Go：disable HTTP chunk mode

Go：disable HTTP chunk mode

---

1.结论

应用层显式设置 Content-Length，可以 disable chunk mode。

	http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
		writer.Header().Set("Content-Length", "2049")
		writer.Write(bytes.Repeat([]byte("."), 2049))
	})

---

2.跑起来

Go demo：

package main

import (
	"bytes"
	"net/http"
)

func main() {
	http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
		writer.Write(bytes.Repeat([]byte("."), 2049))
	})

	http.ListenAndServe(":9080", nil)
}

运行：

$ go run main.go 
^Csignal: interrupt

测试：

$ curl -v http://127.0.0.1:9080/
...
> 
< HTTP/1.1 200 OK
< Date: Wed, 28 Apr 2021 10:39:50 GMT
< Content-Type: text/plain; charset=utf-8
< Transfer-Encoding: chunked
< 
...（2049B）

即，当Go服务端写入2049B时，将使用chunk模式传输（Transfer-Encoding: chunked）。

---

3.查源码

Go版本：

$ go version
go version go1.13.6 linux/amd64

源码定义：

// net/http/server.go
// This should be >= 512 bytes for DetectContentType,
// but otherwise it's somewhat arbitrary.
const bufferBeforeChunkingSize = 2048

// net/http/server.go
// Read next request from connection.
func (c *conn) readRequest(ctx context.Context) (w *response, err error) {
	...
	w = &response{
		conn:          c,
		cancelCtx:     cancelCtx,
		req:           req,
		reqBody:       req.Body,
		handlerHeader: make(Header),
		contentLength: -1,
		...
	}
	...
	w.cw.res = w
	w.w = newBufioWriterSize(&w.cw, bufferBeforeChunkingSize)
	return w, nil
}

在创建http.response时指定buffer的大小：

// A response represents the server side of an HTTP response.
type response struct {
	...
	w  *bufio.Writer // buffers output in chunks to chunkWriter
	cw chunkWriter
	...
}

---

4.disable HTTP chunk mode

源码注释：【重点】

// If the handler didn't declare a Content-Length up front, we either
// go into chunking mode or, if the handler finishes running before
// the chunking buffer size, we compute a Content-Length and send that
// in the header instead.

如果你已经显式声明设置Content-Length，则不会进入chunk mode，否则，受到chunk buffer size限制：

1.超过chunk buffer size，use chunk mode；
2.未超chunk buffer size，Go底层计算Content-Length并填入HTTP应答Header；

因此，如果应用层已知HTTP应答总长度，直接设置Content-Length即可disable chunk：

package main

import (
	"bytes"
	"net/http"
)

func main() {
	http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
		writer.Header().Set("Content-Length", "2049")
		writer.Write(bytes.Repeat([]byte("."), 2049))
	})

	http.ListenAndServe(":9080", nil)
}

此时，应用层写入2049B，超过chunk buffer size（2048），但是使用Content-Length而非chunk。

$ curl -v http://127.0.0.1:9080/
... 
< HTTP/1.1 200 OK
< Content-Length: 2049
< Date: Wed, 28 Apr 2021 11:00:00 GMT
< Content-Type: text/plain; charset=utf-8
< 
...（2049B）

---

5.不建议的方式

修改源码 src/net/http/server.go 中 bufferBeforeChunkingSize 值，例如：

// This should be >= 512 bytes for DetectContentType,
// but otherwise it's somewhat arbitrary.
const bufferBeforeChunkingSize = 1024（原2048）

此时，应用层未设置Content-Length并写入1025B，重新编译运行，也将进入chunk mode：

func main() {
	http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
		writer.Write(bytes.Repeat([]byte("."), 1025))
	})

	http.ListenAndServe(":9080", nil)
}

$ go build main.go
$ ./main

$ curl -v http://127.0.0.1:9080/
...
> 
< HTTP/1.1 200 OK
< Date: Wed, 28 Apr 2021 11:12:53 GMT
< Content-Type: text/plain; charset=utf-8
< Transfer-Encoding: chunked
< 
...（1025B）

由于没有API设置 bufferBeforeChunkingSize 值，因此，只能通过源码方式修改。

这种方法不利于后续运维，因此不建议。