net/http: add Client.CloseIdleConnections method

[bronze1man]

Consider the following code:

package main

import (
	"fmt"
	"time"
	"net/http"
	"io/ioutil"
)

func main(){
	b,err:=GetDataFromMyServer(time.Second*5)
	fmt.Println(string(b),err)
	return
}

func GetDataFromMyServer(timeout time.Duration) (b []byte,err error){
	httpClient:=&http.Client{
		Transport: &http.Transport{
			Dial: net.Dial, // or something with a proxy implement.
		},
		Timeout: timeout,
	}
	defer CloseGoHttpClient(httpClient)
	resp,err:=httpClient.Get("https://www.google.com/")
	if err!=nil{
		return nil,err
	}
	defer resp.Body.Close()
	return ioutil.ReadAll(resp.Body)
}

func CloseGoHttpDefaultIdleConnections() {
	if http.DefaultTransport==nil{
		return
	}
	tran,ok:=http.DefaultTransport.(*http.Transport)
	if !ok{
		return
	}
	tran.CloseIdleConnections()
}

func CloseGoHttpClient(client *http.Client){
	CloseGoHttpDefaultIdleConnections()
	if client.Transport==nil{
		return
	}
	tran,ok:=client.Transport.(*http.Transport)
	if !ok{
		return
	}
	tran.CloseIdleConnections()
}

I know that the *http.Client need to be closed or it will leak memory or connections in the process and make a out of memory error after create a lot of it.
It is too complex for a normal programmer to understand how to close a *http.Client (or even know that the *http.Client object need close).
So I think add a func Close() error method to it, should make the programmer think it looks like a *os.File, it need to be closed after using it.
In our team, several new golang programmers made a out of memory and too many connections from creating too many *http.Client object, and forget to close it.

Here is a bug caused by *http.Client object manage:
softlayer/softlayer-go#88

There is not document of how to close a *http.Client, it just say Clients should be reused instead of created as needed., it do not say the create a lot of *http.Client and do not close it will cause leak memory:
https://golang.org/pkg/net/http/#Client

[bcmills]

Is this a dup of #21978?

See also #24739.

[bradfitz]

@bcmills, doesn't seem like a dup of #21978.

I suppose we could add a Close method on Client that just checks for its Transport having a CloseIdleConnections method (as HTTP/1 and HTTP/2 both do), but that's a bit weird because we don't have a named interface for that anywhere.

Updating the Client documentation to mention Transport.CloseIdleConnections is awkward because https://golang.org/pkg/net/http/#Client.Transport is a RoundTripper, so all callers need to do the type check.

[bradfitz]

How about we name the method "CloseIdleConnections" on Client and name the interface in httputil?

Because "Close" almost implies too much: that it'd close in-flight response bodies, etc.

[rsc]

No one seems to be objecting, so proposal approved.

[gopherbot]

Change https://golang.org/cl/130115 mentions this issue: net/http: add Client.CloseIdleConnections

[bronze1man]

How about call the http.DefaultTransport.CloseIdleConnections when the Transport field is nil?
It looks weird to me that it can not CloseIdleConnections and do nothing with the http.DefaultClient.