Lesson 13 · A body and Content-Length

A body and Content-Length

A POST or PUT carries a body, and the server needs to know how many bytes to read - that is the Content-Length header. Today you attach a body and set its length automatically.

The goal

Attach a body to a request, set Content-Length to its byte count, and append it after the blank line.

Start here - the target

TO DO

Scenario: Framing a request body by length

Givena POST request for "http://example.com/s" with the body "hello" attached

Whenthe request is serialized

Thenthe Content-Length header is "5" and the bytes end with "\r\n\r\nhello"

Andattaching an empty body sets Content-Length "0", while a request with no body attached has no Content-Length header at all

Background

When a request carries content - a form post, a JSON payload - the receiver has to know where the body ends, and the simplest way to say so is the Content-Length header: the exact number of bytes that follow the blank line. Attaching a body of hello sets Content-Length: 5, and serialize writes those five bytes right after the header block. A request with no body attached sends no Content-Length at all - which is why the bodyless GET from last lesson is still correct and unchanged.

Two edges are worth pinning. An empty body is still a body: Content-Length: 0 is a valid, meaningful header. And set the length only if it is not already set, so a later lesson that wants to control the header itself (a HEAD-style request, or a re-sent body) is not overridden. Length-framing is the default; chunked framing, the harder case, is chapter four.

Make it work

// attaching a body sets Content-Length to its byte length
// (only if not already set), and serialize appends the body
// bytes after the header block's blank line.
func (r *Request) SetBody(b []byte) {
  // record the body; Set("Content-Length", len(b)) if absent
}

CheckpointDONE

A request frames its body with a correct Content-Length. Commit and stop here.