如何使用LuaSec在Lua中正确发送POST数据？

尝试在Lua中通过LuaSec发送JSON格式POST数据，按示例操作后无数据发送，GET请求也存在同类问题。怀疑ltn12使用方式有误，测试代码如下：

local ltn12 = require('ltn12')
local https = require('ssl.https')
local json = require("json")

local body = json.encode({
    test =  "test ok"
})

local r = {}
https.request {
    url = 'https://httpbin.org/anything',
    method = "POST",
    headers = {["Content-Type"] = "application/json"},
    source = ltn12.source.string(body),
    sink = ltn12.sink.table(r)
}
print(r[1])

返回结果中data字段为空：

{
  "args": {}, 
  "data": "", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Content-Type": "application/json", 
    "Host": "httpbin.org", 
    "User-Agent": "LuaSocket 3.0-rc1", 
    "X-Amzn-Trace-Id": "..."
  }, 
  "json": null, 
  "method": "POST", 
  "origin": "XX.XX.XX.XX", 
  "url": "https://httpbin.org/anything"
}

核心问题是未在请求头中添加Content-Length字段。LuaSec搭配ltn12的组合不会自动计算并设置请求体长度，服务器无法知晓需要读取多少数据，导致接收为空。

只需在headers中添加Content-Length，值为JSON字符串的长度即可：

修正后的完整代码：

local ltn12 = require('ltn12')
local https = require('ssl.https')
local json = require("json")

local body = json.encode({
    test =  "test ok"
})

local r = {}
local res, code, headers, status = https.request {
    url = 'https://httpbin.org/anything',
    method = "POST",
    headers = {
        ["Content-Type"] = "application/json",
        ["Content-Length"] = tostring(#body) -- 新增Content-Length头
    },
    source = ltn12.source.string(body),
    sink = ltn12.sink.table(r)
}
print(r[1])

此时httpbin返回的结果中，data和json字段会正确显示发送的内容：

{
  "args": {},
  "data": "{\"test\":\"test ok\"}",
  "files": {},
  "form": {},
  "headers": {
    "Content-Length": "17",
    "Content-Type": "application/json",
    "Host": "httpbin.org",
    "User-Agent": "LuaSocket 3.0-rc1",
    "X-Amzn-Trace-Id": "..."
  },
  "json": {
    "test": "test ok"
  },
  "method": "POST",
  "origin": "XX.XX.XX.XX",
  "url": "https://httpbin.org/anything"
}

内容的提问来源于stack exchange，提问作者pguimier