语法: ok, err = ngx.flush(wait?)
环境: rewrite_by_lua*, access_by_lua*, content_by_lua*
向客户端刷新响应输出。
自 v0.3.1rc34 版本开始，ngx.flush 接受一个布尔型可选参数 wait (默认值 false)。当通过默认参数调用时，本函数发起一个异步调用 (将直接返回，不等待输出数据被写入系统发送缓冲区)。当把 wait 参数设置为 true 时，本函数将以同步模式执行。
在同步模式下，本函数不会立即返回，一直到所有输出数据被写入系统输出缓冲区，或者到达发送超时 send_timeout 时间。请注意，因为使用了 Lua 协程机制，本函数即使在同步模式下也不会阻塞 Nginx 事件循环。
当 ngx.flush(true) 在 ngx.print 或 ngx.say 之后被立刻调用时，它将使这两个函数以同步模式执行。这在流式输出时非常有用。
请注意，ngx.flush 在 HTTP 1.0 缓冲输出模式下不起作用。详情请参考 HTTP 1.0 support。
自 v0.8.3 版本开始，本函数执行成功是返回 1，否则返回 nil 和错误信息串。
English Source
syntax: ok, err = ngx.flush(wait?)
context: rewrite_by_lua*, access_by_lua*, content_by_lua*
Flushes response output to the client.
ngx.flush accepts an optional boolean wait argument (Default: false) first introduced in the v0.3.1rc34 release. When called with the default argument, it issues an asynchronous call (Returns immediately without waiting for output data to be written into the system send buffer). Calling the function with the wait argument set to true switches to synchronous mode.
In synchronous mode, the function will not return until all output data has been written into the system send buffer or until the send_timeout setting has expired. Note that using the Lua coroutine mechanism means that this function does not block the Nginx event loop even in the synchronous mode.
When ngx.flush(true) is called immediately after ngx.print or ngx.say, it causes the latter functions to run in synchronous mode. This can be particularly useful for streaming output.
Note that ngx.flush is not functional when in the HTTP 1.0 output buffering mode. See HTTP 1.0 support.
Since v0.8.3 this function returns 1 on success, or returns nil and a string describing the error otherwise.