问题描述：

使用client.do(....)循环发送html" title=http>http 消息的时候，会报html" title=http>http: ContentLength=27 with Body length 0 错误。

原因是：

client.do(...)函数中发送完一次req之后，会将req中的body清空导致的。

func (c *Client) do(req *Request) (retres *Response, reterr error) {

    ....

    req.closeBody()

   ....

}

问题复现例子：

package main


import (
        "fmt"
        "net/html" title=http>http"
        "time"
        "bytes"
)


func main() {
        tr := &html" title=http>http.Transport{
            MaxIdleConns:       10,
            IdleConnTimeout:    30 * time.Second,
            DisableCompression: true,
        }
        client := &html" title=http>http.Client{Transport: tr}
        str := "hello world, I am html" title=http>http body"
        // 这里的req是全局的，所有的发送消息都会使用这个req
        req, err := html" title=http>http.NewRequest("GET", "html" title=http>http://127.0.0.1/", bytes.NewReader([]byte(str)))
        if err != nil {
            fmt.Println(err)
            return
        }
        req.Header.Add("If-None-Match", `W/"wyzzy"`)
        for i:=0;i<5;i++ {
            resp, err := client.Do(req)
            if err != nil {
                fmt.Println(err)
            }
            fmt.Printf("%s \n", resp)
        }
}

输出结果：

$ ./client //第一次发送成功，后面发送都会失败。
&{200 OK %!s(int=200) HTTP/1.1 %!s(int=1) %!s(int=1) map[Content-Length:[23] Content-Type:[text/plain; charset=utf-8] Date:[Tue, 22 Dec 2020 09:19:43 GMT]] %!s(*html" title=http>http.bodyEOFSignal=&{0xc000090040 {0 0} false <nil> 0x12348c0 0x1234850}) %!s(int64=23) [] %!s(bool=false) %!s(bool=false) map[] %!s(*html" title=http>http.Request=&{GET 0xc000132000 HTTP/1.1 1 1 map[If-None-Match:[W/"wyzzy"]] {0xc000066c30} 0x1233870 27 [] false 127.0.0.1 map[] map[] <nil> map[]   <nil> <nil> <nil> 0xc000016088}) %!s(*tls.ConnectionState=<nil>)}
Get "html" title=http>http://127.0.0.1/": html" title=http>http: ContentLength=27 with Body length 0
%!s(*html" title=http>http.Response=<nil>)
Get "html" title=http>http://127.0.0.1/": html" title=http>http: ContentLength=27 with Body length 0
%!s(*html" title=http>http.Response=<nil>)
Get "html" title=http>http://127.0.0.1/": html" title=http>http: ContentLength=27 with Body length 0
%!s(*html" title=http>http.Response=<nil>)
Get "html" title=http>http://127.0.0.1/": html" title=http>http: ContentLength=27 with Body length 0
%!s(*html" title=http>http.Response=<nil>)

代码修复例子：

package main


import (
        "fmt"
        "net/html" title=http>http"
        "time"
        "bytes"
)


func main() {
        tr := &html" title=http>http.Transport{
            MaxIdleConns:       10,
            IdleConnTimeout:    30 * time.Second,
            DisableCompression: true,
        }
        client := &html" title=http>http.Client{Transport: tr}
        str := "hello world, I am html" title=http>http body"


        for i:=0;i<5;i++ {
            req, err := html" title=http>http.NewRequest("GET", "html" title=http>http://127.0.0.1/", bytes.NewReader([]byte(str)))
            if err != nil {
                fmt.Println(err)
                return
            }
            req.Header.Add("If-None-Match", `W/"wyzzy"`)
            resp, err := client.Do(req)
            if err != nil {
                fmt.Println(err)
            }
            fmt.Printf("%s \n", resp)
        }
}

