关于golang http transport的讲解，网上有很多文章读它进行了描述，但很多文章讲的都比较粗，很多代码实现并没有讲清楚。故给出更加详细的实现说明。整体看下来细节实现层面还是比较难懂的。
本次使用golang版本1.12.9
transport实现了RoundTripper接口，该接口只有一个方法RoundTrip()，故transport的入口函数就是RoundTrip()。transport的主要功能其实就是缓存了长连接，用于大量http请求场景下的连接复用，减少发送请求时TCP(TLS)连接建立的时间损耗，同时transport还能对连接做一些限制，如连接超时时间，每个host的最大连接数等。transport对长连接的缓存和控制仅限于TCP+(TLS)+HTTP1，不对HTTP2做缓存和限制。
tranport包含如下几个主要概念：
连接池：在idleConn中保存了不同类型(connectMethodKey)的请求连接(persistConn)。当发生请求时，首先会尝试从连接池中取一条符合其请求类型的连接使用
readLoop/writeLoop：连接之上的功能，循环处理该类型的请求(发送request，返回response)
roundTrip：请求的真正入口，接收到一个请求后会交给writeLoop和readLoop处理。
一对readLoop/writeLoop只能处理一条连接，如果这条连接上没有更多的请求，则关闭连接，退出循环，释放系统资源
下述代码都来自golang源码的src/net/httptransport.go文件
type RoundTripper interface { // RoundTrip executes a single HTTP transaction, returning // a Response for the provided Request. // // RoundTrip should not attempt to interpret the response. In // particular, RoundTrip must return err == nil if it obtained // a response, regardless of the response's HTTP status code. // A non-nil err should be reserved for failure to obtain a // response. Similarly, RoundTrip should not attempt to // handle higher-level protocol details such as redirects, // authentication, or cookies. // // RoundTrip should not modify the request, except for // consuming and closing the Request's Body. RoundTrip may // read fields of the request in a separate goroutine. Callers // should not mutate or reuse the request until the Response's // Body has been closed. // // RoundTrip must always close the body, including on errors, // but depending on the implementation may do so in a separate // goroutine even after RoundTrip returns. This means that // callers wanting to reuse the body for subsequent requests // must arrange to wait for the Close call before doing so. // // The Request's URL and Header fields must be initialized. RoundTrip(*Request) (*Response, error) }