post请求体过大导致ngx.req.get_post_args()取不到参数体的问题

http://nginx.org/en/docs/http/ngx_http_core_module.html#client_body_buffer_size

该地址对于client_body_buffer_size配置做了说明

syntax: client_body_buffer_size size

default: client_body_buffer_size 8k|16k

context: http,server,location

Sets buffer size for reading client request body. In case the request body is larger than the buffer, the whole body or only its part is written to a temporary file. By default, buffer size is equal to two memory pages. This is 8K on x86, other 32-bit platforms, and x86-64. It is usually 16K on other 64-bit platforms.

为读取客户端请求体设置缓存区大小。为了防止请求体超过缓存区大小，将整个请求体或其中一分部写入一个临时文件。默认缓存区大小为two memory pages。在x86、32位机器、x86-64上是8k，其它64位机器上是16k。

解决方案有两种：

1、调大client_body_buffer_size值

client_body_buffer_size 128k

client_max_body_size 100m

这两个参数配合使用，如果总body_size超过这个值，请求会被拒绝

2、去临时文件读取请求体

需要用到ngx.req.get_body_file，从临时文件读取数据

--Nginx服务器中使用lua获取get或post参数
local request_method = ngx.var.request_method
local args = nil
local param = nil
local param2 = nil
--获取参数的值
if "GET" == request_method then
    args = ngx.req.get_uri_args()
elseif "POST" == request_method then
    ngx.req.read_body()
    args = ngx.req.get_post_args()
end
param = args["param"]
param2 = args["param2"]
--升级版(能处理content-type=multipart/form-data的表单)：
local function explode ( _str,seperator )
    local pos, arr = 0, {}
        for st, sp in function() return string.find( _str, seperator, pos, true ) end do
            table.insert( arr, string.sub( _str, pos, st-1 ) )
            pos = sp + 1
        end
    table.insert( arr, string.sub( _str, pos ) )
    return arr
end
local args = {}
local file_args = {}
local is_have_file_param = false
local function init_form_args()
    local receive_headers = ngx.req.get_headers()
    local request_method = ngx.var.request_method
    if "GET" == request_method then
        args = ngx.req.get_uri_args()
    elseif "POST" == request_method then
        ngx.req.read_body()
            --判断是否是multipart/form-data类型的表单
        if string.sub(receive_headers["content-type"],1,20) == "multipart/form-data;" then   
            is_have_file_param = true
            content_type = receive_headers["content-type"]
            --body_data可是符合http协议的请求体，不是普通的字符串
            body_data = ngx.req.get_body_data()
            --请求体的size大于nginx配置里的client_body_buffer_size，则会导致请求体被缓冲到磁盘临时文件里，client_body_buffer_size默认是8k或者16k
            if not body_data then
                local datafile = ngx.req.get_body_file()
                if not datafile then
                    error_code = 1
                    error_msg = "no request body found"
                else
                    local fh, err = io.open(datafile, "r")
                    if not fh then
                        error_code = 2
                        error_msg = "failed to open " .. tostring(datafile) .. "for reading: " .. tostring(err)
                    else
                        fh:seek("set")
                        body_data = fh:read("*a")
                        fh:close()
                        if body_data == "" then
                            error_code = 3
                            error_msg = "request body is empty"
                        end
                    end
                end
            end
            local new_body_data = {}
            --确保取到请求体的数据
            if not error_code then
                local boundary = "--" .. string.sub(receive_headers["content-type"],31)
                local body_data_table = explode(tostring(body_data),boundary)
                local first_string = table.remove(body_data_table,1)
                local last_string = table.remove(body_data_table)
                for i,v in ipairs(body_data_table) do
                    local start_pos,end_pos,capture,capture2 = string.find(v,'Content%-Disposition: form%-data; name="(.+)"; filename="(.*)"')
                    --普通参数
                    if not start_pos then
                        local t = explode(v,"rnrn")
                        local temp_param_name = string.sub(t[1],41,-2)
                        local temp_param_value = string.sub(t[2],1,-3)
                        args[temp_param_name] = temp_param_value
                    else
                    --文件类型的参数，capture是参数名称，capture2是文件名                            
                        file_args[capture] = capture2
                        table.insert(new_body_data,v)
                    end
                end
                table.insert(new_body_data,1,first_string)
                table.insert(new_body_data,last_string)
                --去掉app_key,app_secret等几个参数，把业务级别的参数传给内部的API
                body_data = table.concat(new_body_data,boundary)--body_data可是符合http协议的请求体，不是普通的字符串
            end
        else
            args = ngx.req.get_post_args()
        end
    end
end