修复后的执行结果：

$ ./client
&{200 OK %!s(int=200) HTTP/1.1 %!s(int=1) %!s(int=1) map[Content-Length:[23] Content-Type:[text/plain; charset=utf-8] Date:[Tue, 22 Dec 2020 09:23:42 GMT]] %!s(*html" title=http>http.bodyEOFSignal=&{0xc0001462c0 {0 0} false <nil> 0x12348c0 0x1234850}) %!s(int64=23) [] %!s(bool=false) %!s(bool=false) map[] %!s(*html" title=http>http.Request=&{GET 0xc00016a000 HTTP/1.1 1 1 map[If-None-Match:[W/"wyzzy"]] {0xc000114c30} 0x1233870 27 [] false 127.0.0.1 map[] map[] <nil> map[]   <nil> <nil> <nil> 0xc00012e008}) %!s(*tls.ConnectionState=<nil>)}
&{200 OK %!s(int=200) HTTP/1.1 %!s(int=1) %!s(int=1) map[Content-Length:[23] Content-Type:[text/plain; charset=utf-8] Date:[Tue, 22 Dec 2020 09:23:42 GMT]] %!s(*html" title=http>http.bodyEOFSignal=&{0xc00009c000 {0 0} false <nil> 0x12348c0 0x1234850}) %!s(int64=23) [] %!s(bool=false) %!s(bool=false) map[] %!s(*html" title=http>http.Request=&{GET 0xc00016a200 HTTP/1.1 1 1 map[If-None-Match:[W/"wyzzy"]] {0xc0001150e0} 0x1233870 27 [] false 127.0.0.1 map[] map[] <nil> map[]   <nil> <nil> <nil> 0xc00012e008}) %!s(*tls.ConnectionState=<nil>)}
&{200 OK %!s(int=200) HTTP/1.1 %!s(int=1) %!s(int=1) map[Content-Length:[23] Content-Type:[text/plain; charset=utf-8] Date:[Tue, 22 Dec 2020 09:23:42 GMT]] %!s(*html" title=http>http.bodyEOFSignal=&{0xc0001a4000 {0 0} false <nil> 0x12348c0 0x1234850}) %!s(int64=23) [] %!s(bool=false) %!s(bool=false) map[] %!s(*html" title=http>http.Request=&{GET 0xc0000ae000 HTTP/1.1 1 1 map[If-None-Match:[W/"wyzzy"]] {0xc000096180} 0x1233870 27 [] false 127.0.0.1 map[] map[] <nil> map[]   <nil> <nil> <nil> 0xc00012e008}) %!s(*tls.ConnectionState=<nil>)}
&{200 OK %!s(int=200) HTTP/1.1 %!s(int=1) %!s(int=1) map[Content-Length:[23] Content-Type:[text/plain; charset=utf-8] Date:[Tue, 22 Dec 2020 09:23:42 GMT]] %!s(*html" title=http>http.bodyEOFSignal=&{0xc00009c100 {0 0} false <nil> 0x12348c0 0x1234850}) %!s(int64=23) [] %!s(bool=false) %!s(bool=false) map[] %!s(*html" title=http>http.Request=&{GET 0xc00016a380 HTTP/1.1 1 1 map[If-None-Match:[W/"wyzzy"]] {0xc000115440} 0x1233870 27 [] false 127.0.0.1 map[] map[] <nil> map[]   <nil> <nil> <nil> 0xc00012e008}) %!s(*tls.ConnectionState=<nil>)}
&{200 OK %!s(int=200) HTTP/1.1 %!s(int=1) %!s(int=1) map[Content-Length:[23] Content-Type:[text/plain; charset=utf-8] Date:[Tue, 22 Dec 2020 09:23:42 GMT]] %!s(*html" title=http>http.bodyEOFSignal=&{0xc0001a4080 {0 0} false <nil> 0x12348c0 0x1234850}) %!s(int64=23) [] %!s(bool=false) %!s(bool=false) map[] %!s(*html" title=http>http.Request=&{GET 0xc0000ae180 HTTP/1.1 1 1 map[If-None-Match:[W/"wyzzy"]] {0xc000096510} 0x1233870 27 [] false 127.0.0.1 map[] map[] <nil> map[]   <nil> <nil> <nil> 0xc00012e008}) %!s(*tls.ConnectionState=<nil>)}