {
		err = ErrConnectionClosed
	}
	return err
}
func (c *HostClient) Do(req *Request, resp *Response) error {
	var err error
	var retry bool
	maxAttempts := c.MaxIdemponentCallAttempts
	if maxAttempts <= 0 {
		maxAttempts = DefaultMaxIdemponentCallAttempts
	}
	attempts := 0

	atomic.AddInt32(&c.pendingRequests, 1)
	for {
		retry, err = c.do(req, resp)
		if err == nil || !retry {
			break
		}

		if !isIdempotent(req) {
			// Retry non-idempotent requests if the server closes
			// the connection before sending the response.
			//
			// This case is possible if the server closes the idle
			// keep-alive connection on timeout.
			//
			// Apache and nginx usually do this.
			if err != io.EOF {
				break
			}
		}
		attempts++
		if attempts >= maxAttempts {
			break
		}
	}
	atomic.AddInt32(&c.pendingRequests, -1)

	if err == io.EOF {
		err = ErrConnectionClosed
	}
	return err
}
// Do performs the given http request and sets the corresponding response.
//
// Request must contain at least non-zero RequestURI with full url (including
// scheme and host) or non-zero Host header + RequestURI.
//
// The function doesn't follow redirects. Use Get* for following redirects.
//
// Response is ignored if resp is nil.
//
// ErrNoFreeConns is returned if all HostClient.MaxConns connections
// to the host are busy.
//
// It is recommended obtaining req and resp via AcquireRequest
// and AcquireResponse in performance-critical code.